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

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).

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, feat: replace picojpeg with JPEGDEC for JPEG image decoding #1136 (comment)
• and the conclusion feat: replace picojpeg with JPEGDEC for JPEG image decoding #1136 (comment)
• Proposal to improve quality added at Proposal: Image conversion during upload #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 >