![]() Worst DSSIM was 0.97 (equivalent of 80% quality JPEG).īenchmark ran on 3rd generation iPad with iOS 5.1. 98% of images had DSSIM better than 0.99 (equivalent of 95% quality JPEG). Average similarity (measured in DSSIM metric, using alpha-channel-aware tool) was 0.999. Information about testsĪll ImageOptim optimizations were lossless.ĥ7% of images in the ImageAlpha set were optimized losslessly. Since Xcode conversion can be disabled and iOS supports standard PNGs, such big size and speed savings are possible in actual AppStore applications. Images alone were 65% smaller and were displayed 2.5 times quicker than Xcode-optimized ones. Manually optimizing images with ImageAlpha reduced entire application size by more than a half (33.4 MB down to 16.3 MB). Decoding speed appears to be correlated to image file size more than anything else (most likely savings on byteswapping are negligible compared to additional disk I/O and extra data to decompress.) ConclusionĪlthough Xcode image compression is better than nothing, the “CgBi” images are larger and slower than well optimized, standard PNGs.ĭisabling Xcode conversion and simply using ImageOptim instead was enough to reduce the application size by almost 30% (33.4 MB down to 23.8 MB) and halve initial display time in the benchmark. Xcode-optimized images were significantly slower to display. Loading speedĪlthough Xcode optimisation is assumed to improve decoding speed of PNG images, testing on an actual device disproves that: ( benchmark details at the end) Type of image After conversion images were further optimized with ImageOptim. Images that could not be converted without noticeable quality loss were posterized instead (using ImageAlpha's posterizer with dithering). ImageAlpha+ImageOptim Images converted to PNG8+alpha using ImageAlpha. ImageOptim Images dropped into ImageOptim 1.4.0 and optimized few times until optimization gave no more savings. Xcode default That's the size Tapbots delivered to the AppStore. That's a typical size you'd get from an image processing software such as Photoshop. Unoptimized RGBA image saved with vanilla libpng. Files are ordered by their unoptimized size. Here are results of converting images back to PNG and optimizing them with ImageOptim and ImageAlpha: It has all images compressed with Xcode, which converted them to “CgBi” format. Tweetbot (an excellent, graphically-rich iOS Twitter client) weights 33.4 MB, of which over 26 MB is used for 978 images.
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