ios，无法获得 CGImageRef 的正确像素颜色

Question

我做了一个上下文，然后是一个渐变。之后，我将根据上下文进行绘制。然后我从上下文中收到一个渐变图像，它是正确的，我可以在调试器中看到它。但是当我尝试获取特定值的像素颜色时，它是不正确的。感谢您的帮助，谢谢。这是代码。

- (UIColor*) getColorForPoint: (CGPoint) point imageWidth: (NSInteger) imageHeight
{
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    // create a gradient

    CGPoint startPoint = CGPointMake(0, 0);
    CGPoint endPoint = CGPointMake(imageHeight, 0);

    // create a bitmap context to draw the gradient to, 1 pixel high.
    CGContextRef context = CGBitmapContextCreate(NULL, imageHeight, 1, 8, 0, colorSpace,     kCGImageAlphaPremultipliedLast);

    // draw the gradient into it
    CGContextAddRect(context, CGRectMake(0, 0, imageHeight, 1));
    CGContextClip(context);
    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);

    // Get our RGB bytes into a buffer with a couple of intermediate steps...
    //      CGImageRef -> CFDataRef -> byte array
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    CGDataProviderRef provider = CGImageGetDataProvider(cgImage);
    CFDataRef pixelData = CGDataProviderCopyData(provider);

    // cleanup:
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(cgImage);
    CGContextRelease(context);

    const UInt8* data = CFDataGetBytePtr(pixelData);

    // we got all the data we need.
    // bytes in the data buffer are a succession of R G B A bytes

    CGFloat y = 100 * point.y / imageHeight;
    int pixelIndex = (int)y * 4; // 4 bytes per color
    UIColor *color = [UIColor colorWithRed:data[pixelIndex + 0]/255.0f
                                 green:data[pixelIndex + 1]/255.0f
                                  blue:data[pixelIndex + 2]/255.0f
                                 alpha:data[pixelIndex + 3]/255.0f];

    // done fetching color data, finally release the buffer
    CGDataProviderRelease(provider);

    return color;
}

Answer 1

我测试了你的代码，问题似乎是对 point.y 进行的乘法运算在计算之前 pixelIndex :

CGFloat y = 100 * point.y / imageHeight;
int pixelIndex = (int)y * 4; // 4 bytes per color

我从您的代码中删除了它，它可以正常工作。也改了point到int匹配 imageWidth参数，并将其限制为始终 0 <= point < imageWidth :

- (UIColor*) getColorForPoint: (NSInteger) point imageWidth: (NSInteger) imageWidth
{
    point = MIN(imageWidth - 1, MAX(0, point));

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    // create a gradient

    CGPoint startPoint = CGPointMake(0, 0);
    CGPoint endPoint = CGPointMake(imageWidth, 0);

    // create a bitmap context to draw the gradient to, 1 pixel high.
    CGContextRef context = CGBitmapContextCreate(NULL, imageWidth, 1, 8, 0, colorSpace,     kCGImageAlphaPremultipliedLast);

    // draw the gradient into it
    CGContextAddRect(context, CGRectMake(0, 0, imageWidth, 1));
    CGContextClip(context);
    CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);

    // Get our RGB bytes into a buffer with a couple of intermediate steps...
    //      CGImageRef -> CFDataRef -> byte array
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    CGDataProviderRef provider = CGImageGetDataProvider(cgImage);
    CFDataRef pixelData = CGDataProviderCopyData(provider);

    // cleanup:
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(cgImage);
    CGContextRelease(context);

    const UInt8* data = CFDataGetBytePtr(pixelData);

    // we got all the data we need.
    // bytes in the data buffer are a succession of R G B A bytes

    int pixelIndex = (int)point * 4; // 4 bytes per color
    UIColor *color = [UIColor colorWithRed:data[pixelIndex + 0]/255.0f
                                     green:data[pixelIndex + 1]/255.0f
                                      blue:data[pixelIndex + 2]/255.0f
                                     alpha:data[pixelIndex + 3]/255.0f];

    // done fetching color data, finally release the buffer
    CGDataProviderRelease(provider);

    return color;
}