Mac OS X/iOS: How to write indexed PNG image using RGBA colours through CGImage?

Question

I'm trying to write a PNG file with indexed colours and alpha values (RGBA) using CGImageDestination. In terms of the PNG specification that is an image with color type 3 and a tRNS chunk. I'm getting a matrix of indices and an attached palette from a separate part of the program.

I've successfully managed to write indexed images without alpha values (i.e. only opaque colours). If I add alpha values as per documentation the produced images no longer use color type 3 (palette), but become full RGBA images (color type 6). In the actual PNG file the alpha values are essentially attached to the palette entry. There doesn't seem to be a way to create an explicit RGBA colour space.

I could use libpng or other PNG libraries directly to achieve the desired encoding. However, I'd prefer to only use CGImage and friends if possible.

TL;DR: How do I produce an indexed PNG file with alpha values using CGImage/CGImageDestination?

A very similar question was already asked two years ago (Create and write paletted RGBA PNG using NSImage), but it talked about Cocoa and didn't receive a solution.

My test code follows. If USE_ALPHA is defined at compile time, the produced image uses alpha values. Otherwise it doesn't.

/* Compilation command (tested with Xcode 4.6.3 on Mac OS X 10.8.4):
 *
 * clang++ -std=c++11 -stdlib=libc++ -framework ApplicationServices -DUSE_ALPHA -o demo demo.cpp
 */

#include <utility>
#include <vector>
#include <array>

#include <ApplicationServices/ApplicationServices.h>

/* R, G, B */
const std::vector<std::array<std::uint8_t, 3>> colortable = {
  { 0xF0, 0x10, 0x10 },
  { 0x20, 0xF0, 0x20 },
  { 0x30, 0x30, 0xF0 },
  { 0xF0, 0xF0, 0x40 },
  { 0x50, 0xF0, 0xF0 },
};

/* Colour index, alpha */
const std::vector<std::uint8_t> data = {
  3, 0x30,
  2, 0x20,
  1, 0x10,
  0, 0x00,

  4, 0x40,
  3, 0x30,
  2, 0x20,
  1, 0x10,
};

void safeCfRelease(const ::CFTypeRef ref) {
  if (ref != nullptr) {
    ::CFRelease(ref);
  }
}

int main() {
  const std::shared_ptr<const ::__CFURL>
      url(::CFURLCreateWithFileSystemPath(nullptr, CFSTR("out.png"),
                                          ::kCFURLPOSIXPathStyle, false),
          safeCfRelease);

  const std::shared_ptr<::CGDataProvider>
    prov_data(::CGDataProviderCreateWithData(nullptr, &data[0], data.size(), nullptr),
              safeCfRelease);

  const std::shared_ptr<::CGColorSpace>
    gray(::CGColorSpaceCreateDeviceGray(), safeCfRelease),
    rgb(::CGColorSpaceCreateDeviceRGB(), safeCfRelease),
    indexed(::CGColorSpaceCreateIndexed(rgb.get(), colortable.size(),
                  reinterpret_cast<const unsigned char *>(&colortable[0])),
            safeCfRelease);

  const auto bitmap_info =
#ifdef USE_ALPHA
    ::kCGImageAlphaLast |
#else
    ::kCGImageAlphaNoneSkipLast |
#endif
    ::kCGBitmapByteOrderDefault;

  const std::shared_ptr<::CGImage>
    image(::CGImageCreate(/* width */ 4,
                          /* height */ 2,
                          /* bitsPerComponent */ 8,
                          /* bitsPerPixel */ 16,
                          /* bytesPerRow */ 8,
                          indexed.get(),
                          bitmap_info,
                          prov_data.get(), nullptr, false,
                          ::kCGRenderingIntentDefault),
          safeCfRelease);

  const std::shared_ptr<::CGImageDestination>
    dest(::CGImageDestinationCreateWithURL(url.get(), ::kUTTypePNG, 1, nullptr),
         safeCfRelease);

  ::CGImageDestinationAddImage(dest.get(), image.get(), nullptr);
  ::CGImageDestinationFinalize(dest.get());
}

Description given by file for opaque colours (this is also given for an image with the tRNS chunk produced using ImageMagick):

PNG image data, 4 x 2, 8-bit colormap, non-interlaced

Description with alpha values using CGImageDestination:

PNG image data, 4 x 2, 8-bit/color RGBA, non-interlaced

Answer 1

Don't know if this will work, but have your tried UIImagePNGRepresentation()?

CGImage cgimg;   //initialize this with the CGImage that you want to be a PNG 
UIImage* image = [UIImage imageWithCGImage:cgimg];
NSData* pngData = UIImagePNGRepresentation(image);
CGImageRelease(cgimg);

Hope this helps!