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Help => Graphics => Topic started by: smash on August 02, 2011, 10:45:47 pm

Title: How to create and manipulate a pixelarray?
Post by: smash on August 02, 2011, 10:45:47 pm

Hi guys (sry for my english  :P  :oops:)

I'm coding a simulation of langton's ant at the moment.
Because SetPixel is to slow, i want (i read about it) i want to code it via a pixelarray with 800x600 Pixels.

creating:

Code: [Select]

sf::Uint8 *pixels = new sf::Uint8[800 * 600 * 4]

that should be right.

And now, I want to access it:

Code: [Select]

pixels[x*800 + y +0] = 0; //red
pixels[x*800 + y +1] = 0; //green
pixels[x*800 + y +2] = 0; //blue
pixels[x*800 + y +3] = 0xff; //alpha

but I think it's not right, isn't it?

If the pixelarray is ready I load it via LoadFromPixels() into an Image and Draw it.

But it happens not the right result.
The picture is not how i expected.


Where's my fault?
Hope someone did something similar.

Title: How to create and manipulate a pixelarray?
Post by: Laurent on August 03, 2011, 08:05:44 am

The formula is wrong. It should be:

Code: [Select]

pixels[(y * 800 + x) * 4 + 0] = red;
pixels[(y * 800 + x) * 4 + 1] = green;
pixels[(y * 800 + x) * 4 + 2] = blue;
pixels[(y * 800 + x) * 4 + 3] = alpha;

Title: Re: How to create and manipulate a pixelarray?
Post by: Nexus on August 03, 2011, 10:06:46 am

Quote from: "smash"

that should be right.

Yes, but why don't you use STL containers? They free you from the burden of manual memory management.

Code: [Select]

std::vector<sf::Uint8> pixels(800 * 600 * 4);

Title: Re: How to create and manipulate a pixelarray?
Post by: smash on August 03, 2011, 01:18:17 pm

Quote from: "Laurent"

The formula is wrong. It should be:

Code: [Select]

pixels[(y * 800 + x) * 4 + 0] = red;
pixels[(y * 800 + x) * 4 + 1] = green;
pixels[(y * 800 + x) * 4 + 2] = blue;
pixels[(y * 800 + x) * 4 + 3] = alpha;

ok, thanks this works for me.
But it's also really really slow (not really faster as SetPixel()).

I got (at a resolution of 800x600) a Map  with 200x200 Fields, and every Field consits of 12 Pixels (4x3)

And in every Step I change one of these Fields, draw the whole picture and display it.
Which means I change every Step 12 pixels and so 48 assignments (4 assignments per pixel), in each step.
I show you my Code:

Code: [Select]


void Grid::SetFieldPixel(int xMap, int yMap, sf::Color color)
{
    for(int i=0; i < 4; i++)
    {
        for(int j=0; j < 3; j++)
        {
            //GridImage.SetPixel(xMap*4+i, yMap*3+j, color); // the "old" SetPixel() Idea...

            if(color == sf::Color::Black)
            {
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +0] = 0;
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +1] = 0;
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +2] = 0;
            }
            else
            {
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +0] = 255;
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +1] = 255;
                pixels[ (((xMap*4+i) + 800 * (yMap*3+j)) *4) +2] = 255;
            }

            pixels[((xMap*4+i)+ 800 * (yMap*3+j))*4 +3] = 255;
        }
    }
}


And I dont know, why it's so slow, it's ca. as fast as the SetPixel() variant.

Quote from: "Nexus"

Quote from: "smash"

that should be right.

Yes, but why don't you use STL containers? They free you from the burden of manual memory management.

Code: [Select]

std::vector<sf::Uint8> pixels(800 * 600 * 4);

Don't understood your intention... I knew the STL containers, but what do you mean exactly??


someone an Idea to get this faster?

Title: How to create and manipulate a pixelarray?
Post by: Laurent on August 03, 2011, 10:02:23 pm

Please don't duplicate posts.
http://www.sfml-dev.org/forum/viewtopic.php?t=5477