Please anyone can help me to find source code about how can read HDRI and extract radiance map and then use it as texture map?
To my knowledge, HDRs ARE radiance maps in a certain format. I have never heard of the HRDI format. I know of 3 formats for radiance data:
RGBE, developed by Greg Ward. Simple sample code can be found at the Cornell website. Or just download RADIANCE. http://www.graphics.cornell.edu/online/formats/
Pixar’s Log format and
SGI’s LogLuv format.Both supported by Sam Leffler’s TIFF lib. http://www.libtiff.org/
Than I guess you have RAW(pic,ppm etc.) formats.
Didn’t Lucas just release some open-source library for their file format, too?
You can find simple RGBE loading code at this address: http://www.graphics.cornell.edu/~bjw/rgbe.html
The original description was in Graphics Gems two or four I can’t remember. You might find it along with lots of images and paper about Tiff logLuv on Greg Ward’s homepage.
There is now also openEXR from ILM it is an open format with example code etc… http://www.openexr.net/