How do you generate an RGBA image in Core Graphics?

Question

I\'m trying to generate an RGBA8 image from text to use as an OpenGL ES 2.0 texture.

+(UIImage *)imageFromText:(NSString *)text
{
  UIFont *font = [UIFont sy         


        

Answer 1

I was getting the same unsupported parameter combination error even though I was using kCGImageAlphaPremultipliedLast. The issue in my case turned out to be that the width I was getting was fractional. Turning it into an integer by passing int(width) to CGBitmapContextCreate solved the problem.

--Edit in response to Steven's comment--

The issue with feeding in a fractional width as the second argument is not that CGBitmapContextCreate interprets it as such -- as stated, it gets casted implicitly to the argument's unsigned integer type. Rather, it produces a discrepancy in the bytes_per_row argument, since int(width * 4) is not the same as int(width) * 4. E.g. if the width is 22.5, then width gets truncated to 22, but width * 4 computes to 90, not 88.

Answer 2

    CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

    CGBitmapInfo    bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little;

    CGContextRef theContext = CGBitmapContextCreate(NULL, imgSize.width, imgSize.height, 8, 4*imgSize.width, colorSpace, bitmapInfo);

Answer 3

The color space you specify during creation wouldn't cause an error like that.

The reason you're getting that error is that you've specified 8 bits per component, presumably 4 color components in the 4*size.width value you passed in for bytesPerRow, yet a bitmapInfo parameter of kCGImageAlphaNone. kCGImageAlphaNone means only RGB, not RGBA. If you want RGBA, you should most likely specify kCGImageAlphaLast kCGImageAlphaPremultipliedLast.

[EDIT] sorry. I should have said kCGImageAlphaPremultipliedLast, not kCGImageAlphaLast.

So, something like this:

CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
CGContextRef contextRef =  CGBitmapContextCreate(NULL,
                                                 size.width,
                                                 size.height,
                                                 8,
                                                 4 * size.width,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedLast);