My current iPhone game (more info coming soon, I promise!) has a resource system that allows me to load texture maps when I need them and unload them when I don’t. This helps makes the game very memory efficient, but it had one annoying flaw. At program startup I had to load and unload several textures to get size information I needed to set up other game structures. To avoid this wasteful operation I started looking at ways to extract information from a png without loading the whole thing.

This StackOverflow discussion was useful, along with this guide to the png format from the W3C, but unfortunately, Apple threw a spanner in the works.

If you’ve been programming the iPhone for a while you’ll know that by default, Xcode premultiplies the alpha channel of PNG files before adding them to the application bundle. It also adds its own undocumented data chunk to the beginning of the file, breaking PNG guidelines which state that the IHDR chunk should always be at the beginning.

This Objective C function will return the size of an image for both standard and Apple PNGs. It scans through the file looking for the IHDR chunk, making no assumptions about its position. It should be easy enough to convert to other members of the C family.

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#include <stdio.h>
 
//----------------------------------------------------------------------------
// GetPNGSize() - gets PNG Dimensions without loading any data
//----------------------------------------------------------------------------
BOOL GetPNGSize(const char *fileName,CGSize *pngSize)
{
    // PNG Signature
    unsigned char png_signature[8] = {137, 80, 78, 71, 13, 10, 26, 10};
    // IHDR chunk identifier
    unsigned char ihdr_signature[4]={'I','H','D','R'};
 
    FILE *fp;
    if( (fp = fopen(fileName,"rb")) == NULL )
    {
        return NO;
    }
 
    // load png signature and check it
    unsigned char sigBuf[8];
    fread(sigBuf, 1, 8, fp);
 
    if(memcmp(&sigBuf[0], &png_signature[0], 8))
    {
        fclose(fp);
        return NO;
    }
 
    // examine chunks until we find IHDR
    BOOL done = NO;
    while(!done && !feof(fp))
    {
        // get chunk length and type
        unsigned char chunkInfo[8];
        fread(&chunkInfo, 1, 8, fp);
 
        long length = (chunkInfo[0] << 24) | (chunkInfo[1] << 16) | (chunkInfo[2] << 8) | chunkInfo[3];
 
        // is this the IHDR?
        if(!memcmp(&chunkInfo[4], &ihdr_signature[0], 4))
        {
            unsigned char sizeInfo[8];
            fread(sizeInfo,1,8,fp);
 
            pngSize->width = (sizeInfo[0] << 24) | (sizeInfo[1] << 16) | (sizeInfo[2] << 8) | sizeInfo[3]; 
            pngSize->height= (sizeInfo[4] << 24) | (sizeInfo[5] << 16) | (sizeInfo[6] << 8) | sizeInfo[7];
            done = YES;
        }
        else 
        {
            // skip over the data and checksum and go to next chunk
            fseek(fp, length+4 , SEEK_CUR);
        }
    }
    fclose(fp);
    return done;
}
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