Any possible way to compress PNG or use indexed RGB on node-canvas?

[hunt]

I'm not sure is it possible or not? by node-canvas api after we use "createPNGStream" image will be in indexed rgb format or PNG-8

Thank you!

[kkoopa]

See #306

[mathiasbynens]

@kkoopa Could you clarify how #306 enables saving an indexed PNG8 file? It doesn’t seem like any of the filters map to that directly.

The only PNG8 example I see is using node-mapnik.

[kkoopa]

It does not enable indexed PNG8, just setting zlib compression level and filters.

[mathiasbynens]

Thanks for clarifying, @kkoopa!

I guess this issue could be reopened and repurposed/renamed as “Make it possible to use PNG8 palette”, then.

[zbjornson]

@mathiasbynens FWIW, I've been starting to use this library with indexed PNG encoding by using one of the RGBA channels (e.g. A) as our palette indices, calling toBuffer("raw"), extracting and packing every 4th byte, and then using a different PNG encoding function to write the indexed PNG (we made our own; you can bind to libpng or lodepng for example).

I was toying with what an API in node-canvas could look like, e.g.

var c = new Canvas(w, h, "palette8" /* or "alpha8" or "png8" or ... */);
c.pngStream({palette: /* a Uint8ClampedArray of RGBA values */});

which seems all and well until you get to getImageData and putImageData -- if the returned pixel array is just one channel, it's compact and easy to work with but doesn't follow the Canvas specification; if it is unpacked and returns all four channels, it's bigger and harder to work with but follows the Canvas spec.

[zbjornson]

Actually, I just discovered PR #678, which lets you set any of Cairo's native formats (including alpha8). @mathiasbynens that would be a good place to discuss this further.

[LinusU]

Reopening this until we can get it resolved :)