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February 2007
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2007.02.27 19:02 "Re: convert grey-scale to RGB image", by Thierry Pierron
maria fernandez wrote: > I have a black&white tiff image. > I read this image with TIFFReadScanline and I show > pixels value, I obtain values since 0 until 255. I > don't know why because like I have said before is a > black&white image. > To be sure, just check the TIFF tag TIFFTAG_BITSPERSAMPLE (run tiffinfo on it). If it is 8, you have a grayscale image, if it is 1, it is black and white. And, well, if you look at 1bit data as a chunk of 8bit, ... you'll have any value between 0 and 255. TIFFReadScanline won't convert magically 1bit data into 8bit. > I have opened the image with the GIMP and I have seen > that the image was in grey-scale, then I have saved it > as RGB image. > GIMP does not support 1bit TIFF, they are automatically converted into 8bit. At least with version I have (2.2.13). > I have read this last image with TIFFReadScanline and > now the pixels value are 0 or 255, that it's ok > because the image always has been a black&white image. > > Is there a method that converts a grey-scale image to > RGB image? > ImageMagick has certainly a tool for that, or you can write your own. Using libtiff, it can be done around 20 lines of C. But, well, what waste of space ! Converting 1bit B'nW into RGB 24bits, will obviously give you raw data 24 times bigger, with no added quality. -- Thierry Pierron