How do you generate an RGBA image in Core Graphics?

问题

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 systemFontOfSize:20.0];  
  CGSize size  = [text sizeWithFont:font];

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGContextRef contextRef =  CGBitmapContextCreate (NULL,
                                                    size.width, size.height,
                                                    8, 4*size.width,
                                                    colorSpace,
                                                    kCGImageAlphaLast
                                                    );
  CGColorSpaceRelease(colorSpace);
  UIGraphicsPushContext(contextRef);

  [text drawAtPoint:CGPointMake(0.0, 0.0) withFont:font];
  UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

  UIGraphicsPopContext();

  return image;
}

Unfortunately, there's no CGColorSpaceCreateDeviceRGBA, and CGColorSpaceCreateDeviceRGB results in the following error:

CGBitmapContextCreate: unsupported parameter combination: 8 integer bits/component; 32 bits/pixel; 3-component color space; kCGImageAlphaLast; 448 bytes/row.

What am I missing to create the proper RGBA8 format that OpenGL wants here?

Update: I changed the last parameter of CGBitmapContextCreate from kCGImageAlphaNone (which it was when I copy pasted the code) to kCGImageAlphaLast, which is one of several variations I've tried in error.

Update 2: UIGraphicsGetImageFromCurrentImageContext() returns nil if the context was not created with UIGraphicsBeginImageContext(), so it is necessary to extract the image differently: [UIImage imageWithCGImage:CGBitmapContextCreateImage(contextRef)].

回答1:

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);

回答2:

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.

回答3:

    CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

    CGBitmapInfo    bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little;

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

来源：https://stackoverflow.com/questions/6861891/how-do-you-generate-an-rgba-image-in-core-graphics

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iphone

objective-c

opengl-es

core-graphics