Using OpenMP and Magick++ for pixel-by-pixel image processing - c++

Question

I'm working on a C++ code for image manipulation that works pixel-by-pixel (using Magick++), and I want to use it with OpenMP, but I have the next issue:
Magick: Semaphore operation failed (unable to destroy semaphore) [Dispositivo o recurso ocupado].
img_test: magick/pixel_cache.c:2765: ModifyCache: La declaración `image->cache != (Cache) ((void *)0)' no se cumple.
And, also, it keeps stuck in an infinite loop.
Here is the code snippet:
int main(int argc,char **argv)
{
InitializeMagick(*argv);
Image img1, img2;
img1.read(argv[1]);
img2.read(argv[2]);
int sx = img1.columns();
int sy = img1.rows();
Image out;
out.size(Geometry(sx,sy));
cout << "Processing pictures..." << endl;
int iy;
#pragma omp for private(iy)
for (iy=0;iy<sy;iy++)
{
#pragma omp parallel for
for (int ix=0;ix<sx;ix++)
{
double _r = 0.0, _g = 0.0, _b = 0.0;
ColorRGB ppix1(img1.pixelColor(ix,iy));
ColorRGB ppix2(img2.pixelColor(ix,iy));
// do some image processing...
ColorRGB opix(_r*MaxRGB,_g*MaxRGB,_b*MaxRGB);
out.pixelColor(ix,iy,opix);
}
}
out.write("Output.png");
}
Is there a way to solve this?

Answer 1

Is there a way to solve this?
For this example, you would want to use schedule ordered.
cout << "Processing pictures..." << endl;
int iy;
#pragma omp for schedule(static) ordered
for (iy=0;iy<sy;iy++)
{
#pragma omp ordered
for (int ix=0;ix<sx;ix++)
{
double _r = 0.0, _g = 0.0, _b = 0.0;
ColorRGB ppix1(img1.pixelColor(ix,iy));
ColorRGB ppix2(img2.pixelColor(ix,iy));
// do some image processing...
ColorRGB opix(_r*MaxRGB,_g*MaxRGB,_b*MaxRGB);
out.pixelColor(ix,iy,opix);
}
}
out.write("Output.png");
Edit
If you do want to work with low-level pixel information across parallel, #NoseKnowsAll is correct for a single region of iy. However you'll run into issues calling out.pixelColor as the internal cache may fall out of sync. I would suggest export the pixels data, perform work in parallel, and import the final results.
// Allocate three buffers the total size of x * y * RGB
double * buffer1 = new double[sx * sy * 3];
double * buffer2 = new double[sx * sy * 3];
double * buffer3 = new double[sx * sy * 3];
// Write pixel data to first two buffers
img1.write(0,0, sx, sy, "RGB", DoublePixel, buffer1);
img2.write(0,0, sx, sy, "RGB", DoublePixel, buffer2);
cout << "Processing pictures..." << endl;
int iy;
#pragma omp parallel for
for (iy=0;iy<sy;iy++)
{
for (int ix=0;ix<sx;ix++)
{
// Find where in buffer the current pixel is located at
size_t idx = (iy * sx + ix) * 3;
// For fun, let's alternate which source to assing to the
// third buffer.
if ((iy % 2 && ix % 2) || (!(iy % 2) && !(ix % 2))) {
buffer3[idx+0] = buffer1[idx+0]; // R
buffer3[idx+1] = buffer1[idx+1]; // G
buffer3[idx+2] = buffer1[idx+2]; // B
} else {
buffer3[idx+0] = buffer2[idx+0]; // R
buffer3[idx+1] = buffer2[idx+1]; // G
buffer3[idx+2] = buffer2[idx+2]; // B
}
}
}
// Import the third buffer into out Image
out.read(sx, sy, "RGB", DoublePixel, buffer3);
out.write("Output.png");
YMMV