JPEG Extract DCT tables

Question

I am developing an image viewer/editor using VS2008 ,MFC and WIC and I would like to recompress jpegs as closely (quality-wize) as possible to their original, after image processing. Does anyone know how to extract luminance and chrominance tables to set in
http://msdn.microsoft.com/en-us/library/gg430026%28v=vs.85%29.aspx WITHOUT having to use an additional large library (like libjpeg)?

Answer 1

The JPEG standard specification covers the details of the luminance and chrominance table.

In JPEG terminology, those tables are both referred to as "quantization table" (DQT). The luminance quantization table is applied on the "Y" channel and the chrominance quantization table is applied to "Cb" and "Cr" channels.

This website lists the quantization tables for a large number of camera manufacturers and JPEG authoring software.

To recompress an image using the same quantization tables as another existing image:

• Query and enumerate all IWICMetadataBlockReader objects from the source image (IWICBitmapFrameDecode).
• Find the metadata blocks which have the types GUID_MetadataFormatJpegChrominance and GUID_MetadataFormatJpegLuminance
• Extract their binary data, and use those data to initialize the Luminance and Chrominance properties of the JPEG encoder options by putting them into the IPropertyBag when calling IWICBitmapFrameEncode::Initialize.