6c3a615fac
## Summary
- Add embedded image support to EPUB rendering with JPEG and PNG
decoders
- Implement pixel caching system to cache decoded/dithered images to SD
card for faster re-rendering
- Add 4-level grayscale support for display
## Changes
### New Image Rendering System
- Add `ImageBlock` class to represent an image with its cached path and
display dimensions
- Add `PageImage` class as a new `PageElement` type for images on pages
- Add `ImageToFramebufferDecoder` interface for format-specific image
decoders
- Add `JpegToFramebufferConverter` - JPEG decoder with Bayer dithering
and scaling
- Add `PngToFramebufferConverter` - PNG decoder with Bayer dithering and
scaling
- Add `ImageDecoderFactory` to select appropriate decoder based on file
extension
- Add `getRenderMode()` to GfxRenderer for grayscale render mode queries
### Dithering and Grayscale
- Implement 4x4 Bayer ordered dithering for 4-level grayscale output
- Stateless algorithm works correctly with MCU block decoding
- Handles scaling without artifacts
- Add grayscale render mode support (BW, GRAYSCALE_LSB, GRAYSCALE_MSB)
- Image decoders and cache renderer respect current render mode
- Enables proper 4-level e-ink grayscale when anti-aliasing is enabled
### Pixel Caching
- Cache decoded/dithered images to `.pxc` files on SD card
- Cache format: 2-bit packed pixels (4 pixels per byte) with
width/height header
- On subsequent renders, load directly from cache instead of re-decoding
- Cache renderer supports grayscale render modes for multi-pass
rendering
- Significantly improves page navigation speed for image-heavy EPUBs
### HTML Parser Integration
- Update `ChapterHtmlSlimParser` to process `<img>` tags and extract
images from EPUB
- Resolve relative image paths within EPUB ZIP structure
- Extract images to cache directory before decoding
- Create `PageImage` elements with proper scaling to fit viewport
- Fall back to alt text display if image processing fails
### Build Configuration
- Add `PNG_MAX_BUFFERED_PIXELS=6402` to support up to 800px wide images
### Test Script
- Generate test EPUBs with annotated JPEG and PNG images
- Test cases cover: grayscale (4 levels), centering, scaling, cache
performance
## Test plan
- [x] Open EPUB with JPEG images - verify images display with proper
grayscale
- [x] Open EPUB with PNG images - verify images display correctly and no
crash
- [x] Navigate away from image page and back - verify faster load from
cache
- [x] Verify grayscale tones render correctly (not just black/white
dithering)
- [x] Verify large images are scaled down to fit screen
- [x] Verify images are centered horizontally
- [x] Verify page serialization/deserialization works with images
- [x] Verify images rendered in landscape mode
## Test Results
[png](https://photos.app.goo.gl/5zFUb8xA8db3dPd19)
[jpeg](https://photos.app.goo.gl/SwtwaL2DSQwKybhw7)
---
### AI Usage
Did you use AI tools to help write this code? _**< YES >**_
---------
Co-authored-by: Matthías Páll Gissurarson <mpg@mpg.is>
Co-authored-by: Dave Allie <dave@daveallie.com>
41 lines
1.3 KiB
C
41 lines
1.3 KiB
C
#pragma once
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#include <GfxRenderer.h>
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#include <stdint.h>
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// 4x4 Bayer matrix for ordered dithering
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inline const uint8_t bayer4x4[4][4] = {
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{0, 8, 2, 10},
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{12, 4, 14, 6},
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{3, 11, 1, 9},
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{15, 7, 13, 5},
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};
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// Apply Bayer dithering and quantize to 4 levels (0-3)
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// Stateless - works correctly with any pixel processing order
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inline uint8_t applyBayerDither4Level(uint8_t gray, int x, int y) {
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int bayer = bayer4x4[y & 3][x & 3];
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int dither = (bayer - 8) * 5; // Scale to +/-40 (half of quantization step 85)
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int adjusted = gray + dither;
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if (adjusted < 0) adjusted = 0;
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if (adjusted > 255) adjusted = 255;
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if (adjusted < 64) return 0;
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if (adjusted < 128) return 1;
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if (adjusted < 192) return 2;
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return 3;
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}
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// Draw a pixel respecting the current render mode for grayscale support
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inline void drawPixelWithRenderMode(GfxRenderer& renderer, int x, int y, uint8_t pixelValue) {
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GfxRenderer::RenderMode renderMode = renderer.getRenderMode();
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if (renderMode == GfxRenderer::BW && pixelValue < 3) {
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renderer.drawPixel(x, y, true);
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} else if (renderMode == GfxRenderer::GRAYSCALE_MSB && (pixelValue == 1 || pixelValue == 2)) {
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renderer.drawPixel(x, y, false);
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} else if (renderMode == GfxRenderer::GRAYSCALE_LSB && pixelValue == 1) {
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renderer.drawPixel(x, y, false);
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}
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}
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