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# EPUB Reader Architectural Decisions
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**Date:** 2026-01-23 19:47:23
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**Status:** Active
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**Based on:** EPUB 3.3 Compliance Audit
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---
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## Purpose
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This document captures architectural decisions about which EPUB 3.3 features to implement, intentionally omit, or defer. Each decision includes rationale based on:
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- Hardware constraints (ESP32-C3, 800x480 4-level grayscale e-ink)
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- Memory limitations (~400KB SRAM, no PSRAM)
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- User experience goals
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- Implementation complexity
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---
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## ADR-001: Inline Image Support
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**Status:** RECOMMENDED FOR IMPLEMENTATION
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### Context
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EPUBs frequently contain images for:
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- Cover art
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- Chapter illustrations
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- Diagrams and figures
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- Decorative elements
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Current implementation displays `[Image: alt_text]` placeholder.
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### Decision
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**Implement inline image rendering** using existing infrastructure.
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### Rationale
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1. **Infrastructure Exists:**
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- `Bitmap` class handles BMP parsing with grayscale conversion and dithering
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- `JpegToBmpConverter` converts JPEG to BMP (most common EPUB image format)
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- `GfxRenderer::drawBitmap()` already renders bitmaps to e-ink
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- `ZipFile` can extract files from EPUB archive
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- Home screen cover rendering demonstrates the pattern works
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2. **Memory Management Pattern:**
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- Convert and cache images to SD card (like thumbnail generation)
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- Load one image at a time during page render
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- Use streaming conversion to minimize RAM usage
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3. **High User Impact:**
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- Many EPUBs contain important visual content
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- Technical books rely on diagrams
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- Children's books heavily use illustrations
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### Implementation Architecture
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```
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┌────────────────────────────────────────────────────────────────┐
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│ Image Processing Pipeline │
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├────────────────────────────────────────────────────────────────┤
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│ │
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│ EPUB ZIP ──► Extract Image ──► Convert to BMP ──► Cache to SD │
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│ │ │ │
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│ │ ├─ JPEG: JpegToBmpConverter│
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│ │ └─ BMP: Direct copy │
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│ │ │
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│ └─► During page render: │
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│ Load cached BMP ──► drawBitmap() │
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│ │
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└────────────────────────────────────────────────────────────────┘
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```
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### Page Element Structure
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```cpp
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// New PageImage element (alongside PageLine)
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class PageImage final : public PageElement {
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std::string cachedBmpPath; // Path to converted BMP on SD
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int16_t width;
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int16_t height;
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public:
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void render(GfxRenderer& renderer, int fontId, int xOffset, int yOffset) override;
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bool serialize(FsFile& file) override;
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static std::unique_ptr<PageImage> deserialize(FsFile& file);
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};
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```
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### Constraints
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- **No PNG support** initially (would require adding `pngle` library)
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- **Maximum image size:** Scale to viewport width, max 800x480
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- **Memory budget:** ~10KB row buffer during conversion
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---
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## ADR-002: JavaScript/Scripting Support
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**Status:** INTENTIONALLY OMITTED
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### Context
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EPUB 3.3 allows JavaScript in content documents for interactive features.
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### Decision
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**Do not implement JavaScript execution.**
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### Rationale
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1. **Security Risk:**
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- Untrusted code execution on embedded device
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- No sandboxing infrastructure
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- Potential for malicious EPUBs
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2. **Hardware Limitations:**
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- E-ink display unsuitable for interactive content
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- Limited RAM for JavaScript engine
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- No benefit for static reading experience
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3. **Minimal EPUB Use:**
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- Most EPUBs don't use JavaScript
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- Interactive textbooks target tablets, not e-readers
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4. **Implementation Complexity:**
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- Would require embedding V8/Duktape/QuickJS
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- DOM manipulation engine
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- Event handling system
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### Alternative
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`<script>` elements are silently ignored. Content remains readable.
