Update the jpegli README

lib/jpegli/README.md

@@ -1,28 +1,49 @@
-# Improved JPEG decoder implementation
+# Improved JPEG encoder and decoder implementation
 
-This subdirectory contains a JPEG decoder implementation that is API and ABI
-compatible with libjpeg62.
+This subdirectory contains a JPEG encoder and decoder implementation that is
+API and ABI compatible with libjpeg62.
 
-*NOTE*: This is still a work in progress, currently only API functions called
-from libjxl's benchmark_xl tool are implemented.
+## Building
 
-To decompress an ```input.jpg``` file with this new library:
+When building the parent libjxl project, two binaries, `tools/cjpegli` and
+`tools/djpegli` will be built, as well as a
+`lib/jpegli/libjpeg.so.62.3.0` shared library that can be used as a drop-in
+replacement for the system library with the same name.
 
-```
-(from the libjxl root directory)
-$ ./ci.sh opt
-$ LD_PRELOAD=./build/libjpeg.so.62 ./build/tools/benchmark_xl --input input.jpg --codec=jpeg --decode_only --save_decompressed --output_dir .
-```
+## Encoder improvements
 
-The decompressed file will be saved as ```input.jpg.jpeg.png```.
+Improvements and new features used by the encoder include:
 
-To benchmark the jpeg encoding-decoding round-trip on an ```input.png``` with
-the new library, first build a statically linked ```cjpeg-static``` binary,
-which is found in ```$PATH```, and then run:
+* Support for 16-bit unsigned and 32-bit floating point input buffers.
 
-```
-(from the libjxl root directory)
-$ ./ci.sh opt
-$ LD_PRELOAD=./build/libjpeg.so.62 ./build/tools/benchmark_xl --input input.png --codec=jpeg:cjpeg-static:q90
-```
+* Color space conversions, chroma subsampling and DCT are all done in floating
+  point precision, the conversion to integers happens first when producing
+  the final quantized DCT coefficients.
 
+* The desired quality can be indicated by a distance parameter that is
+  analogous to the distance parameter of JPEG XL. The quantization tables
+  are chosen based on the distance and the chroma subsampling mode, with
+  different positions in the quantization matrix scaling differently, and the
+  red and blue chrominance channels have separate quantization tables.
+
+* Adaptive dead-zone quantization. On noisy parts of the image, quantization
+  thresholds for zero coefficients are higher than on smoother parts of the
+  image.
+
+* Support for more efficient compression of JPEGs with an ICC profile
+  representing the XYB colorspace. These JPEGs will not be converted to the
+  YCbCr colorspace, but specialized quantization tables will be chosen for
+  the original X, Y, B channels.
+
+## Decoder improvements
+
+* Support for 16-bit unsigned and 32-bit floating point output buffers.
+
+* Non-zero DCT coefficients are dequantized to the expectation value of their
+  respective quantization intervals assuming a Laplacian distribution of the
+  original unquantized DCT coefficients.
+
+* After dequantization, inverse DCT, chroma upsampling and color space
+  conversions are all done in floating point precision, the conversion to
+  integer samples happens only in the final output phase (unless output to
+  floating point was requested).