Last modified: 2010-05-15 14:35:57 UTC
http://en.wikipedia.org/wiki/Image:Harvard_shield.png displays the image at full size (1200 x 1398), regardless of user preferences on size or whether one is logged in or not. The "Download high resolution version" link appears, even though the image is being displayed at full size.
URLs of images that have this probelm: http://de.wikipedia.org/wiki/Bild:TEST.png http://commons.wikimedia.org/wiki/Image:The_german_language_since_1945.PNG the strange thing is that thumbnails of those images work correctly. So, what is the problem here? Maybe the software does not detect that the image *should* be scaled. but in that case, the "high resultion" link would not appear, right?
Something else: it appears that the software indeed does not try to scale the image, or it fails trying: The URL of the supposedly scaled image is the same as the URL of the "original" version. I.e. no temporary/cached scaled version of the files is created/used.
JeLuF told me this happens when an image gets bigger (in bytes) when scaled down. This is the case for images that have a low color-depth (like 4 or 8 bit), which are rendered in 24 bit depth when scaling (because auf antialiasing), creating a file bigger than the original. In this case the software does not use the scaled image, because it would be against the original purpose of this feature, namely to preserve bandwidth. However, as this feature is also very useful to allow large images to be seen "at once" even if the original is bigger than the users screnn, and this is percived as the main advantage by many users, this is confusing. Also, as images with a low color-depth are not very frequent, i would argue that enabling scaling even in cases where the scaled file is bigger than the original would profit the users and wouldn't hurt too much bandwidth-wise.
Even if the behavior is left as-is, it needs to be better documented so it doesn't confuse anyone else in the future.
This bug was introduced by an attempted solution to bug 1218. I suggest two things: * Always resize images. * Use ImageMagick's "identify" to count the colors in an image (possibly only if the input image was indexed or grayscale); if it's 256 or less, convert the PNG to an indexed PNG (losslessly). The first cures the user hassle; the second will probably avoid most of the cases where the image actually gets bigger (the original motivation for the unresized images).
The behaviour for image resize has been changed in HEAD (and note that this is a change of behaviour, not a bug as such.)
Changed the summary to reflect the issue.