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---
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## ADR-003: Fixed Layout (FXL) Support
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**Status:** INTENTIONALLY OMITTED
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### Context
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EPUB Fixed Layout provides pixel-precise page positioning for:
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- Comic books
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- Children's picture books
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- Magazines
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- Technical drawings with precise layout
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### Decision
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**Do not implement Fixed Layout support.**
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### Rationale
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1. **Display Mismatch:**
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- FXL designed for high-resolution color tablets
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- 800x480 grayscale e-ink would require heavy downscaling
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- Visual quality would be poor
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2. **User Experience:**
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- FXL EPUBs expect pan/zoom interaction
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- E-ink refresh rate makes this impractical
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- Text would be too small to read without zoom
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3. **Implementation Complexity:**
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- Requires full CSS positioning engine
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- Viewport meta tag handling
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- Coordinate transformation system
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### Alternative
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FXL EPUBs will open but may display incorrectly. Users should use reflowable EPUBs on this device.
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---
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## ADR-004: Audio/Video and Media Overlays
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**Status:** HARDWARE LIMITED - CANNOT IMPLEMENT
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### Context
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EPUB 3.3 supports:
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- `<audio>` and `<video>` elements
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- Media Overlays (SMIL synchronization)
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- Text-to-speech hints
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### Decision
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**Cannot implement due to hardware constraints.**
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### Rationale
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1. **No Audio Hardware:**
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- Device has no speaker or audio DAC
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- No audio output jack
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2. **No Video Capability:**
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- E-ink refresh rate (~1 Hz) incompatible with video
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- No video decoding hardware
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### Alternative
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- Audio/video elements are ignored
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- Alt text or fallback content displayed if available
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- Media Overlays not processed
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---
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## ADR-005: Color CSS Properties
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**Status:** HARDWARE LIMITED - SIMPLIFIED HANDLING
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### Context
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EPUBs use CSS colors for:
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- Text color (`color`)
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- Background color (`background-color`)
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- Border colors
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### Decision
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**Ignore color properties; display in grayscale.**
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### Rationale
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1. **Hardware Constraint:**
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- Display is 4-level grayscale only
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- Cannot render colors
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2. **Acceptable Degradation:**
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- Text remains readable in black
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- Background remains white
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- Colored elements appear as gray variations
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### Implementation
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Color CSS properties are parsed but not applied. Default black text on white background used.
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---
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## ADR-006: Table Rendering
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**Status:** DEFERRED - OPTIONAL IMPLEMENTATION
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### Context
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Tables appear in:
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- Technical documentation
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- Reference material
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- Data presentations
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Current implementation shows `[Table omitted]` placeholder.
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### Decision
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**Implement simple text-based table rendering as an optional enhancement.**
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### Rationale
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1. **Moderate Impact:**
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- Some EPUBs use tables, but not majority
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- Technical users would benefit most
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2. **Complexity vs. Benefit:**
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- Full table layout is complex (colspan, rowspan, sizing)
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- Simple tables can be rendered as text columns
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### Implementation Approach (if implemented)
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```
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┌──────────────────────────────────────┐
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│ Text-Based Table Rendering │
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├──────────────────────────────────────┤
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│ │
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│ Header1 │ Header2 │ Header3 │
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│ ─────────────────────────────────────│
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│ Data 1 │ Data 2 │ Data 3 │
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│ Data 4 │ Data 5 │ Data 6 │
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│ │
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└──────────────────────────────────────┘
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```
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### Constraints
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- Equal-width columns (no complex sizing)
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- No colspan/rowspan support
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- Truncate wide content
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- Maximum 4-5 columns before overflow
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---
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## ADR-007: SVG and MathML
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**Status:** INTENTIONALLY OMITTED
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### Context
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- SVG: Scalable Vector Graphics for illustrations
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- MathML: Mathematical notation markup
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### Decision
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**Do not implement SVG or MathML rendering.**
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### Rationale
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1. **Implementation Complexity:**
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- SVG requires full vector graphics engine
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- MathML requires specialized math typesetting
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2. **Limited Use:**
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- Most EPUBs use raster images, not SVG
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- MathML primarily in academic texts
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3. **Alternative Exists:**
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- Many EPUBs include fallback PNG for SVG
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- MathML often has image fallback
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### Alternative
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Display alt text or fallback image if available.
