The “JPEG-Compression” compression technology is used by virtually all digital cameras. So, how does this thing function, exactly?
In order to understand the algorithm, it must be broken down into its constituent parts. Here we just demonstrate the compression procedure; the decompression procedure follows the opposite sequence. Only the most typical compression methods are shown. 8-bit RGB data is compressed using a lossy algorithm. Lossy compression means that in addition to saving space, part of the original image data is lost (in opposite to lossless compression).
Subsampling and Color Conversion
The RGB data is then decomposed into its brightness and colour components. Yuv is the data’s new home. This process does not lower data size because it only alters how the data is shown. Subsampling the colour information does not significantly reduce the visible image’s information content since the human eye is far more sensitive to the intensity information than the colour information. The amount of information, however, is drastically decreased.
Methods of Block Processing and Discrete Cosine Transform
After subsampling the color components, the resulting Yuv picture data is chopped up into 8×8 pixel squares. These pixel grids will henceforth undergo the whole algorithm. Discrete Cosine Transforms are applied to each individual block (DCT). The meaning of this is unclear. The data is described digitally for each pixel in the spatial domain (i.e. before the transformation). Therefore, we use a list of pixel numbers and values to describe the picture data. Coefficients of spatial frequencies for vertical and horizontal orientations characterize the visual content after the transformation. To that end, we’ll need to keep track of 64-pixel values in the spatial domain. The frequency domain requires us to keep track of 64 frequency coefficients. We haven’t done any data reduction yet.
Quantization
As a result of quantization, just 64 bits are required to store all 64 coefficients. Quantization is a process in which some information is lost while others are gained, and the amount of loss depends on the size of the quantization steps. In most cases, the user will have the option to set the JPEG compression level. This user data is relevant during the quantization stage (remaining image quality and file size).
Encoding of varying lengths and reordering
As quantization progresses, a larger fraction of coefficients will be set to zero. Furthermore, these zero-valued coefficients are very certainly located in the more extreme frequency range. For this reason, we rearrange the data so that the values occur first for low spatial frequencies and last for high ones. It’s quite probable that after the reordering, we’ll see a few numbers up front and a large number of zeros after that. Keeping track of “0 0 0 0 0 0 0 0 0 0” is cumbersome, whereas keeping track of “10x 0” is a breeze. As a result, a lot of information is cut down on as well.
Tools For JPEG Compression.
JPEG Compressor
Simple to use, no software download is necessary for the online JPEG Compressor. This tool can reduce the size of JPEG files as well as resize them. Each picture can have its own compression ratio set by you.
iLoveIMG
You may use iLoveIMG, a handy website, to enhance your photos in a number of ways. Users are allowed to post up to 30 photos in either JPEG, PNG, or GIF format at once. You have the option of saving your compressed photographs locally on your computer, uploading them to a cloud storage service, or receiving a QR code to see the images on your mobile device. Picture resizing, rotation, editing, and watermarking are just some of iLoveIMG’s other features for improving image quality. There are both free and premium tiers of service available on iLoveIMG.
AnyWebP
Both a downloaded software for MacOS and Windows and a web-based interface are available to users of AnyWebP. Images in JPEG, PNG, and ICO formats are all supported by this optimization programme. AnyWebP is a file format converter that also includes resizing and compression tools.
JPEGmini
With support for Linux, MacOS, and a few others, JPEGmini is available for download. The quality of your photos will be maintained while their file sizes are reduced thanks to the software’s usage of a unique image compression algorithm. You may try out JPEGmini before you buy it by downloading a free trial version.
Image Resizing
You may optimize your pictures without spending a dime by using Resize Photos. Users may select a compression level and upload files in many different formats, such as JPEG, PNG, GIF, and BMP. You may also perform simple things like adding text, filters, and shadows to your pictures.
Conclusion
In order to compress images, JPEG uses a block-based method. Subsampling of color data, quantization of DCT coefficients, and Huffman coding are used to reduce data size (reorder and coding). With the help of the settings, the user may manage the degree of quality loss in the reduced image (or chose presets). Skipping subsampling and using an appropriate quantization matrix ensures a high-quality compression with little data loss. To achieve a high level of compression, subsampling is activated, and the quantization matrix is chosen such that the majority of coefficients are set to 0. After decompression, artefacts become prominent in such a picture.
