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It came up in the discussion on GitHub PR #94 that, by default, Oxipng (the PNG encoder used by Spreet) will perform compression trials using four filters: none, sub, entropy, and bigrams. And it turns out that None (i.e. no filter) is almost always the best choice for spritesheets (where best means smallest PNG). That means there's an optimsation available to Spreet. Instead of using the defaults and letting Oxipng try all four filters, Spreet could just limit it to only using None. Benchmarking Spreet using a handful of spritesheets suggests this can speed Spreet up by 7-12% overall.
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Of the project's 16 fixtures, 12 were unchanged, three were smaller, and one was larger. I tried this out on icons from osm-bright-gl-style, openstreetmap-americana, Mapbox's basic, and whaam styles, and the ~1000 icons from openstreetmap-carto; spritesheets generated from all those remained unchanged in file size. I'm not sure why three fixtures are smaller. Running oxipng with |
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For future reference, OptiPNG's guide to PNG optimisation was useful for understanding the encoding process. |
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It came up in the discussion on #94 that, by default, Oxipng (the PNG encoder used by Spreet) will perform compression trials using four filters: None, Sub, Entropy, and Bigrams. And it turns out that None (i.e. no filter) is almost always the best choice for spritesheets (where best means smallest PNG output).
That means there's an optimsation available to Spreet. Instead of using the defaults and letting Oxipng try all four filters, Spreet could just limit it to only using None. Benchmarking Spreet using a handful of sprite collections suggests this can speed things up by 7-12% overall.