GetPixel, any faster method of access?

[sparkzi]

Hi,
I want to compare images using SFML I tried to do using shaders (http://en.sfml-dev.org/forums/index.php?topic=12239.0) and now I want to make non-shader version.
Currently i have something like this:

                // drawing some polygons on texture

                texture.display();
                sf::Image& generatedImage = texture.getTexture().copyToImage();

               
                long double diff = 0;
                int j = 0;
                for(int j = 0; j<300; j++)
                {
                        for(int i = 0; i<300; i++)
                        {
                                diff += std::abs(generatedImage.getPixel(j,i).r - originalImage.getPixel(j,i).r) +
                                std::abs(generatedImage.getPixel(j,i).g - originalImage.getPixel(j,i).g) +
                                std::abs(generatedImage.getPixel(j,i).b - originalImage.getPixel(j,i).b);
                        }
                }
 

as you can see I'm using

sf::Image& generatedImage = texture.getTexture().copyToImage();

and then when I want access pixel values:

generatedImage.getPixel(x,y).r

I have run profiler and it shows that program spend almost whole time in getPixel() insted of copyToImage(). I thpught that when i call .getTexture.copyToImage() the whole image will be copied from GPU to CPU and then accessing pixels will be very quick. Maybe I do something wrong? Can anyone help me?

[Laurent]

First thing to optimize: call getPixel twice instead of 6 times.

Second thing to possibly optimize: getPixel is not optimized, you may get better results by accessing pixels directly (image.getPixelsPtr() + offset).

[sparkzi]

Thank you for quick answer.
How it works: data is transfered from CPU to GPU during

 texture.getTexture().copyToImage()

or when getPixel is called?

[Laurent]

Data is transfered to CPU in copyToImage(). getPixel does nothing more than accessing an element in an array, but it could be faster.