iphone - 我如何在 iPhone 中使用 Objective-C 计算 UIImage 中的红色像素？

Question

我是iphone软件开发的初学者。我正在开发关于皮肤癌的应用程序，其中我想计算或计算 iPhone 相机捕获的 UIImage 中的红色像素。可以从 UIImage 中计算红色像素吗？

Answer 1

由于这是一个几乎每周都会被问到的问题，我决定制作一个小示例项目来展示如何做到这一点。您可以查看代码:

http://github.com/st3fan/iphone-experiments/tree/master/Miscellaneous/PixelAccess/

重要的一点是下面的代码，它接受一个UIImage，然后计算纯红色像素的数量。这是一个示例，您可以使用它并根据您自己的算法修改它:

/**
 * Structure to keep one pixel in RRRRRRRRGGGGGGGGBBBBBBBBAAAAAAAA format
 */

struct pixel {
    unsigned char r, g, b, a;
};

/**
 * Process the image and return the number of pure red pixels in it.
 */

- (NSUInteger) processImage: (UIImage*) image
{
    NSUInteger numberOfRedPixels = 0;

    // Allocate a buffer big enough to hold all the pixels

    struct pixel* pixels = (struct pixel*) calloc(1, image.size.width * image.size.height * sizeof(struct pixel));
    if (pixels != nil)
    {
        // Create a new bitmap

        CGContextRef context = CGBitmapContextCreate(
            (void*) pixels,
            image.size.width,
            image.size.height,
            8,
            image.size.width * 4,
            CGImageGetColorSpace(image.CGImage),
            kCGImageAlphaPremultipliedLast
        );

        if (context != NULL)
        {
            // Draw the image in the bitmap

            CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), image.CGImage);

            // Now that we have the image drawn in our own buffer, we can loop over the pixels to
            // process it. This simple case simply counts all pixels that have a pure red component.

            // There are probably more efficient and interesting ways to do this. But the important
            // part is that the pixels buffer can be read directly.

            NSUInteger numberOfPixels = image.size.width * image.size.height;

            while (numberOfPixels > 0) {
                if (pixels->r == 255) {
                    numberOfRedPixels++;
                }
                pixels++;
                numberOfPixels--;
            }

            CGContextRelease(context);
        }

        free(pixels);
    }

    return numberOfRedPixels;
}

关于如何调用它的简单示例:

- (IBAction) processImage
{
    NSUInteger numberOfRedPixels = [self processImage: [UIImage imageNamed: @"DutchFlag.png"]];
    label_.text = [NSString stringWithFormat: @"There are %d red pixels in the image", numberOfRedPixels];
}

Github 上的示例项目包含一个完整的工作示例。