Goal: Make an image with a transparent background for your web-page, clip-art, etc. Issue: PNG file size is bloated by losslessly saving "invisible" data. . Acquire image, . Select the backdrop, leaving the subject of the picture untouched, . ensure the drawable has an Alpha channel, . Erase or Clear the selection, . save as PNG. Thing is, gimp does erase/clear by setting the alpha component to fully transparent, but doesn't touch the other channels. So you can load up that image again, move the alpha channel back to opaque, and the background you erased comes back in full detail. This isn't a bug, as png is a lossless format, but saving all that detail which you intended to discard is a feature that does increase your file size. Solution? Perhaps just a footnote in the manual and the help page for the PNG save-filter. Or maybe semi-lossy filter to optionally run upon export, which replaces the data in these invisible pixels with more compressible stuff. The "GIMP That's Smarter Than You Are" (sponsored by the Corporation for Using Technology to Further Human Inferiority Complexes) could detect this fairly trivially, and ask if you really meant to be saving that stuff. The semi-lossy filter could be as simple as this mathmap[1] filter: if alpha(origVal(xy)) == 0 then rgba:[0,0,0,0] else origVal(xy) end although it's imaginable that some color which matched the non-invisible parts of the image would be more compressible than 0,0,0. A more complex "lossy filter" would require intimate knowledge of the export format, and (now I'm starting to stray into the jpeg thread) could do things like the "jpeg-blur" mentioned recently. I do like the neatness of the idea of being able to select which parts of the image may have their quality sacfiriced to achieve better compression, while the subject of the image retains most of its quality, but I feel like that problem might get rather tricky and the returns in practice may be fairly small... [1] MathMap plug-in: http://www.complang.tuwien.ac.at/~schani/mathmap/ A plug-in which lets you rapidly make a variety of filters. Good fun and useful in developing your own filters and effects. -- Kevin Turner <acapnotic@xxxxxxxxxxxxxxxxxxxxx> | OpenPGP encryption welcome here
