For every pixel we see in the digital image, the file must remember the color of that pixel. Compression is when the process regicides that two or more adjacent pixels or pixel which are side by side, are so similar in appearance, that they are changed to become the same color. This reduces the size of the file.
However, if over compressed, the image quality may suffer. Compression artifacts are visible on highly compressed images and appear as blocky chunks or color or noise that is unpleasing to the eye.
In some cases, compression can greatly reduce your file size without any differences. And in others, it essentially destroyed the appearance of your photograph.
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