Every decoder stage is now built - time to wire them into one pass that turns decoded planes into an RGB image. Today you assemble the full image, completing the decoder.
Combine the luma and upsampled chroma planes into an RGB image by converting every pixel, cropping to the declared size.
This is where the decoder becomes whole. Each component’s plane is assembled from its blocks; the chroma planes are upsampled to full resolution; and then, pixel by pixel, the aligned Y, Cb, and Cr are run through the color conversion to produce R, G, B. A region that decoded to neutral samples - luma and chroma all 128 - comes out as flat gray (128,128,128), the sanity check that every plane lines up.
The final detail is cropping. Because MCUs tile in whole 8-or-16-pixel steps, the decoded planes can extend past the image’s true width and height; those extra samples were encoder padding and are simply dropped when packing the final image. With that, the decoder is finished: signature and marker walk, table and frame parsing, the full entropy scan, dequantize, inverse DCT, level shift, upsample, and color conversion, all cooperating to turn a baseline JPEG into pixels. What remains in the project is the mirror image - an encoder that runs this pipeline backward.
// planeW is the padded plane stride (MCU tiling overshoots the frame edge),// so cropping to w,h drops the padding samples.// for each pixel (x,y) up to width,height:// y_,cb,cr := luma[y*planeW+x], cbPlane[..], crPlane[..] // chroma upsampled// r,g,b := ycbcrToRGB(y_, cb, cr)// pack into the RGB image; ignore samples past width/height.func assemble(luma, cb, cr []byte, planeW, w, h int) *Image { }