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---
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## ADR-008: CSS Selector Support
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**Status:** CURRENT IMPLEMENTATION SUFFICIENT
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### Context
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CSS selectors enable targeting elements for styling. Current support:
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- Element selectors (`p`, `div`)
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- Class selectors (`.classname`)
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- Element.class selectors (`p.intro`)
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### Decision
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**Maintain current limited selector support; do not expand.**
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### Rationale
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1. **Sufficient for Most EPUBs:**
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- 90%+ of EPUB styling uses simple selectors
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- Complex selectors rarely affect core readability
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2. **Implementation Complexity:**
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- Descendant selectors require DOM tree
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- Pseudo-selectors need state tracking
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- Specificity calculation is complex
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3. **Memory Constraints:**
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- DOM tree would consume significant RAM
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- Current streaming parser is memory-efficient
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### Not Implemented
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- Descendant selectors (`div p`)
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- Child selectors (`ul > li`)
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- Sibling selectors (`h1 + p`)
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- Pseudo-classes (`:first-child`, `:hover`)
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- Pseudo-elements (`::before`, `::after`)
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- Attribute selectors (`[type="text"]`)
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---
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## ADR-009: Internal Link Navigation
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**Status:** RECOMMENDED FOR IMPLEMENTATION (PHASE 2)
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### Context
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EPUBs use internal links for:
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- Footnotes
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- Cross-references
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- Table of contents
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- Index entries
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### Decision
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**Implement internal link navigation in Phase 2.**
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### Rationale
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1. **User Value:**
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- Footnotes are common in non-fiction
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- Reference navigation improves usability
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2. **Complexity:**
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- Requires anchor parsing and storage
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- Needs selection UI for link activation
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- Cross-chapter navigation adds complexity
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### Implementation Architecture
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```
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Link Navigation Flow:
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1. Parse <a href="#id"> during HTML parsing
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2. Store link targets in PageLine metadata
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3. Add link highlighting (underline or marker)
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4. User selects link via UI
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5. Resolve target: same chapter (anchor) or cross-chapter (spine + anchor)
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6. Navigate to target page/position
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```
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### Deferred
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- External links (http://) - no network navigation on e-reader
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---
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## ADR-010: DRM and Encryption
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**Status:** INTENTIONALLY OMITTED
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### Context
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EPUB supports DRM through:
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- `encryption.xml` in META-INF
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- Adobe DRM
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- Various proprietary schemes
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### Decision
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**Do not implement DRM support.**
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### Rationale
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1. **Licensing Complexity:**
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- Adobe DRM requires licensing agreements
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- Proprietary schemes have legal restrictions
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2. **User Expectation:**
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- Open-source e-reader users expect DRM-free content
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- DRM conflicts with device modification philosophy
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3. **Implementation Complexity:**
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- Each DRM scheme is different
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- Secure key storage required
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- Regular updates needed for scheme changes
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### Alternative
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Users should remove DRM from purchased content using legal tools before loading to device.
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---
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## ADR-011: List Rendering Enhancements
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**Status:** RECOMMENDED FOR IMPLEMENTATION (PHASE 1)
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### Context
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Current implementation:
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- `<li>` renders bullet character `•`
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- No numbered list support
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- No nesting indentation
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### Decision
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**Enhance list rendering with ordered numbers and nesting.**
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### Rationale
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1. **Low Complexity:**
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- Track list type (`<ol>` vs `<ul>`) in parser state
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- Maintain counter for ordered lists
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- Apply indentation based on nesting depth
