# EPUB Reader Architectural Decisions

**Date:** 2026-01-23 19:47:23  
**Status:** Active  
**Based on:** EPUB 3.3 Compliance Audit

---

## Purpose

This document captures architectural decisions about which EPUB 3.3 features to implement, intentionally omit, or defer. Each decision includes rationale based on:

- Hardware constraints (ESP32-C3, 800x480 4-level grayscale e-ink)
- Memory limitations (~400KB SRAM, no PSRAM)
- User experience goals
- Implementation complexity

---

## ADR-001: Inline Image Support

**Status:** RECOMMENDED FOR IMPLEMENTATION

### Context

EPUBs frequently contain images for:
- Cover art
- Chapter illustrations
- Diagrams and figures
- Decorative elements

Current implementation displays `[Image: alt_text]` placeholder.

### Decision

**Implement inline image rendering** using existing infrastructure.

### Rationale

1. **Infrastructure Exists:**
   - `Bitmap` class handles BMP parsing with grayscale conversion and dithering
   - `JpegToBmpConverter` converts JPEG to BMP (most common EPUB image format)
   - `GfxRenderer::drawBitmap()` already renders bitmaps to e-ink
   - `ZipFile` can extract files from EPUB archive
   - Home screen cover rendering demonstrates the pattern works

2. **Memory Management Pattern:**
   - Convert and cache images to SD card (like thumbnail generation)
   - Load one image at a time during page render
   - Use streaming conversion to minimize RAM usage

3. **High User Impact:**
   - Many EPUBs contain important visual content
   - Technical books rely on diagrams
   - Children's books heavily use illustrations

### Implementation Architecture

```
┌────────────────────────────────────────────────────────────────┐
│                    Image Processing Pipeline                    │
├────────────────────────────────────────────────────────────────┤
│                                                                │
│  EPUB ZIP ──► Extract Image ──► Convert to BMP ──► Cache to SD │
│                   │                  │                         │
│                   │                  ├─ JPEG: JpegToBmpConverter│
│                   │                  └─ BMP: Direct copy        │
│                   │                                            │
│                   └─► During page render:                       │
│                       Load cached BMP ──► drawBitmap()         │
│                                                                │
└────────────────────────────────────────────────────────────────┘
```

### Page Element Structure

```cpp
// New PageImage element (alongside PageLine)
class PageImage final : public PageElement {
  std::string cachedBmpPath;  // Path to converted BMP on SD
  int16_t width;
  int16_t height;
  
public:
  void render(GfxRenderer& renderer, int fontId, int xOffset, int yOffset) override;
  bool serialize(FsFile& file) override;
  static std::unique_ptr<PageImage> deserialize(FsFile& file);
};
```

### Constraints

- **No PNG support** initially (would require adding `pngle` library)
- **Maximum image size:** Scale to viewport width, max 800x480
- **Memory budget:** ~10KB row buffer during conversion

---

## ADR-002: JavaScript/Scripting Support

**Status:** INTENTIONALLY OMITTED

### Context

EPUB 3.3 allows JavaScript in content documents for interactive features.

### Decision

**Do not implement JavaScript execution.**

### Rationale

1. **Security Risk:**
   - Untrusted code execution on embedded device
   - No sandboxing infrastructure
   - Potential for malicious EPUBs

2. **Hardware Limitations:**
   - E-ink display unsuitable for interactive content
   - Limited RAM for JavaScript engine
   - No benefit for static reading experience

3. **Minimal EPUB Use:**
   - Most EPUBs don't use JavaScript
   - Interactive textbooks target tablets, not e-readers

4. **Implementation Complexity:**
   - Would require embedding V8/Duktape/QuickJS
   - DOM manipulation engine
   - Event handling system

### Alternative

`<script>` elements are silently ignored. Content remains readable.

---

## ADR-003: Fixed Layout (FXL) Support

**Status:** INTENTIONALLY OMITTED

### Context

EPUB Fixed Layout provides pixel-precise page positioning for:
- Comic books
- Children's picture books
- Magazines
- Technical drawings with precise layout

### Decision

**Do not implement Fixed Layout support.**

### Rationale

1. **Display Mismatch:**
   - FXL designed for high-resolution color tablets
   - 800x480 grayscale e-ink would require heavy downscaling
   - Visual quality would be poor

2. **User Experience:**
   - FXL EPUBs expect pan/zoom interaction
   - E-ink refresh rate makes this impractical
   - Text would be too small to read without zoom

3. **Implementation Complexity:**
   - Requires full CSS positioning engine
   - Viewport meta tag handling
   - Coordinate transformation system

### Alternative

FXL EPUBs will open but may display incorrectly. Users should use reflowable EPUBs on this device.

