// Homework 1
// Color to Greyscale Conversion
//A common way to represent color images is known as RGBA - the color
//is specified by how much Red, Green, and Blue is in it.
//The 'A' stands for Alpha and is used for transparency; it will be
//ignored in this homework.
//Each channel Red, Blue, Green, and Alpha is represented by one byte.
//Since we are using one byte for each color there are 256 different
//possible values for each color. This means we use 4 bytes per pixel.
//Greyscale images are represented by a single intensity value per pixel
//which is one byte in size.
//To convert an image from color to grayscale one simple method is to
//set the intensity to the average of the RGB channels. But we will
//use a more sophisticated method that takes into account how the eye
//perceives color and weights the channels unequally.
//The eye responds most strongly to green followed by red and then blue.
//The NTSC (National Television System Committee) recommends the following
//formula for color to greyscale conversion:
//I = .299f * R + .587f * G + .114f * B
//Notice the trailing f's on the numbers which indicate that they are
//single precision floating point constants and not double precision
//constants.
//You should fill in the kernel as well as set the block and grid sizes
//so that the entire image is processed.
#include "reference_calc.cpp"
#include "utils.h"
#include <stdio.h>
__global__
void rgba_to_greyscale(const uchar4* const rgbaImage,
unsigned char* const greyImage,
int numRows, int numCols)
{
//TODO
//Fill in the kernel to convert from color to greyscale
//the mapping from components of a uchar4 to RGBA is:
// .x -> R ; .y -> G ; .z -> B ; .w -> A
//
//The output (greyImage) at each pixel should be the result of
//applying the formula: output = .299f * R + .587f * G + .114f * B;
//Note: We will be ignoring the alpha channel for this conversion
//First create a mapping from the 2D block and grid locations
//to an absolute 2D location in the image, then use that to
//calculate a 1D offset
int thread_x = blockIdx.x * blockDim.x + threadIdx.x;
int thread_y = blockIdx.y * blockDim.y + threadIdx.y;
int mapped_id = thread_y * numCols + thread_x;
const uchar4& rgba = rgbaImage[mapped_id];
float channelSum = .299f * rgba.x + .587f * rgba.y + .114f * rgba.z;
greyImage[mapped_id] = channelSum;
}
void your_rgba_to_greyscale(const uchar4 * const h_rgbaImage, uchar4 * const d_rgbaImage,
unsigned char* const d_greyImage, size_t numRows, size_t numCols)
{
//You must fill in the correct sizes for the blockSize and gridSize
//currently only one block with one thread is being launched
const int block_x = 16;
const int block_y = 16;
int grid_x = numCols / block_x;
if (grid_x * block_x < numCols) {
++grid_x;
}
int grid_y = numRows / block_y;
if (grid_y * block_y < numRows) {
++grid_y;
}
const dim3 blockSize(block_x, block_y, 1); //TODO
const dim3 gridSize(grid_x, grid_y, 1); //TODO
rgba_to_greyscale<<<gridSize, blockSize>>>(d_rgbaImage, d_greyImage, numRows, numCols);
cudaDeviceSynchronize(); checkCudaErrors(cudaGetLastError());
}
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