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2. **Clear User Benefit:**
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- Numbered lists convey sequence
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- Indentation shows hierarchy
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### Implementation
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```cpp
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// Parser state additions
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int listDepth = 0;
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int orderedListCounter = 0;
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bool isOrderedList = false;
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// On <ol> start
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isOrderedList = true;
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orderedListCounter = 1;
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listDepth++;
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// On <li> in ordered list
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addWord(std::to_string(orderedListCounter++) + ".", REGULAR);
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// Apply indent
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textIndent = listDepth * 20; // pixels per level
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```
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---
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## Summary Table
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| Feature | Decision | Rationale |
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|---------|----------|-----------|
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| Inline Images | IMPLEMENT | High impact, infrastructure ready |
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| JavaScript | OMIT | Security risk, no benefit for e-ink |
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| Fixed Layout | OMIT | Display mismatch, poor UX |
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| Audio/Video | CANNOT | No hardware support |
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| Color CSS | IGNORE | Grayscale display |
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| Tables | DEFER | Moderate impact, high complexity |
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| SVG/MathML | OMIT | High complexity, limited use |
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| Complex CSS Selectors | OMIT | Memory constraints, limited benefit |
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| Internal Links | IMPLEMENT (Phase 2) | User value for references |
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| DRM | OMIT | Licensing, philosophy conflict |
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| List Enhancements | IMPLEMENT (Phase 1) | Low complexity, clear benefit |
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---
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## Implementation Priority
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### Phase 1 (Quick Wins)
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- [x] Basic bullet rendering (already implemented)
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- [ ] Ordered list numbering
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- [ ] Nested list indentation
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- [ ] Line-height CSS support
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### Phase 2 (Image Support)
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- [ ] Image extraction from EPUB
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- [ ] JPEG to BMP conversion for inline images
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- [ ] PageImage element integration
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- [ ] Image scaling and layout
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### Phase 3 (Navigation)
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- [ ] Internal link parsing
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- [ ] Link selection UI
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- [ ] Anchor navigation
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### Deferred/Not Planned
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- PNG support (would need library addition)
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- Table rendering
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- Complex CSS selectors
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- JavaScript, FXL, DRM
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@ -1,262 +0,0 @@
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# EPUB 3.3 Compliance Feature Prioritization
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|
||||
**Date:** 2026-01-23 19:46:02
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**Based on:** EPUB 3.3 Compliance Audit
|
||||
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||||
---
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||||
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## Overview
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This document reviews the audit findings from the CrossPoint Reader EPUB 3.3 compliance audit and prioritizes features for implementation based on:
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1. **User Impact** - How much the feature improves the reading experience
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2. **Implementation Complexity** - Level of effort required
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3. **Existing Infrastructure** - Available code that can be leveraged
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4. **Hardware Feasibility** - Whether the e-ink display can support it
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---
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## Priority 1: High Impact, Infrastructure Ready
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These features have existing code that can be leveraged and provide significant user value.
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### 1.1 Inline Image Rendering
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**Current State:** Images show placeholder text `[Image: alt_text]`
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**Infrastructure Available:**
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- `Bitmap` class (`lib/GfxRenderer/Bitmap.h`) - BMP parsing with grayscale conversion and dithering
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- `JpegToBmpConverter` (`lib/JpegToBmpConverter/`) - JPEG to BMP conversion with prescaling
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- `GfxRenderer::drawBitmap()` - Already renders bitmaps to e-ink display
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- `ZipFile` - Can extract images from EPUB archive
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||||
**Implementation Approach:**
|
||||
1. Extend `ChapterHtmlSlimParser` to extract image `src` attributes
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2. Create `PageImage` element (alongside existing `PageLine`)
|
||||
3. Extract and cache images from EPUB ZIP to SD card as BMP
|
||||
4. Integrate image blocks into page layout calculations
|
||||
5. Render images inline with text during page display
|
||||
|
||||
**Complexity:** Medium
|
||||
**User Impact:** High - Many EPUBs have important diagrams, illustrations, and decorative images
|
||||
|
||||
**Supported Formats:**
|
||||
- JPEG (via `picojpeg`) - Most common in EPUBs
|
||||
- BMP (native support)
|
||||
- PNG - **Not currently supported** (would need `pngle` or similar library)
|
||||
|
||||
---
|
||||
|
||||
### 1.2 List Markers (Bullets and Numbers)
|
||||
|
||||
**Current State:** List items render without visual markers; `<li>` already adds bullet character
|
||||
|
||||
**Infrastructure Available:**
|
||||
- `ChapterHtmlSlimParser` already handles `<li>` tags (line 248 adds bullet)
|
||||
- Font system supports Unicode characters
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Track list type (`<ol>` vs `<ul>`) in parser state
