From Tracker Software PDF SDK
The AutoContrast method automatically adjusts the overall contrast and mixture of colors within a page, or specified rectangular area of a page. Because it does not adjust channels individually, AutoContrast does not remove color casts. It maps the lightest and darkest pixels in the image to white and black which makes highlights appear lighter and shadows appear darker.
HRESULT AutoContrast([in] RECT* stRect);
- [in] Pointer to RECT structure which defines the rectangular area where the function should be applied. If this parameter is NULL, the entire page will be processed.
Note:Please note that partial processing of an area of a page is not supported for all image formats. For example, it is not supported for indexed images, because each pixel in such images does not contain a color, but an index into a color table (or palette) which is common for the entire page.
Returns S_OK if operation was successful or error code in other cases.