Scaling black and white image

Question

I fetch an image from the web and show it in an UIImage, but first i scaled it down. The thing is, if the image is black and white, the result is pretty bad, with lots of white lines crossing the pic.

I've tried many variations but i see no improvement, always the same poor quality result. Any tips?

Some of the things i tried (remember i'm working without an UIImageView):

http://iosdevelopertips.com/graphics/how-to-scale-an-image-using-an-objective-c-category.html

How to scale a UIImageView proportionally?

What's the easiest way to resize/optimize an image size with the iPhone SDK?

http://www.iphonedevsdk.com/forum/iphone-sdk-development/5204-resize-image-high-quality.html

Assuming your image is 1-bit-per-pixel, are you able to convert to grayscale (8 BPP or better) before scaling? — cbranch, Oct 30 '11 at 21:29
@cbranch That might be it, because i don't. But how do i recognize if it is in the first place, to make the conversion? — ferostar, Oct 30 '11 at 21:41
If your solution is working, please post it as answer and accept it. — vikingosegundo, Oct 30 '11 at 22:53

score 2 · Accepted Answer · answered Nov 13 '11 at 19:57

Turns out @cbranch was right, so first here is the solution i found (without releases):

- (BOOL) isGrayscaleImage:(UIImage *)image
{
    CGImageRef imgRef = [image CGImage];
    CGColorSpaceModel clrMod = CGColorSpaceGetModel(CGImageGetColorSpace(imgRef));

    switch (clrMod) {
        case kCGColorSpaceModelMonochrome : 
            return YES;
        default:
            return NO;
    }
}

- (UIImage *) imageToGrayscale:(UIImage *)image
{
    CGSize size = image.size; 
    CGRect rect = CGRectMake(0.0f, 0.0f, size.width, size.height); 
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray(); 
    CGContextRef context = CGBitmapContextCreate(nil, size.width, size.height, 8, 0, colorSpace, kCGImageAlphaNone); 
    CGColorSpaceRelease(colorSpace); 
    CGContextDrawImage(context, rect, [image CGImage]); 
    CGImageRef grayscale = CGBitmapContextCreateImage(context);
    UIImage *img = [UIImage imageWithCGImage:grayscale];
    return img;
}

Notice that this isn't necessary for iOS5, don't know why (nothing about this in the documentation).

Scaling black and white image

1 Answers1