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Handle 16x16 MCU blocks in JPEG decoding (#120)

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## Summary

* Handle 16x16 MCU blocks in JPEG decoding
* We were only correctly handling 8x8 blocks, which means that we did
not correctly support a lot of JPGs leading to an interlacing style on
the images

## Additional Context

* Fixes https://github.com/daveallie/crosspoint-reader/issues/118
This commit is contained in:
Dave Allie
2025-12-24 22:21:41 +11:00
committed by GitHub
parent 1107590b56
commit 27035b2b91
1 changed files with 18 additions and 4 deletions
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22
lib/JpegToBmpConverter/JpegToBmpConverter.cpp
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@@ -182,6 +182,9 @@ bool JpegToBmpConverter::jpegFileToBmpStream(File& jpegFile, Print& bmpOut) {
}
// Process MCU block into MCU row buffer
// MCUs are composed of 8x8 blocks. For 16x16 MCUs, there are four 8x8 blocks:
// Block layout for 16x16 MCU: [0, 64] (top row of blocks)
// [128, 192] (bottom row of blocks)
for (int blockY = 0; blockY < mcuPixelHeight; blockY++) {
for (int blockX = 0; blockX < mcuPixelWidth; blockX++) {
const int pixelX = mcuX * mcuPixelWidth + blockX;
@@ -191,16 +194,27 @@ bool JpegToBmpConverter::jpegFileToBmpStream(File& jpegFile, Print& bmpOut) {
continue;
}
// Calculate which 8x8 block and position within that block
const int block8x8Col = blockX / 8; // 0 or 1 for 16-wide MCU
const int block8x8Row = blockY / 8; // 0 or 1 for 16-tall MCU
const int pixelInBlockX = blockX % 8;
const int pixelInBlockY = blockY % 8;
// Calculate byte offset: each 8x8 block is 64 bytes
// Blocks are arranged: [0, 64], [128, 192]
const int blockOffset = (block8x8Row * (mcuPixelWidth / 8) + block8x8Col) * 64;
const int mcuIndex = blockOffset + pixelInBlockY * 8 + pixelInBlockX;
// Get grayscale value
uint8_t gray;
if (imageInfo.m_comps == 1) {
// Grayscale image
gray = imageInfo.m_pMCUBufR[blockY * mcuPixelWidth + blockX];
gray = imageInfo.m_pMCUBufR[mcuIndex];
} else {
// RGB image - convert to grayscale
const uint8_t r = imageInfo.m_pMCUBufR[blockY * mcuPixelWidth + blockX];
const uint8_t g = imageInfo.m_pMCUBufG[blockY * mcuPixelWidth + blockX];
const uint8_t b = imageInfo.m_pMCUBufB[blockY * mcuPixelWidth + blockX];
const uint8_t r = imageInfo.m_pMCUBufR[mcuIndex];
const uint8_t g = imageInfo.m_pMCUBufG[mcuIndex];
const uint8_t b = imageInfo.m_pMCUBufB[mcuIndex];
// Luminance formula: Y = 0.299*R + 0.587*G + 0.114*B
// Using integer approximation: (30*R + 59*G + 11*B) / 100
gray = (r * 30 + g * 59 + b * 11) / 100;