// Homework 2
// Image Blurring
//
// In this homework we are blurring an image. To do this, imagine that we have
// a square array of weight values. For each pixel in the image, imagine that we
// overlay this square array of weights on top of the image such that the center
// of the weight array is aligned with the current pixel. To compute a blurred
// pixel value, we multiply each pair of numbers that line up. In other words, we
// multiply each weight with the pixel underneath it. Finally, we add up all of the
// multiplied numbers and assign that value to our output for the current pixel.
// We repeat this process for all the pixels in the image.
// To help get you started, we have included some useful notes here.
//****************************************************************************
// For a color image that has multiple channels, we suggest separating
// the different color channels so that each color is stored contiguously
// instead of being interleaved. This will simplify your code.
// That is instead of RGBARGBARGBARGBA... we suggest transforming to three
// arrays (as in the previous homework we ignore the alpha channel again):
// 1) RRRRRRRR...
// 2) GGGGGGGG...
// 3) BBBBBBBB...
//
// The original layout is known an Array of Structures (AoS) whereas the
// format we are converting to is known as a Structure of Arrays (SoA).
// As a warm-up, we will ask you to write the kernel that performs this
// separation. You should then write the "meat" of the assignment,
// which is the kernel that performs the actual blur. We provide code that
// re-combines your blurred results for each color channel.
//****************************************************************************
// You must fill in the gaussian_blur kernel to perform the blurring of the
// inputChannel, using the array of weights, and put the result in the outputChannel.
// Here is an example of computing a blur, using a weighted average, for a single
// pixel in a small image.
//
// Array of weights:
//
// 0.0 0.2 0.0
// 0.2 0.2 0.2
// 0.0 0.2 0.0
//
// Image (note that we align the array of weights to the center of the box):
//
// 1 2 5 2 0 3
// -------
// 3 |2 5 1| 6 0 0.0*2 + 0.2*5 + 0.0*1 +
// | |
// 4 |3 6 2| 1 4 -> 0.2*3 + 0.2*6 + 0.2*2 + -> 3.2
// | |
// 0 |4 0 3| 4 2 0.0*4 + 0.2*0 + 0.0*3
// -------
// 9 6 5 0 3 9
//
// (1) (2) (3)
//
// A good starting place is to map each thread to a pixel as you have before.
// Then every thread can perform steps 2 and 3 in the diagram above
// completely independently of one another.
// Note that the array of weights is square, so its height is the same as its width.
// We refer to the array of weights as a filter, and we refer to its width with the
// variable filterWidth.
//****************************************************************************
// Your homework submission will be evaluated based on correctness and speed.
// We test each pixel against a reference solution. If any pixel differs by
// more than some small threshold value, the system will tell you that your
// solution is incorrect, and it will let you try again.
// Once you have gotten that working correctly, then you can think about using
// shared memory and having the threads cooperate to achieve better performance.
//****************************************************************************
// Also note that we've supplied a helpful debugging function called checkCudaErrors.
// You should wrap your allocation and copying statements like we've done in the
// code we're supplying you. Here is an example of the unsafe way to allocate
// memory on the GPU:
//
// cudaMalloc(&d_red, sizeof(unsigned char) * numRows * numCols);
//
// Here is an example of the safe way to do the same thing:
//
// checkCudaErrors(cudaMalloc(&d_red, sizeof(unsigned char) * numRows * numCols));
//
// Writing code the safe way requires slightly more typing, but is very helpful for
// catching mistakes. If you write code the unsafe way and you make a mistake, then
// any subsequent kernels won't compute anything, and it will be hard to figure out
// why. Writing code the safe way will inform you as soon as you make a mistake.
// Finally, remember to free the memory you allocate at the end of the function.
//****************************************************************************
#include "reference_calc.cpp"
#include "utils.h"
__global__
void gaussian_blur(const unsigned char* const inputChannel,
unsigned char* const outputChannel,
int numRows, int numCols,
const float* const filter, const int filterWidth)
{
// TODO
// NOTE: Be sure to compute any intermediate results in floating point
// before storing the final result as unsigned char.
