How can I compose (adjoin) multiple JPEG files without re-compression?
I know there is jpegtran that can losslessly crop and resize JPEG images, so I wonder if there is similar tool to adjoin images lossless?
Their size is a multiple 1 MCU block (16 pixels in both directions).
What you want to do is certainly possible, but in order for it to be truly lossless, both images must use the same quantization tables and color subsampling option. If, for example, the 2 images come from the same source (e.g. camera), then you can be pretty sure that it would be possible. You would need to entropy-decode each MCU in both images, rearrange them the way you want and then re-entropy code them and output the JPEG bitstream. If you want to overlay one on the other or crop/merge them on non-MCU boundaries, then you hit the same problem as above and must do a full decode and re-encode.