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Custom Kernel with GpuMat No Effect

asked 2014-07-03 03:48:27 -0600

Jon1111 gravatar image

Hi so I'm a beginner trying to write a pretty simple kernel. I just want to take the arc cosine of the pixels of an image and then return those. I'm using GpuMat objects but I cannot get the kernel to do anything.

//.cpp code
void gpu_acos(const gpu::GpuMat &src, gpu::GpuMat &dst){
    float* srcptr = (float *);
    float* dstptr = (float *);
    acos_func(srcptr,dstptr, src.step,dst.step, src.cols,src.rows);


bool test_acos(){
    Mat input = imread("corgi.jpg",0);
    int rows = input.rows;
    int cols = input.cols;
    Size in_size(rows,cols);
    gpu::GpuMat src, dst;
    cout << "baack from gpu call\n";

    Mat test_out;;
    cout << "this is the test: " << test_out << endl;
    return true;


 //.cu code
#include "custom_kernels.h"
#include <iostream>
using namespace std;
using namespace cv; 
__global__ void acosKernel(const float* srcptr, float* dstptr, size_t srcstep, size_t dststep, int cols, int rows){
    int rowInd = blockIdx.y*blockDim.y+threadIdx.y;
    int colInd = blockIdx.x*blockDim.x+threadIdx.x;
    if(rowInd >= rows || colInd >= cols)
    const float* rowsrcPtr = srcptr+rowInd*srcstep;
    float* rowdstPtr = dstptr+rowInd*dststep;

    float pixVal = rowsrcPtr[colInd];

    if( ((int) pixVal % 90)==0)
            rowdstPtr[colInd] = acos(pixVal);


int divUp(int a, int b){ 
    return (a+b-1)/b;


void acos_func(const float* srcptr, float* dstptr, size_t srcstep, size_t dststep, int cols, int rows){
    dim3 blDim(32,8);
    dim3 grDim(divUp(cols,blDim.x),divUp(rows,blDim.y));

    acosKernel<<<grDim, blDim>>>(srcptr,dstptr,srcstep,dststep,cols,rows);

} ~

Even when I've replaced the acos logic with something simple (for example all 0's) I get as output the input image. I'm sure I'm missing something obvious, but any help would be greatly appreciated. Thanks in advance!

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answered 2014-07-06 05:29:23 -0600

You are using a float pointer in the CPP function and the kernel, but the GpuMat src is a char image as you are uploading it from input, which is loaded from a JPG image. Use a char pointer instead of a float for src (not dst which is created as a float image) and it should get fine. This doesn't explain (or does it?) the behavior you get when you change the logic, but this is definitely an issue.

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Hi thanks for the response. I changed the src pointer to a char pointer but it still does not work. I still could not get the dstptr values to be zero. I tried by both defining the GpuMat dst as a CV_8UC1 type and using unsigned char* and also as a CV_32FC1 using float* but neither type had an impact. This seems pretty basic so am I just missing something really obvious?

Jon1111 gravatar imageJon1111 ( 2014-07-07 04:27:10 -0600 )edit

Nevermind I was not stepping correctly when I switched from unsigned chars to floats. Thanks for the tip that GpuMat src is a char image

Jon1111 gravatar imageJon1111 ( 2014-07-08 03:24:33 -0600 )edit

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Asked: 2014-07-03 03:48:27 -0600

Seen: 1,827 times

Last updated: Jul 06 '14