iOS,无法获得CGImageRef的正确像素颜色 [英] ios, can't get correct pixel color of CGImageRef
问题描述
我做了一个背景,然后是一个渐变.之后,我将参考上下文.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,这是不正确的.感谢您的帮助,谢谢.这是代码.
I made a context, then a gradient. After that I am drawing to the context. Then I receive an gradiented image from context and it is correct, I can see it in the debugger. But when I try to get a pixel color for a specific value it isn't correct. I appreciate any help, thanks. Here is the code.
- (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;
}
推荐答案
我测试了您的代码,问题似乎是在计算pixelIndex
之前对point.y
进行了乘法运算:
I tested your code and the problem seems to be the bit of multiplication done to point.y
before calculating 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此代码,并且它可以正常工作.还将point
更改为int
以匹配imageWidth
参数,并将其始终限制为0 <= point < imageWidth
:
I removed this from your code, and it works. Also changed point
to an int
to match the imageWidth
parameter, and bounded it to always be 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;
}
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