Illtud Daniel on wrote... | Hi, | | I've tried to google, but I can't answer this specific question. | I suspect the answer is in the filtering, but I can't find one | that works for me. If anybody knows the answer I'd appreciate a | point in the right direction. | | I have a 4-bit (16 color) greyscale tiff that's 3755390 bytes. | Photoshop's 'save for web' conversion to gif saves it as a | 16-color non-interlaced LZW gif of size 226014 bytes with the | original dimensions. | If the original image only has 16 colors, and its the original dimensions the only change that should have possibly been made is changing from a 16 bit color of unknown profile or colorspace to 8 bit RGB color.
| There are indeed 16 colors, but it's done some kind of | sharpening/dithering, which I would have expected to decrease the | compressibility. The Histogram and the mean have changed slightly. | No bruuring should have been made. Dithering should ONLY have occured if there has been too many colors to fit in the final 256 color limit for GIF. That is obviously not the case. HOWEVER: It has been reported and noted in IM Examples bugs that the dither routine sometimes does not properly short-circuit itself when no action is required... See http://imagemagick.org/Usage/bugs/tesing/#dither This has been reported previously some time ago. | I can't get Imagemagick to create a gif smaller than 392366 | bytes, significantly bigger. I've tried -strip, -filter | Lanczos, -unsharp, -sampling-factor, - everything I could find | but I can't get smaller. What am I missing? The only thing I can | think that Photoshop is doing is prefiltering intelligently to | increase the redundancy so that it LZW compresses better. Is | there any way of doing that in Imagemagick? | You may be able to try a -compress LZW option to ensure that GIF is compressing the image. However if the image is being dithered (when it shouldn't be) that will cause LWZ to fail badly. A Error-correction Dither is essentually randomized, so it does not compress well as it does not repeat. This gets worse for GIF animations, as frames also do not repeat the same color patterns. | I can post the 3 images somewhere if that would help. | PLEASE!!!! I can then compare the images at pixel level to see what has happened. Best idea is to start a thread on the IM bugs Forum with links to the images in question. Anthony Thyssen ( System Programmer ) <[EMAIL PROTECTED]> ----------------------------------------------------------------------------- No dragon can resist the fascination of riddling talk, and of wasting time trying to understand it. -- JRR Tolkein, ``The Hobbit'' ----------------------------------------------------------------------------- Anthony's Home is his Castle http://www.cit.gu.edu.au/~anthony/ _______________________________________________ Magick-users mailing list [email protected] http://studio.imagemagick.org/mailman/listinfo/magick-users
