Create Paletted CGImageRef

Question

I'm trying to create a paletted (indexed) PNG using CoreGraphics.

The best I've found is that I can use:

CGColorSpaceRef colorSpace = CGColorSpaceCreateIndexed(CGImageGetColorSpace(maskedImage), 255, <#const unsigned char *colorTable#>);

Then:

CGImageRef palettedImage = CGImageCreateCopyWithColorSpace(maskedImage, colorSpace)

However I'm not sure what to put as the colorTable. I'd like to leverage some pre-made (fast) quantization algorithm - such as the one that is already built in to ImageIO when calling CGImageDestinationCreateWithURL(url, kUTTypeGIF , 1, NULL);

How do I create a palette for the PNG?

score 1 · Answer 1 · answered Jul 05 '13 at 00:55

1

if your color space is for example RGB you would set the colorTable like so:

{R, G, B, R, G, B, R, G, B, ...}

answered Jul 05 '13 at 00:55

Danilo

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Is there a way to generate the color table automatically? – Chris Nolet Jul 05 '13 at 00:58
with or without you knowing the colors that should be in your color table? – Danilo Jul 05 '13 at 01:01
Without :) Something 'auto-magic'. I'm afraid my own quantization algorithms will be too slow. – Chris Nolet Jul 05 '13 at 01:50
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I don't think the sdk can do that for you. I am not sure. But you could use a library like http://pngquant.org/lib/ to do the conversion. – Danilo Jul 05 '13 at 09:44

Chris Nolet · Accepted Answer · 2013-07-21T21:40:12.370

So the final solution was to do something like this:

// Create an 8-bit palette for the bitmap via libimagequant (http://pngquant.org/lib)
liq_attr *liqAttr = liq_attr_create();
liq_image *liqImage = liq_image_create_rgba(liqAttr, bitmap, (int)width, (int)height, 0);
liq_result *liqRes = liq_quantize_image(liqAttr, liqImage);

liq_write_remapped_image(liqRes, liqImage, bitmap, bytesPerRow * height);
const liq_palette *liqPal = liq_get_palette(liqRes);

// Transpose the result into an rgba array
unsigned char colorTable[1024];
for (NSInteger n = 0; n < liqPal->count; n++) {
    colorTable[4 * n] = liqPal->entries[n].r;
    colorTable[4 * n + 1] = liqPal->entries[n].g;
    colorTable[4 * n + 2] = liqPal->entries[n].b;
    colorTable[4 * n + 3] = liqPal->entries[n].a;
}

// Release
liq_attr_destroy(liqAttr);
liq_image_destroy(liqImage);
liq_result_destroy(liqRes);

My hope was to use the color table to create a CGContextRef. However, according to this article: http://developer.apple.com/library/mac/#qa/qa1037/_index.html that isn't possible in any case.

You're using palette after freeing it (move `liq_result_destroy(liqRes);` to bottom). Also you could use `liqPal->entries` as the color table, so you don't even need the copy operation. — Kornel, Jul 21 '13 at 20:08

Create Paletted CGImageRef

2 Answers2