CGBitmapContextCreate and interlaced? image

Question

I'm converting some image drawing code from Cairo to Quartz and I'm slowly making progress and learning Quartz along the way but I've run into a problem with the image format.

In the Cairo version it works like this:

unsigned short *d = (unsigned short*)imageSurface->get_data();
int stride = imageSurface->get_stride() >> 1;

int height = imageHeight;
int width = imageWidth;
do {

    d = *p++; // p = raw image data
    width --;

    if( width == 0 ) {
        height --;
        width = imageWidth;
        d += stride;
    }

} while( height );

Now this produces the image as expected on the Cairo::ImageSurface. I've converted this over to how use Quartz and it is making progress but I'm not sure where I'm going wrong:

NSInteger pixelLen = (width * height) * 8;
unsigned char *d = (unsigned char*)malloc(pixelLen);
unsigned char *rawPixels = d;

int height = imageHeight;
int width = imageWidth;
do {

    d = *p++; // p = raw image data
    width --;

    if( width == 0 ) {
        height --;
        width = imageWidth;
    }

} while( height );

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(rawPixels, imageWidth, imageHeight, 8, tileSize * sizeof(int), colorSpace, kCGBitmapByteOrderDefault);
CGImageRef image = CGBitmapContextCreateImage(context);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
UIImage *resultUIImage = [UIImage imageWithCGImage:image];
CGImageRelease(image);

Now this is obviously heading in the right direction as it produces something that looks a bit like the desired image but it creates 4 copies of the image in a row, each with different pixels filled in so I'm assuming this is an interlaced image (I don't know a great deal about image formats) and that I need to somehow combine them somehow to create a complete image but I don't know how to do that with Quartz.

I think the stride has something to do with the problem but from what I understand this is the byte distance from one row of pixels to another which would not be relevant in the context of Quartz?

Ken Aspeslagh Ken Aspeslagh · Accepted Answer · 2011-12-20T14:53:52

It sounds like stride would correspond to rowBytes or bytesPerRow. This value is important because it is not necessarily equal to width * bytesPerPixel because rows might be padded to optimized offsets.

It's not completely obvious what the Cairo code is doing, and it doesn't look quite correct either. Either way, without the stride part, your loop makes no sense because it makes an exact copy of the bytes.

The loop in the Cairo code is copying a row of bytes, then jumping over the next row of data.

CGBitmapContextCreate and interlaced? image

1 Answers