Adds a PlatformIO pre-build script to patch JPEGDEC library. When
decoding progressive JPEGs with AC coefficients, MCU_SKIP (-8) causes
array index 0xFFFFF8, creating a wild pointer ~33MB past the sMCUs
array. The patch redirects pMCU to sMCUs[0] when MCU_SKIP is active,
preventing store-access faults while maintaining correct behavior for
JPEG_SCALE_EIGHTH decoding. Devices with larger framebuffers (like the
x3) (792×528 = 52,272 bytes vs 800×480 = 48,000 bytes) have less free
heap, shifting the allocation and changing where the wild pointer lands.
Commit 8628297 guarded the DC coefficient write (pMCU[0]) with if (iMCU
>= 0), which prevents crashes for progressive JPEGs whose first scan is
DC-only (iScanEnd == 0). However, if the first scan includes AC
coefficients (iScanEnd > 0), the AC decode loop still writes through the
wild pointer and crashes.
## Summary
The progressive JPEG fixes (AC table skip, MCU_SKIP guard) from PR #1136
have been fixed upstream in bitbank2/JPEGDEC@8628297. Pin to that commit
and remove the pre-build patch script.
## Additional Context
Tested on Strange Pictures epub
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## Summary
Replaces the picojpeg library with bitbank2/JPEGDEC for JPEG decoding in
the EPUB image pipeline. JPEGDEC provides built-in coarse scaling (1/2,
1/4, 1/8), 8-bit grayscale output, and streaming block-based decoding
via callbacks.
Includes a pre-build patch script for two JPEGDEC changes affecting
progressive JPEG support and EIGHT_BIT_GRAYSCALE mode.
Closes#912
## Additional Context
# Example progressive jpeg
<img
src="https://github.com/user-attachments/assets/e63bb4f8-f862-4aa0-a01f-d1ef43a4b27a"
width="400" height="800" />
Good performance increase from JPEGDEC over picojpeg cc @bitbank2 thanks
## Baseline JPEG Decode Performance: picojpeg vs JPEGDEC (float in
callback) vs JPEGDEC (fixed-point in callback)
Tested with `test_jpeg_images.epub` on device (ESP32-C3), first decode
(no cache).
| Image | Source | Output | picojpeg | JPEGDEC float | JPEGDEC
fixed-point | vs picojpeg | vs float |
|-------|--------|--------|----------|---------------|---------------------|-------------|----------|
| jpeg_format.jpg | 350x250 | 350x250 | 313 ms | 256 ms | **104 ms** |
**3.0x** | **2.5x** |
| grayscale_test.jpg | 400x600 | 400x600 | 768 ms | 661 ms | **246 ms**
| **3.1x** | **2.7x** |
| gradient_test.jpg | 400x500 | 400x500 | 707 ms | 597 ms | **247 ms** |
**2.9x** | **2.4x** |
| centering_test.jpg | 350x400 | 350x400 | 502 ms | 412 ms | **169 ms**
| **3.0x** | **2.4x** |
| scaling_test.jpg | 1200x1500 | 464x580 | 5487 ms | 1114 ms | **668
ms** | **8.2x** | **1.7x** |
| wide_scaling_test.jpg | 1807x736 | 464x188 | 4237 ms | 642 ms | **497
ms** | **8.5x** | **1.3x** |
| cache_test_1.jpg | 400x300 | 400x300 | 422 ms | 348 ms | **141 ms** |
**3.0x** | **2.5x** |
| cache_test_2.jpg | 400x300 | 400x300 | 424 ms | 349 ms | **142 ms** |
**3.0x** | **2.5x** |
### Summary
- **1:1 scale (fixed-point vs float)**: ~2.5x faster — eliminating
software float on the FPU-less ESP32-C3 is the dominant win
- **1:1 scale (fixed-point vs picojpeg)**: ~3.0x faster overall
- **Downscaled images (vs picojpeg)**: 8-9x faster — JPEGDEC's coarse
scaling + fixed-point draw callback
- **Downscaled images (fixed-point vs float)**: 1.3-1.7x — less dramatic
since JPEG library decode dominates over the draw callback for fewer
output pixels
- The fixed-point optimization alone (vs float JPEGDEC) saved **~60% of
render time** on 1:1 images, confirming that software float emulation
was the primary bottleneck in the draw callback
- See thread for discussions on quality of progressive images,
https://github.com/crosspoint-reader/crosspoint-reader/pull/1136#issuecomment-3952952315
- and the conclusion
https://github.com/crosspoint-reader/crosspoint-reader/pull/1136#issuecomment-3959379386
- Proposal to improve quality added at
https://github.com/crosspoint-reader/crosspoint-reader/discussions/1179
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**< PARTIALLY >**_
---------
Co-authored-by: Dave Allie <dave@daveallie.com>
Justin Mitchell
martin brook