Creating 2bits color depth PNG images with Libpng

Question

I would like to generate PNG images with 1bits (2 colors) or 2bit (4 colors) depth with the library Libpng.

Does any one know how to do it ? I have tested examples, and they all seem to work with 8bit color depth ?

I know png_set_IHDR but in the example I test, when I change the depth parameter in png_set_IHDR from 8 to 2 or 1, my program draw one pixels of 2 or 4. I think, it's due to the memory allocation created with the png_malloc function.

In the example I try to modify (http://www.lemoda.net/c/write-png/), the png_malloc function allocates all pixels of the image with sizeof uint8_t.

png_malloc (png_ptr, sizeof (uint8_t) * bitmap->width * pixel_size);

Can you tell to me how to allocate 1bit or 2bits pixels ?

Thank's Jo2s

It is directly done by using `png_set_IHDR` with a bit depth of either 1 or 2. What else do you need ? — mmgp, Feb 14 '13 at 22:48

score 1 · Answer 1 · answered May 22 '13 at 02:30

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In png_set_IHDR, the bit_depth parameter sets the bit depth of all color components. Together with PNG_COLOR_TYPE_RGB, you end up with one bit for the red component, one for the green and one for the blue.

You should call png_set_IHDR with PNG_COLOR_TYPE_PALETTE, and then if bit_depth is one, you must have a palette with two colors (0 and 1), and if bit_depth is 2, one with four colors (0 to 3.)

answered May 22 '13 at 02:30

André Leiradella

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For PNG_COLOR_TYPE_RGB, only bit depths 8 and 16 are allowed by the PNG spec. So yes, you should use PNG_COLOR_TYPE_PALETTE as Andre said. – Glenn Randers-Pehrson Feb 18 '14 at 00:18

Creating 2bits color depth PNG images with Libpng

1 Answers1