I'm trying to get the pixel color for a C4 image, how do I access the image color matrix? c4framework

Question

When discussing color colorMatrix: bias, he reads:

This filter performs the matrix transformation as follows to convert the color vector:

sr = point (s, redVector) sg = point (s, green vector) sb = point (s, blue vector) sa = point (s, alphaVector) s = s + offset

Is there a way to access data values ​​for different color vectors?

Answer 1

The discussion you refer to in the C4 documentation relates to a process that uses a filter to calculate matrix multiplication. This is actually just a description of what the filter does with the colors in the image when it is applied.

In fact, what happens under the hood is that the colorMatrix: method installs a CIFilter called CIColorMatrix and applies it to C4Image . Unfortunately, the source code for the CIColorMatrix filter CIColorMatrix not provided by Apple.

So, the long-awaited answer to your question:

You cannot access the color components for pixels in C4Image through the CIColorMatrix filter. But the C4Image class has a CGImage property (for example, yourC4Image.CGImage ), which you can use to get the data in pixels.

A good, simple technique can be found HERE

EDIT: I was obsessed with this question yesterday and added these two methods to the C4Image class:

Way to load pixel data:

 -(void)loadPixelData { NSUInteger width = CGImageGetWidth(self.CGImage); NSUInteger height = CGImageGetHeight(self.CGImage); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); bytesPerPixel = 4; bytesPerRow = bytesPerPixel * width; rawData = malloc(height * bytesPerRow); NSUInteger bitsPerComponent = 8; CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big); CGColorSpaceRelease(colorSpace); CGContextDrawImage(context, CGRectMake(0, 0, width, height), self.CGImage); CGContextRelease(context); } 

And the pixel color access method:

 -(UIColor *)colorAt:(CGPoint)point { if(rawData == nil) { [self loadPixelData]; } NSUInteger byteIndex = bytesPerPixel * point.x + bytesPerRow * point.y; CGFloat r, g, b, a; r = rawData[byteIndex]; g = rawData[byteIndex + 1]; b = rawData[byteIndex + 2]; a = rawData[byteIndex + 3]; return [UIColor colorWithRed:RGBToFloat(r) green:RGBToFloat(g) blue:RGBToFloat(b) alpha:RGBToFloat(a)]; } 

How I would apply the methods from another post that I mentioned.