JPG's and BMP's have three channels (RGB), with values between 0 and 255 (0 being black, and 255 being red, green, or blue). The GI in C2 uses 24bit images only. In a JPG or BMP or PNG, or whatever other 24 bit image type, clouds and the sun wil most likely both be white. The GI is done using the color values of the pixels, so the white clouds will cast as much light as the white sun. In HDRI, instead of a value between 0 and 255, the format supports a value between 0 and a one followed by 38 zeros (someone correct me if I'm wrong as to the actual number, but still, it's basically a really big number). This means that the clouds can look white, and the sun can also, but the suns actual value in the HDRI image can be much greater then that of the clouds, and therefore it will actually cast much more light. The only real difference between JPG's and BMP's is that JPG's have better compression, so that the files are smaller. EMC