JPEG Sandbox is a very barebones editor that supports clicking on any pixel in a JPEG image to see the 64 DCT coefficients that make up that 8×8 block. Each individual coefficient can be edited through text. You can also use the slider to zero out coefficients one by one starting with the lowest absolute value (which is what you’d do during compression) or reverse that to see what it would look like to throw away the most important coefficients first.
Omar Shehata created this to help with explaining how JPEG works for a talk at CodeBar 2022. See their Unraveling the JPEG article for more on JPEG.
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