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Why JPEG compression processes image by 8x8 blocks instead of applying Discrete Cosine Transform to the whole image?
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The image is split into blocks of 8×8 pixels, and for each block, each of the Y, CB, and CR data undergoes the discrete cosine transform (DCT).
Image divided into 8x8 blocks of pixels From here on out, JPG does all operations on 8x8 blocks of pixels.
JPEG is a lossy image compression method. It employs a transform coding method using the DCT (Discrete Cosine Transform). An image is a function of i and j (or conventionally x and y) in the spatial domain.
The JPEG compression is a block based compression. The data reduction is done by the subsampling of the color information, the quantization of the DCT-coefficients and the Huffman-Coding (reorder and coding). The user can control the amount of image quality loss due to the data reduction by setting (or chose presets).
8 X 8 was chosen after numerous experiments with other sizes.
The conclusions of experiments are: 1. Any matrices of sizes greater than 8 X 8 are harder to do mathematical operations (like transforms etc..) or not supported by hardware or take longer time. 2. Any matrices of sizes less than 8 X 8 dont have enough information to continue along with the pipeline. It results in bad quality of the compressed image.
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