As I mentioned in the last week’s blog, the game assets have CgBI PNGs in it which we need to decode.
Initially, I thought about converting it to a standard PNG to have the PNGDecoder decode it but since we have the raw pixels extracted, we can just draw the surface.
How different is the CgBI format than standard PNG
Apple had created this format to optimize for the native pixel format of the iPhone’s early PowerVR GPUs. It contains significant differences like-
- An extra critical chunk(CgBI) – Right after the 8 byte standard PNG header and 4 bytes for length, CgBI format has also a CgBI type field from bytes 12-15.
- Swapped pixel format – Instead of a standard PNG file’s pixel format, i.e., RGBA, CgBI format has the pixel format swapped to BGRA. The red and the blue channel are swapped, presumably for high speed direct blitting to the framebuffer.
- It contains raw deflate data. It has the zlib header, footer removed from the IDAT chunk.
- It has every channel pre-multiplied with the alpha channel.
- Pre-multiplied with the alpha channel during encoding: color’ = color * alpha / 255
I have extracted the raw pixels by removing the CgBI header, un-premultiplying the alpha channel and swapping the red and blue channel all while working on raw deflate data. Next, I will be drawing the surface and we’ll have the help window displayed on our screen. The immediate next step after that would be to render the help contents on it. Hopefully, I will be briefing about that in the next blog 😉
Thanks for reading 🙂