RGBtoGray.cu

/*
* Provides function to turn RGB image to gray image.
*/
#include<stdint.h>

// Turn off mangling.
extern "C"{

// Function takes in RGB color image size of of width
// and heights and outputs gray image to imageout.
// DO NOT USE SAME MEMORY AREA AS INPUT AND OUTPUT!
__global__ void rgbToGray(uint8_t * __restrict__ image,uint8_t * __restrict__ imageout,uint32_t width,uint32_t height){

	// libpng manual says Y = 0.2126 * R + 0.7152 * G + 0.0722 * B where
	// Y is grayscale and R,G, and B are from RGB.
	// PNG also has cHRM chunk where one can get weights but don't have
	// time to deal with that.
	// http://www.libpng.org/pub/png/libpng-manual.txt

	uint32_t xindex=blockIdx.x*blockDim.x+threadIdx.x;
	uint32_t yindex=blockIdx.y*blockDim.y+threadIdx.y;

	if(xindex<width && yindex<height){
		// Index to the pixel.
		uint32_t index=yindex*width+xindex;
		imageout[index]=(0.2126*image[index*3+0]+0.7152*image[index*3+1]+0.0722*image[index*3+2]);
	}

}

}