---

## ADR-004: Audio/Video and Media Overlays

**Status:** HARDWARE LIMITED - CANNOT IMPLEMENT

### Context

EPUB 3.3 supports:
- `<audio>` and `<video>` elements
- Media Overlays (SMIL synchronization)
- Text-to-speech hints

### Decision

**Cannot implement due to hardware constraints.**

### Rationale

1. **No Audio Hardware:**
   - Device has no speaker or audio DAC
   - No audio output jack

2. **No Video Capability:**
   - E-ink refresh rate (~1 Hz) incompatible with video
   - No video decoding hardware

### Alternative

- Audio/video elements are ignored
- Alt text or fallback content displayed if available
- Media Overlays not processed

---

## ADR-005: Color CSS Properties

**Status:** HARDWARE LIMITED - SIMPLIFIED HANDLING

### Context

EPUBs use CSS colors for:
- Text color (`color`)
- Background color (`background-color`)
- Border colors

### Decision

**Ignore color properties; display in grayscale.**

### Rationale

1. **Hardware Constraint:**
   - Display is 4-level grayscale only
   - Cannot render colors

2. **Acceptable Degradation:**
   - Text remains readable in black
   - Background remains white
   - Colored elements appear as gray variations

### Implementation

Color CSS properties are parsed but not applied. Default black text on white background used.

---

## ADR-006: Table Rendering

**Status:** DEFERRED - OPTIONAL IMPLEMENTATION

### Context

Tables appear in:
- Technical documentation
- Reference material
- Data presentations

Current implementation shows `[Table omitted]` placeholder.

### Decision

**Implement simple text-based table rendering as an optional enhancement.**

### Rationale

1. **Moderate Impact:**
   - Some EPUBs use tables, but not majority
   - Technical users would benefit most

2. **Complexity vs. Benefit:**
   - Full table layout is complex (colspan, rowspan, sizing)
   - Simple tables can be rendered as text columns

### Implementation Approach (if implemented)

```
┌──────────────────────────────────────┐
│ Text-Based Table Rendering           │
├──────────────────────────────────────┤
│                                      │
│ Header1   │ Header2   │ Header3      │
│ ─────────────────────────────────────│
│ Data 1    │ Data 2    │ Data 3       │
│ Data 4    │ Data 5    │ Data 6       │
│                                      │
└──────────────────────────────────────┘
```

### Constraints

- Equal-width columns (no complex sizing)
- No colspan/rowspan support
- Truncate wide content
- Maximum 4-5 columns before overflow

---

## ADR-007: SVG and MathML

**Status:** INTENTIONALLY OMITTED

### Context

- SVG: Scalable Vector Graphics for illustrations
- MathML: Mathematical notation markup

### Decision

**Do not implement SVG or MathML rendering.**

### Rationale

1. **Implementation Complexity:**
   - SVG requires full vector graphics engine
   - MathML requires specialized math typesetting

2. **Limited Use:**
   - Most EPUBs use raster images, not SVG
   - MathML primarily in academic texts

3. **Alternative Exists:**
   - Many EPUBs include fallback PNG for SVG
   - MathML often has image fallback

### Alternative

Display alt text or fallback image if available.

---

## ADR-008: CSS Selector Support

**Status:** CURRENT IMPLEMENTATION SUFFICIENT

### Context

CSS selectors enable targeting elements for styling. Current support:
- Element selectors (`p`, `div`)
- Class selectors (`.classname`)
- Element.class selectors (`p.intro`)

### Decision

**Maintain current limited selector support; do not expand.**

### Rationale

1. **Sufficient for Most EPUBs:**
   - 90%+ of EPUB styling uses simple selectors
   - Complex selectors rarely affect core readability

2. **Implementation Complexity:**
   - Descendant selectors require DOM tree
   - Pseudo-selectors need state tracking
   - Specificity calculation is complex

3. **Memory Constraints:**
   - DOM tree would consume significant RAM
   - Current streaming parser is memory-efficient