// NOTE: Be careful not to try to access memory that is outside the bounds of
// the image. You'll want code that performs the following check before accessing
// GPU memory:
//
// if ( absolute_image_position_x >= numCols ||
// absolute_image_position_y >= numRows )
// {
// return;
// }
const int2 thread_2D_pos = make_int2( blockIdx.x * blockDim.x + threadIdx.x,
blockIdx.y * blockDim.y + threadIdx.y);
const int thread_1D_pos = thread_2D_pos.y * numCols + thread_2D_pos.x;
//make sure we don't try and access memory outside the image
//by having any threads mapped there return early
if (thread_2D_pos.x >= numCols || thread_2D_pos.y >= numRows) {
return;
}
// NOTE: If a thread's absolute position 2D position is within the image, but some of
// its neighbors are outside the image, then you will need to be extra careful. Instead
// of trying to read such a neighbor value from GPU memory (which won't work because
// the value is out of bounds), you should explicitly clamp the neighbor values you read
// to be within the bounds of the image. If this is not clear to you, then please refer
// to sequential reference solution for the exact clamping semantics you should follow.
float color = 0.0f;
const int half_filter_width = filterWidth >> 1;
for(int f_y = 0; f_y < filterWidth; ++f_y) {
for(int f_x = 0; f_x < filterWidth; ++f_x) {
int c_x = thread_2D_pos.x + f_x - half_filter_width;
int c_y = thread_2D_pos.y + f_y - half_filter_width;
c_x = min(max(c_x, 0), numCols - 1);
c_y = min(max(c_y, 0), numRows - 1);
float filter_value = filter[f_y*filterWidth + f_x];
color += filter_value * inputChannel[c_y*numCols + c_x];
}
}
outputChannel[thread_1D_pos] = color;
}
//This kernel takes in an image represented as a uchar4 and splits
//it into three images consisting of only one color channel each
__global__
void separateChannels(const uchar4* const inputImageRGBA,
int numRows,
int numCols,
unsigned char* const redChannel,
unsigned char* const greenChannel,
unsigned char* const blueChannel)
{
// TODO
//
// NOTE: Be careful not to try to access memory that is outside the bounds of
// the image. You'll want code that performs the following check before accessing
// GPU memory:
//
// if ( absolute_image_position_x >= numCols ||
// absolute_image_position_y >= numRows )
// {
// return;
// }
const int2 thread_2D_pos = make_int2( blockIdx.x * blockDim.x + threadIdx.x,
blockIdx.y * blockDim.y + threadIdx.y);
const int thread_1D_pos = thread_2D_pos.y * numCols + thread_2D_pos.x;
//make sure we don't try and access memory outside the image
//by having any threads mapped there return early
if (thread_2D_pos.x >= numCols || thread_2D_pos.y >= numRows) {
return;
}
const uchar4& rgba = inputImageRGBA[thread_1D_pos];
redChannel[thread_1D_pos] = rgba.x;
greenChannel[thread_1D_pos] = rgba.y;
blueChannel[thread_1D_pos] = rgba.z;
}
//This kernel takes in three color channels and recombines them
//into one image. The alpha channel is set to 255 to represent
//that this image has no transparency.
__global__
void recombineChannels(const unsigned char* const redChannel,
const unsigned char* const greenChannel,
const unsigned char* const blueChannel,
uchar4* const outputImageRGBA,
int numRows,
int numCols)
{
const int2 thread_2D_pos = make_int2( blockIdx.x * blockDim.x + threadIdx.x,
blockIdx.y * blockDim.y + threadIdx.y);
const int thread_1D_pos = thread_2D_pos.y * numCols + thread_2D_pos.x;
//make sure we don't try and access memory outside the image
//by having any threads mapped there return early
if (thread_2D_pos.x >= numCols || thread_2D_pos.y >= numRows)
return;
unsigned char red = redChannel[thread_1D_pos];
unsigned char green = greenChannel[thread_1D_pos];
unsigned char blue = blueChannel[thread_1D_pos];
//Alpha should be 255 for no transparency
uchar4 outputPixel = make_uchar4(red, green, blue, 255);
outputImageRGBA[thread_1D_pos] = outputPixel;
}
unsigned char *d_red, *d_green, *d_blue;
float *d_filter;
void allocateMemoryAndCopyToGPU(const size_t numRowsImage, const size_t numColsImage,
const float* const h_filter, const size_t filterWidth)
{
//allocate memory for the three different channels
//original
checkCudaErrors(cudaMalloc(&d_red, sizeof(unsigned char) * numRowsImage * numColsImage));
checkCudaErrors(cudaMalloc(&d_green, sizeof(unsigned char) * numRowsImage * numColsImage));
checkCudaErrors(cudaMalloc(&d_blue, sizeof(unsigned char) * numRowsImage * numColsImage));
//TODO:
//Allocate memory for the filter on the GPU
//Use the pointer d_filter that we have already declared for you
//You need to allocate memory for the filter with cudaMalloc
//be sure to use checkCudaErrors like the above examples to
//be able to tell if anything goes wrong
//IMPORTANT: Notice that we pass a pointer to a pointer to cudaMalloc
size_t filter_size = sizeof(float) * filterWidth * filterWidth;
checkCudaErrors(cudaMalloc(&d_filter, filter_size));
//TODO:
//Copy the filter on the host (h_filter) to the memory you just allocated
//on the GPU. cudaMemcpy(dst, src, numBytes, cudaMemcpyHostToDevice);
//Remember to use checkCudaErrors!
checkCudaErrors(cudaMemcpy(d_filter, h_filter, filter_size, cudaMemcpyHostToDevice));
}
void your_gaussian_blur(const uchar4 * const h_inputImageRGBA, uchar4 * const d_inputImageRGBA,
uchar4* const d_outputImageRGBA, const size_t numRows, const size_t numCols,
unsigned char *d_redBlurred,
unsigned char *d_greenBlurred,
unsigned char *d_blueBlurred,
const int filterWidth)
{
//TODO: Set reasonable block size (i.e., number of threads per block)
//TODO:
//Compute correct grid size (i.e., number of blocks per kernel launch)
//from the image size and and block size.
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
//TODO: Launch a kernel for separating the RGBA image into different color channels
separateChannels<<<gridSize, blockSize>>>(d_inputImageRGBA, numRows, numCols, d_red, d_green, d_blue);
// Call cudaDeviceSynchronize(), then call checkCudaErrors() immediately after
// launching your kernel to make sure that you didn't make any mistakes.
cudaDeviceSynchronize(); checkCudaErrors(cudaGetLastError());
//TODO: Call your convolution kernel here 3 times, once for each color channel.
gaussian_blur<<<gridSize, blockSize>>>(d_red, d_redBlurred, numRows, numCols, d_filter, filterWidth);
gaussian_blur<<<gridSize, blockSize>>>(d_green, d_greenBlurred, numRows, numCols, d_filter, filterWidth);
gaussian_blur<<<gridSize, blockSize>>>(d_blue, d_blueBlurred, numRows, numCols, d_filter, filterWidth);
// Again, call cudaDeviceSynchronize(), then call checkCudaErrors() immediately after
// launching your kernel to make sure that you didn't make any mistakes.
cudaDeviceSynchronize(); checkCudaErrors(cudaGetLastError());
// Now we recombine your results. We take care of launching this kernel for you.
//
// NOTE: This kernel launch depends on the gridSize and blockSize variables,
// which you must set yourself.
recombineChannels<<<gridSize, blockSize>>>(d_redBlurred,
d_greenBlurred,
d_blueBlurred,
d_outputImageRGBA,
numRows,
numCols);
cudaDeviceSynchronize(); checkCudaErrors(cudaGetLastError());
}
//Free all the memory that we allocated
//TODO: make sure you free any arrays that you allocated
void cleanup() {
checkCudaErrors(cudaFree(d_red));
checkCudaErrors(cudaFree(d_green));
checkCudaErrors(cudaFree(d_blue));
}
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