Question

我有以下代码试图测试像素矩形中的每个像素

px，py =触摸的位置
dx，dy =要测试的矩形的大小

UIImage *myImage; // image from which pixels are read
int ux = px + dx;
int uy = py + dy;
for (int x = (px-dx); x <= ux; ++x)
{
    for (int y = (py-dy); y <= uy; ++y)
    {
        unsigned char pixelData[] = { 0, 0, 0, 0 };
        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        CGContextRef context = CGBitmapContextCreate(pixelData, 1, 1, 8, 4, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
        CGImageRef cgimage = myImage.CGImage;
        int imageWidth = CGImageGetWidth(cgimage);
        int imageHeight = CGImageGetHeight(cgimage);
        CGContextDrawImage(context, CGRectMake(-px, py - imageHeight, imageWidth, imageHeight), cgimage);
        CGColorSpaceRelease(colorSpace);
        CGContextRelease(context);

        // Here I have latest pixel, with RBG values stored in pixelData that I can test
    }
}

内循环中的代码抓取位置x，y处的像素。是否有更有效的方法从UIImage（myImage）中抓取（x-dx，y-dy），（x + dx，y + dy）的整个像素矩形？

Answer 1

哇，你的回答很简单。

问题是您要为扫描的每个像素创建一个全新的位图上下文。仅创建一个位图上下文是昂贵的，连续数千次这样做对于性能来说真的很糟糕。

首先创建一个位图上下文（使用自定义数据对象），然后扫描该数据。它会更快。

UIImage *myImage; // image from which pixels are read CGSize imageSize = [myImage size]; NSUInteger bytesPerPixel = 4; NSUInteger bytesPerRow = bytesPerPixel * imageSize.width; NSUInteger bitsPerComponent = 8; unsigned char *pixelData = (unsigned char*) calloc(imageSize.height * imageSize.width * bytesPerPixel, sizeof(unsigned char)) CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); CGContextRef context = CGBitmapContextCreate(pixelData, imageSize.width, 1imageSize.height bitsPerComponent, bytesPerPixel, colorSpace, kCGImageAlphaPremultipliedLast); CGContextDrawImage(context, CGRectMake(0, 0, imageSize.width, imageSize.height), myImage.CGImage); // you can tweak these 2 for-loops to get whichever section of pixels from the image you want for reading. for (NSInteger x = 0; x < imageSize.width; x++) { for (NSInteger y = 0; y < imageSize.height; y++) { int pixelIndex = (bytesPerRow * yy) + xx * bytesPerPixel; CGFloat red = (pixelData[pixelIndex] * 1.0) / 255.0; CGFloat green = (pixelData[pixelIndex + 1] * 1.0) / 255.0; CGFloat blue = (pixelData[pixelIndex + 2] * 1.0) / 255.0; CGFloat alpha = (pixelData[pixelIndex + 3] * 1.0) / 255.0; pixelIndex += 4; UIColor *yourPixelColor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha]; } } } // be a good memory citizen CGColorSpaceRelease(colorSpace); CGContextRelease(context); free(pixelData);

在ios中获取矩形像素的最有效方法是什么？

1 个答案: