JPEGkompressio

JPEGkompressio is a method for lossy compression of digital images, particularly photographs. Developed by the Joint Photographic Experts Group, it is one of the most widely used image file formats on the internet. The compression works by reducing the amount of data required to represent the image, which results in a smaller file size. This is achieved through several stages.

The first stage involves converting the image from its original color space, typically RGB, to a luminance-chrominance color space such as YCbCr. This is beneficial because the human eye is more sensitive to changes in brightness (luminance) than to changes in color (chrominance). Next, the chrominance components are downsampled, meaning their resolution is reduced. This discards some color information that is less likely to be noticed by viewers.

The core of JPEG compression lies in the discrete cosine transform (DCT). The image is divided into 8x8 pixel blocks, and the DCT is applied to each block. This transform converts the spatial information of the block into frequency components. High-frequency components represent fine details and sharp edges, while low-frequency components represent smoother areas.

Following the DCT, quantization takes place. This is the lossy step where the DCT coefficients are divided by values from a quantization table and then rounded. Larger values in the quantization table result in more aggressive compression and greater data loss. The quantized coefficients are then encoded using lossless compression techniques, such as Huffman coding or arithmetic coding, to further reduce the file size. The result is a JPEG file that is significantly smaller than the original, with a trade-off in image quality that can be adjusted by the user.