Image Compression-Reduce the file size without changing the image size, and support batch processing of 20 images

There are four main compression methods in JPEG, one of which is lossless compression based on spatial DPCM and the other three are lossy compression based on DCT. DCT-based sequential coding: Based on the principle of DCT transform, the image data is encoded and compressed in the order of top-down and left-to-right. When the receiver receives the data, it then decodes it in this order, and there is image loss in this process, so it is lossy compression. DCT-based progressive coding: It is also based on the DCT transform principle, but the image is scanned several times so that further data compression is performed on the image. So when the image is restored, the image is seen as a rough image, and then gradually refined until the end. DCT-based hierarchical coding: The coding is performed based on the image resolution, starting from a low resolution and gradually increasing the resolution until it is the same as that of the original image. The image is decoded in the same step. Lossless compression based on spatial DPCM: Prediction method and Huffman coding (or arithmetic coding) are used to ensure that the reconstructed image is exactly the same as the original image (set the mean square error to zero). Compression of PNG images in two stages. Prediction: This stage is a pre-processing of the png image, which makes it more convenient for subsequent compression. Compression: Deflate compression is performed, which combines the LZ77 algorithm and the Huffman algorithm to encode the image.