是否可以以编程方式反转图像的颜色？

问题描述

我想在iOS中拍摄图像并反转颜色。

I want to take an image and invert the colors in iOS.

推荐答案

扩展quixoto的答案，因为我有来自我自己的项目的相关源代码，如果你需要放弃CPU上的像素操作，那么我已经添加了以下内容，应该这样做：

To expand on quixoto's answer and because I have relevant source code from a project of my own, if you were to need to drop to on-CPU pixel manipulation then the following, which I've added exposition to, should do the trick:

@implementation UIImage (NegativeImage)

- (UIImage *)negativeImage
{
    // get width and height as integers, since we'll be using them as
    // array subscripts, etc, and this'll save a whole lot of casting
    CGSize size = self.size;
    int width = size.width;
    int height = size.height;

    // Create a suitable RGB+alpha bitmap context in BGRA colour space
    CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *memoryPool = (unsigned char *)calloc(width*height*4, 1);
    CGContextRef context = CGBitmapContextCreate(memoryPool, width, height, 8, width * 4, colourSpace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colourSpace);

    // draw the current image to the newly created context
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self CGImage]);

    // run through every pixel, a scan line at a time...
    for(int y = 0; y < height; y++)
    {
        // get a pointer to the start of this scan line
        unsigned char *linePointer = &memoryPool[y * width * 4];

        // step through the pixels one by one...
        for(int x = 0; x < width; x++)
        {
            // get RGB values. We're dealing with premultiplied alpha
            // here, so we need to divide by the alpha channel (if it
            // isn't zero, of course) to get uninflected RGB. We
            // multiply by 255 to keep precision while still using
            // integers
            int r, g, b;
            if(linePointer[3])
            {
                r = linePointer[0] * 255 / linePointer[3];
                g = linePointer[1] * 255 / linePointer[3];
                b = linePointer[2] * 255 / linePointer[3];
            }
            else
                r = g = b = 0;

            // perform the colour inversion
            r = 255 - r;
            g = 255 - g;
            b = 255 - b;

            // multiply by alpha again, divide by 255 to undo the
            // scaling before, store the new values and advance
            // the pointer we're reading pixel data from
            linePointer[0] = r * linePointer[3] / 255;
            linePointer[1] = g * linePointer[3] / 255;
            linePointer[2] = b * linePointer[3] / 255;
            linePointer += 4;
        }
    }

    // get a CG image from the context, wrap that into a
    // UIImage
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    UIImage *returnImage = [UIImage imageWithCGImage:cgImage];

    // clean up
    CGImageRelease(cgImage);
    CGContextRelease(context);
    free(memoryPool);

    // and return
    return returnImage;
}

@end

这样就增加了一个类别方法UIImage：

So that adds a category method to UIImage that:

1. 创建一个清晰的CoreGraphics位图上下文，它可以访问内存


2. 将UIImage绘制到它


3. 遍历每个像素，从预乘的alpha转换为未反射的RGB，分别反转每个通道，再次乘以alpha并存储回来


4. 从上下文中获取图像并将其包装到UIImage中


5. 自行清理，然后返回UIImage

1. creates a clear CoreGraphics bitmap context that it can access the memory of
2. draws the UIImage to it
3. runs through every pixel, converting from premultiplied alpha to uninflected RGB, inverting each channel separately, multiplying by alpha again and storing back
4. gets an image from the context and wraps it into a UIImage
5. cleans up after itself, and returns the UIImage

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