### Not Implemented

- Descendant selectors (`div p`)
- Child selectors (`ul > li`)
- Sibling selectors (`h1 + p`)
- Pseudo-classes (`:first-child`, `:hover`)
- Pseudo-elements (`::before`, `::after`)
- Attribute selectors (`[type="text"]`)

---

## ADR-009: Internal Link Navigation

**Status:** RECOMMENDED FOR IMPLEMENTATION (PHASE 2)

### Context

EPUBs use internal links for:
- Footnotes
- Cross-references
- Table of contents
- Index entries

### Decision

**Implement internal link navigation in Phase 2.**

### Rationale

1. **User Value:**
   - Footnotes are common in non-fiction
   - Reference navigation improves usability

2. **Complexity:**
   - Requires anchor parsing and storage
   - Needs selection UI for link activation
   - Cross-chapter navigation adds complexity

### Implementation Architecture

```
Link Navigation Flow:
1. Parse <a href="#id"> during HTML parsing
2. Store link targets in PageLine metadata
3. Add link highlighting (underline or marker)
4. User selects link via UI
5. Resolve target: same chapter (anchor) or cross-chapter (spine + anchor)
6. Navigate to target page/position
```

### Deferred

- External links (http://) - no network navigation on e-reader

---

## ADR-010: DRM and Encryption

**Status:** INTENTIONALLY OMITTED

### Context

EPUB supports DRM through:
- `encryption.xml` in META-INF
- Adobe DRM
- Various proprietary schemes

### Decision

**Do not implement DRM support.**

### Rationale

1. **Licensing Complexity:**
   - Adobe DRM requires licensing agreements
   - Proprietary schemes have legal restrictions

2. **User Expectation:**
   - Open-source e-reader users expect DRM-free content
   - DRM conflicts with device modification philosophy

3. **Implementation Complexity:**
   - Each DRM scheme is different
   - Secure key storage required
   - Regular updates needed for scheme changes

### Alternative

Users should remove DRM from purchased content using legal tools before loading to device.

---

## ADR-011: List Rendering Enhancements

**Status:** RECOMMENDED FOR IMPLEMENTATION (PHASE 1)

### Context

Current implementation:
- `<li>` renders bullet character `•`
- No numbered list support
- No nesting indentation

### Decision

**Enhance list rendering with ordered numbers and nesting.**

### Rationale

1. **Low Complexity:**
   - Track list type (`<ol>` vs `<ul>`) in parser state
   - Maintain counter for ordered lists
   - Apply indentation based on nesting depth

2. **Clear User Benefit:**
   - Numbered lists convey sequence
   - Indentation shows hierarchy

### Implementation

```cpp
// Parser state additions
int listDepth = 0;
int orderedListCounter = 0;
bool isOrderedList = false;

// On <ol> start
isOrderedList = true;
orderedListCounter = 1;
listDepth++;

// On <li> in ordered list
addWord(std::to_string(orderedListCounter++) + ".", REGULAR);

// Apply indent
textIndent = listDepth * 20; // pixels per level
```

---

## Summary Table

| Feature | Decision | Rationale |
|---------|----------|-----------|
| Inline Images | IMPLEMENT | High impact, infrastructure ready |
| JavaScript | OMIT | Security risk, no benefit for e-ink |
| Fixed Layout | OMIT | Display mismatch, poor UX |
| Audio/Video | CANNOT | No hardware support |
| Color CSS | IGNORE | Grayscale display |
| Tables | DEFER | Moderate impact, high complexity |
| SVG/MathML | OMIT | High complexity, limited use |
| Complex CSS Selectors | OMIT | Memory constraints, limited benefit |
| Internal Links | IMPLEMENT (Phase 2) | User value for references |
| DRM | OMIT | Licensing, philosophy conflict |
| List Enhancements | IMPLEMENT (Phase 1) | Low complexity, clear benefit |

---

## Implementation Priority

### Phase 1 (Quick Wins)
- [x] Basic bullet rendering (already implemented)
- [ ] Ordered list numbering
- [ ] Nested list indentation
- [ ] Line-height CSS support

### Phase 2 (Image Support)
- [ ] Image extraction from EPUB
- [ ] JPEG to BMP conversion for inline images
- [ ] PageImage element integration
- [ ] Image scaling and layout

### Phase 3 (Navigation)
- [ ] Internal link parsing
- [ ] Link selection UI
- [ ] Anchor navigation

### Deferred/Not Planned
- PNG support (would need library addition)
- Table rendering
- Complex CSS selectors
- JavaScript, FXL, DRM