Imagemagick Convert creating a very pixelated image

Question

I use imagemagick to create thumbnails from images on my website using convert like so: convert -size 220x220 %s -resize 220 -profile '*' %s", $image, $thumb and this has worked great for a long time. Thousands of images have been processed and all the thumbnails look great ... except for one. For some reason this image produces a very ugly thumbnail and I can't figure out why.

Original image: http://i.imgur.com/fCbAN.jpg

Generated Thumbnail: http://i.imgur.com/MdLCs.jpg

Does anyone have any insight as to why this might happen with my convert code?

Answer 1

The thumbnail has been saved with very low quality (approximately 10-15, 99 being close to lossless). I think the question is, "why did that happen".

I can think of some reasons, but you will have to experiment. I assume the images you posted are the real images (not copies done converting e.g. PNG to JPG, I mean), and the command line is complete and describes the complete image workflow.

• your ImageMagick setup attempts to keep estimated image quality. You do not set a quality explicitly (e.g. -quality 75), so the thumbnail gets the same quantizer setting as the source image. Suppose the source has a low quantizer, but you do not see it due to the high-frequency component (the image is "noisy" due to scanning). When resampling, the background loses its noise and becomes a smooth gradient, which was absent in the source. And a smooth gradient is hell on low quantizers. Try explicitly setting a quality factor (40 to 99, 40 is better compressed but chunkier, 99 is very high quality but bigger file).

• there is some kind of interference between the resampler and the Moiré pattern that the scanner creates in the acquired image. This is less likely, because I see a "wavelength" of about 8 pixels which isn't at all uncommon, nor do I think that with so many images you acquired, none had approximately the same size and aspect ratio of this one; which in this scenario ought to have triggered the same behaviour. You say it didn't happen, so if this image isn't uncommon for size, aspect ratio, or source (e.g. one of the very few images scanned with a Scan-o-matic 600 scanner in the batch), this scenario becomes pretty unlikely. But if it is correct, then add a Gaussian blur before resizing and it ought to fix things: e.g. -blur 2x2.

• there is bad juju in the file name, and for some reason this gets the ImageMagick wrapper to interpret a command of "set quantizer to its crappiest value". REALLY unlikely (if the interpret interprets a part of the filename as an option, it shouldn't interpret it as a filename, and the rest of the filename is no longer the true filename; resulting in a "File not found" error which we don't observe. All the same, if the original file name is something like "--progressive-swedish-music.jpg", try renaming it before thumbnailing.

I'm putting my money on option #1, anyway.

Another test which you could attempt is to run the same command from ImageMagick (command line) and not from PHP.