如何在特定的点击点获取UIImage的RGBA值

Question

我正在开发一个iPad应用程序，在一个视图控制器中有多个UIImageViews。每个图像都有一些透明的部分。当用户点击图像时，我想测试他点击的图像区域是否不透明，然后我想做一些动作

搜索后，我得出的结论是，我必须访问图像的原始数据，并检查用户点击

点的alpha值

我使用了在这里找到的解决方案，它帮助很大。我修改了代码，这样如果用户点击的点是透明的(alpha <1)，那么就会显示0，否则打印1。但是，结果在运行时并不准确。我有时得到0点点击点是不透明的，反之亦然。我认为有一个问题与byteIndex值我不确定它返回的颜色数据在点的用户点击

这是我的代码

   CGPoint touchPoint;
    - (void)viewDidLoad
{
    [super viewDidLoad];
    [logo addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleSingleTap:)]];
    }
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [[event allTouches] anyObject];
     touchPoint = [touch locationInView:self.view];
}
- (void)handleSingleTap:(UITapGestureRecognizer *)recognizer
{
    int x = touchPoint.x;
    int y = touchPoint.y;
    [self getRGBAsFromImage:img atX:x andY:y];
}
- (void)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy {
    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                                 bitsPerComponent, bytesPerRow, colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);
    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;

        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
        byteIndex += 4;
        if (alpha < 1) {
            NSLog(@"0");
// here I should add the action I want
        }
        else NSLog(@"1");

    free(rawData);
}

提前感谢您

Answer 1

这种方法很糟糕，因为它每次都要绘制完整的图像，而且还依赖于精确的字节布局。

我建议只绘制到1x1上下文中，并得到alpha

---

- (void)handleSingleTap:(UITapGestureRecognizer *)recognizer
{
int x = touchPoint.x;
int y = touchPoint.y;
CGFloat alpha = [self getAlphaFromImage:img atX:x andY:y];
if(alpha<1)
   …
}
- (CGFloat)getAlphaFromImage:(UIImage*)image atX:(NSInteger)xx andY:(NSInteger)yy {
// Cancel if point is outside image coordinates
if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), CGPointMake(xx,yy))) {
    return 0;
}
// Create a 1x1 pixel byte array and bitmap context to draw the pixel into.
// Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage
NSInteger pointX = xx;
NSInteger pointY = yy;
CGImageRef cgImage = self.CGImage;
NSUInteger width = self.size.width;
NSUInteger height = self.size.height;
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel * 1;
NSUInteger bitsPerComponent = 8;
unsigned char pixelData[4] = { 0, 0, 0, 0 };
CGContextRef context = CGBitmapContextCreate(pixelData, 
                                             1,
                                             1,
                                             bitsPerComponent, 
                                             bytesPerRow, 
                                             colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextSetBlendMode(context, kCGBlendModeCopy);
// Draw the pixel we are interested in onto the bitmap context
CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
CGContextRelease(context);
CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
return alpha;
}