EIGHT_BIT_GRAYSCALE decode of a 3-component progressive JPEG calls
JPEGDecodeMCU_P with MCU_SKIP for Cb and Cr after every Y MCU. The
existing safe-pMCU patch redirects the wild pointer to &sMCUs[0] but
leaves the DC store unguarded, so each chroma skip overwrites the
just-decoded Y DC with the chroma DC predictor. Output reads sMCUs[0],
gets the trailing Cr DC (~0), and renders an all-black image.
Add `if (iMCU >= 0)` guards to the two pMCU[0] writes (main DC store and
successive-approximation update). The pointer redirect stays as the AC
wild-pointer defense; the new guards stop the silent corruption at
sMCUs[0]. The two fixes are independent and both required.
fixes the progressive 8bit grayscale jpeg regression in 1.3.0
Did you use AI tools to help write this code? partial
Jeremy Klein
acc1ed4358