|
||||
2. For `<ol>`: maintain counter, render as "1.", "2.", etc.
|
||||
3. For `<ul>`: use bullet character (already implemented: `\xe2\x80\xa2`)
|
||||
4. Apply text indentation for list item content
|
||||
|
||||
**Current Code (already adds bullet):**
|
||||
```cpp
|
||||
if (strcmp(name, "li") == 0) {
|
||||
self->currentTextBlock->addWord("\xe2\x80\xa2", EpdFontFamily::REGULAR);
|
||||
}
|
||||
```
|
||||
|
||||
**Complexity:** Low
|
||||
**User Impact:** Medium - Improves readability of enumerated content
|
||||
|
||||
---
|
||||
|
||||
### 1.3 Nested List Indentation
|
||||
|
||||
**Current State:** Nested lists are flattened to same indentation level
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Track nesting depth in parser (`listDepth` counter)
|
||||
2. Apply progressive `text-indent` based on depth (e.g., 20px per level)
|
||||
3. Store depth in `BlockStyle` for rendering
|
||||
|
||||
**Complexity:** Low
|
||||
**User Impact:** Medium - Important for technical documentation and outlines
|
||||
|
||||
---
|
||||
|
||||
## Priority 2: Medium Impact, Moderate Complexity
|
||||
|
||||
### 2.1 Basic Table Rendering
|
||||
|
||||
**Current State:** Tables show `[Table omitted]` placeholder
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Parse table structure (`<table>`, `<tr>`, `<td>`, `<th>`)
|
||||
2. Calculate column widths based on content or proportional division
|
||||
3. Render as text rows with column separators (e.g., `|` character)
|
||||
4. Apply header styling (bold) for `<th>` elements
|
||||
|
||||
**Constraints:**
|
||||
- Fixed-width font may be needed for alignment
|
||||
- Complex tables (colspan, rowspan) would remain unsupported
|
||||
- Wide tables may need horizontal truncation
|
||||
|
||||
**Complexity:** Medium-High
|
||||
**User Impact:** Medium - Some EPUBs have data tables, but many don't
|
||||
|
||||
---
|
||||
|
||||
### 2.2 Internal Link Navigation
|
||||
|
||||
**Current State:** Links render as plain text, not interactive
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Parse `<a href="#id">` attributes during HTML parsing
|
||||
2. Store link targets in page elements
|
||||
3. Add UI for link selection (highlight/underline links)
|
||||
4. Implement navigation to target anchor when activated
|
||||
5. Handle cross-chapter links (resolve to spine + anchor)
|
||||
|
||||
**Complexity:** Medium-High (requires UI changes)
|
||||
**User Impact:** Medium - Important for reference material, footnotes
|
||||
|
||||
---
|
||||
|
||||
### 2.3 Line Height CSS Support
|
||||
|
||||
**Current State:** Line height is fixed based on font metrics
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Add `line-height` property to `CssParser`
|
||||
2. Store in `CssStyle` struct
|
||||
3. Apply multiplier to `getLineHeight()` during layout
|
||||
|
||||
**Complexity:** Low
|
||||
**User Impact:** Medium - Improves text density matching to publisher intent
|
||||
|
||||
---
|
||||
|
||||
## Priority 3: Lower Impact or Higher Complexity
|
||||
|
||||
### 3.1 Font Size CSS Support
|
||||
|
||||
**Current State:** Font size controlled by reader settings only
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Parse `font-size` in `CssParser` (relative units: em, rem, %)
|
||||
2. Map to available font sizes (snap to nearest)
|
||||
3. Apply during text rendering
|
||||
|
||||
**Constraints:**
|
||||
- Limited font size options in bitmap fonts
|
||||
- Large size changes may cause layout issues
|
||||
|
||||
**Complexity:** Medium
|
||||
**User Impact:** Low-Medium - Most content uses default sizing
|
||||
|
||||
---
|
||||
|
||||
### 3.2 Page-List Navigation
|
||||
|
||||
**Current State:** Only TOC navigation supported
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Parse `<nav epub:type="page-list">` in navigation document
|
||||
2. Store page reference mappings
|
||||
3. Add UI to jump to specific page numbers
|
||||
|
||||
**Complexity:** Medium
|
||||
**User Impact:** Low - Useful for academic/reference texts with page citations
|
||||
|
||||
---
|
||||
|
||||
### 3.3 Landmarks Navigation
|
||||
|
||||
**Current State:** Not implemented
|
||||
|
||||
**Implementation Approach:**
|
||||
1. Parse `<nav epub:type="landmarks">`
|
||||
2. Extract semantic markers (cover, toc, bodymatter, etc.)
|
||||
3. Add quick-nav UI
|
||||
|
||||
**Complexity:** Low-Medium
|
||||
**User Impact:** Low - Convenience feature
|
||||
|
||||
---
|
||||
|
||||
## Features NOT Recommended for Implementation
|
||||
|
||||
These features are either hardware-limited or provide minimal value for the target use case.
