Lossless JPEG
Adobe's DNG SDK provides a software library for encoding and decoding lossless JPEG with up to 16 bits per sample.
This mode exists because the discrete cosine transform (DCT) based form cannot guarantee that encoder input would exactly match decoder output.
The first selection value in the table, zero, is only used for differential coding in the hierarchical mode of operation.
[5] This mode is quite popular in the medical imaging field, and defined as an option in DNG standard, but otherwise it is not very widely used because of complexity of doing arithmetics on 10, 12, or 14bpp values on typical embedded 32-bit processor and a little resulting gain in space.
[citation needed] JPEG-LS is a lossless or near-lossless compression standard for continuous-tone images.
[7] It is a simple and efficient baseline algorithm which consists of two independent and distinct stages called modeling and encoding.
Total decorrelation cannot be achieved by first order entropy of the prediction residuals employed by these inferior standards.
Besides lossless compression, JPEG-LS also provides a lossy mode ("near-lossless") where the maximum absolute error can be controlled by the encoder.
Contexts are determined by obtaining the differences of the neighboring samples which represents the local gradient: The local gradient reflects the level of activities such as smoothness and edginess of the neighboring samples.
Notice that these differences are closely related to the statistical behavior of prediction errors.
Each one of the differences found in the above equation is then quantized into roughly equiprobable and connected regions.
The purpose of the quantization is to maximize the mutual information between the current sample value and its context such that the high-order dependencies can be captured.
In LOCO-I algorithm, this procedure is modified and improved such that the number of subtractions and additions are reduced.
In the regular mode of JPEG-LS, the standard uses Golomb–Rice codes which are a way to encode non-negative run lengths.
Since Golomb–Rice codes are quite inefficient for encoding low entropy distributions because the coding rate is at least one bit per symbol, significant redundancy may be produced because the smooth regions in an image can be encoded at less than 1 bit per symbol.
This is the “run” mode of JPEG-LS and it is executed once a flat or smooth context region characterized by zero gradients is detected.
JPEG 2000 includes a lossless mode based on a special integer wavelet filter (biorthogonal 3/5).
[citation needed] JPEG XL includes a lossless/near-lossless/responsive mode called Modular which optionally uses a modified Haar transform (called "squeeze") and which is also used to encode the DC (1:8 scale) image in VarDCT mode as well as various auxiliary images such as adaptive quantization fields or additional channels like alpha.
