Dan Wood: The Eponymous Weblog (Archives)
Dan Wood is co-owner of Karelia Software, creating programs for the Macintosh computer. He is the father of two kids, lives in the Bay Area of California USA, and prefers bicycles to cars. This site is his older weblog, which mostly covers geeky topics like Macs and Mac Programming. Go visit the current blog here.
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Also on this website A Basic Course in Dvorak · The Pickle Machine · The Mentos Gallery · Cocoa Table Tester · Making your Wedding Make a Difference ·
Sites I also work on BikeAlameda · A Progressive Alamedan (my political blog) · Karelia Software
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PNG compression performance
permanent link
· Topic/Cocoa
I've been hearing complaints about how big PNGs are when saved by Sandvox, but I finally got around to testing out this issue. Check out these results.
| Source | Dimensions | File Size |
|---|---|---|
| JPEG original before reducing & sharpening | 1200 x 1600 | 912.1 KB |
| -[NSBitmapImageRep representationUsingType: NSJPEGFileType] (quality 0.7) | 480x640 | 101.6 KB |
| -[NSBitmapImageRep representationUsingType: NSPNGFileType] | 480x640 | 376.1 KB |
| Above PNG then processed by PNGCrusher | 480x640 | 303.7 KB |
Notice that the PNG file when saved from Cocoa are almost four times the size of the JPEG. Of course, the quality (0.7) is going to impact the JPEG size, but that's still quite a hit!
When I processed the output file through PNGCrusher, It was able to squeeze a good chunk of wasted space out of the file. This means that the compression algorithm supplied by Cocoa isn't very optimal. (It would be nice to have a parameter to pass in to determine how much to optimize, the tradeoff being speed of course.)
I wanted to get a sense of how Preview.app would do with this. I saved the scaled-down image from my program as lossless TIFF, then opened and saved as PNG from preview. The results were almost the same, just a few bytes in difference. It's good to know that at least Cocoa's API isn't worse than what Preview uses. As with the API, it might be nice to have some control over how optimized the compression should be!
I think I'm going to file two bugs to Apple. One requesting API access to better PNG optimization; another requesting Preview to control this optimization.