|
||||
|
||||
### Hardware Limited (Cannot Implement)
|
||||
|
||||
| Feature | Reason |
|
||||
|---------|--------|
|
||||
| Color images/CSS | Monochrome 4-level grayscale display |
|
||||
| Audio playback | No audio hardware |
|
||||
| Video playback | No video hardware, e-ink refresh too slow |
|
||||
| Animations | E-ink refresh latency incompatible |
|
||||
| Media Overlays | Requires audio hardware |
|
||||
|
||||
### Not Worth Implementing
|
||||
|
||||
| Feature | Reason |
|
||||
|---------|--------|
|
||||
| JavaScript | Security concerns, minimal EPUB use, high complexity |
|
||||
| Fixed Layout (FXL) | Designed for tablets; poor e-ink experience |
|
||||
| SVG | Complex parser needed, limited use in text EPUBs |
|
||||
| MathML | Extremely complex, niche use case |
|
||||
| CSS Grid/Flexbox | Overkill for text layout |
|
||||
| @font-face | Memory constraints, limited benefit |
|
||||
| DRM | Licensing complexity, user expectation of DRM-free |
|
||||
| Forms | Interactive elements unsuitable for e-ink |
|
||||
|
||||
---
|
||||
|
||||
## Implementation Roadmap
|
||||
|
||||
### Phase 1: Quick Wins (Low Effort, High Value)
|
||||
1. ~~List bullet rendering~~ (already implemented)
|
||||
2. Ordered list numbering
|
||||
3. Nested list indentation
|
||||
4. Line-height CSS support
|
||||
|
||||
### Phase 2: Image Support (Medium Effort, High Value)
|
||||
1. JPEG image extraction and caching
|
||||
2. BMP image support
|
||||
3. `PageImage` element integration
|
||||
4. Image scaling to viewport width
|
||||
5. (Optional) PNG support via external library
|
||||
|
||||
### Phase 3: Enhanced Navigation (Medium Effort, Medium Value)
|
||||
1. Internal link parsing
|
||||
2. Link highlighting/selection UI
|
||||
3. Anchor navigation within chapters
|
||||
4. Cross-chapter link navigation
|
||||
|
||||
### Phase 4: Table Support (High Effort, Medium Value)
|
||||
1. Basic table parsing
|
||||
2. Simple column layout
|
||||
3. Text-based table rendering
|
||||
4. Header styling
|
||||
|
||||
---
|
||||
|
||||
## Summary
|
||||
|
||||
| Priority | Feature | Complexity | Impact | Dependencies |
|
||||
|----------|---------|------------|--------|--------------|
|
||||
| 1.1 | Inline Images | Medium | High | Bitmap, JpegToBmpConverter |
|
||||
| 1.2 | Ordered List Numbers | Low | Medium | None |
|
||||
| 1.3 | Nested List Indent | Low | Medium | None |
|
||||
| 2.1 | Basic Tables | Medium-High | Medium | None |
|
||||
| 2.2 | Internal Links | Medium-High | Medium | UI changes |
|
||||
| 2.3 | Line Height CSS | Low | Medium | CssParser |
|
||||
| 3.1 | Font Size CSS | Medium | Low-Medium | Font system |
|
||||
| 3.2 | Page-List Nav | Medium | Low | Navigation system |
|
||||
| 3.3 | Landmarks Nav | Low-Medium | Low | Navigation system |
|
||||
|
||||
**Recommended First Implementation:** Start with Phase 1 quick wins (list improvements, line-height), then proceed to Image Support as it has the highest user-visible impact and leverages existing infrastructure.
|
||||
Loading…
x
Reference in New Issue
Block a user