具有灰色颜色空间的CoreText

Question

I am trying to use CoreText on iOS to render OpenGL textures. CoreText renders in a CoreGraphics Bitmap context, which is then loaded in OpenGL using glTexImage2D .

When I create a bitmap context using an RGB color space evertyhing works fine

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
uint8_t *data = (uint8_t *)calloc(height, 4 * width);
CGContextRef context = CGBitmapContextCreate(data, width, height, 8, 4 * width, colorSpace, kCGImageAlphaNoneSkipLast);
CGColorSpaceRelease(colorSpace);

However, I would like to use a grayscale only color space. When I do the text does not appear.

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
uint8_t *data = (uint8_t *)calloc(height, width);
CGContextRef context = CGBitmapContextCreate(data, width, height, 8, width, colorSpace, kCGImageAlphaNone);
CGColorSpaceRelease(colorSpace);

The text I am rendering is black. In both cases I can draw in the context using CoreGraphics methods.

I draw the text using the fllowing code:

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)text);

CGSize dimensions = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, CFRangeMake(0, [text length]), NULL, CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX), NULL);

CGContextSetTextMatrix(context, CGAffineTransformIdentity);
CGContextTranslateCTM(context, 0, dimensions.height);
CGContextScaleCTM(context, 1.0, -1.0);

CGRect box = CGRectMake(0, 0, dimensions.width, dimensions.height);

CGMutablePathRef path = CGPathCreateMutable();
CGPathAddRect(path, NULL, box );

CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, [text length]), path, NULL);

CTFrameDraw(frame, context);

CFRelease(frame);
CFRelease(path);
CFRelease(framesetter);

Is there a special setting in CoreText to make this work ?

Thanks

Answer 1

OK, i managed to reproduce the problem. In your NSAttributeString property dictionary, you shouldn't use a UIColor (NSColor?), but a CGColorRef. This way the CGContext APIs will know how to treat your color depending on the colorspace. If you just do the following, you should be ok to go.

CGColorRef col = [UIColor blackColor].CGColor;
NSDictionary *attributesDict = [NSDictionary dictionaryWithObjectsAndKeys:
                                          //whatever attributes you need
                                          col, kCTForegroundColorAttributeName,
                                          nil];

NSAttributedString *stringToDraw = [[NSAttributedString alloc] initWithString:yourText
                                                                   attributes:attributesDict];

I hope this helps, let me know if it works!