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What is the meaning of DC and AC in jpeg file?

I am writing a decompressor for jpeg files, I know that this is a very complicated process, but in any case I read this tutorial http://www.impulseadventure.com/photo/jpeg-huffman-coding.html it was very useful until I achieved this

The image content is expected to be 3 components (Y, Cb, Cr). Within each component, the sequence always has a constant current value followed by 63 alternating current values.

What is the meaning of DC and AC?

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It is simple that the first element of the discrete cosine transform (DCT) is a constant or direct current, which is the average value of a pixel. The remaining elements, members of AC or AC, are independent of the average. Therefore, if you add the same number to all pixel values, only the DC-DC DCT will change. AC conditions have not changed.

This is similar to filtering the DC component of the signal with a capacitor, allowing only AC components to pass through.

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Source: https://habr.com/ru/post/1501128/

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