Reorganize image/picture arrays in OpenGL to fit power of 2 textures size

Question

I am having troubles in OpenGL due to the fact that textures have to be power of 2 in OpenGL. What I am doing is the following:

I Load a PNG file into an array of unsigned char, using PNGLIB or SOIL. The idea is that I can run though this array and "Select" the parts that are relevant for me. For example, imagining I've loaded a person, but I just want to store the head in a separate texture. So im looping through the array and selecting only the necessary parts.

First Question: I believe that the data in the array is stored in RGBA mode, but I'm yet not sure if the data is filled rowise or columnwise. Is it possible to know this information?

Second Question: Since there is the need to always create power of 2 textures, it can happen that i have an image with 513pixels width so that I will need a texture with 1024px width. So what is happening is that the picture looks like it gets completly "destroyed" because the pixels are not on the places they should be - The texture has a different size than the relevant data filled in the array. So how can I manage to reorganize the array in order to get the contents of the image again? I tried the following but it doesn't work:

unsigned char* new_memory = 0;

    int index = 0;
    int new_index = 0;
    new_memory = new unsigned char[new_tex_width * new_tex_height * 4];

    for(int i=0; i<picture.width; i++) // WIDTH
    {
        for(int j=0; j<picture.height; j++) // HEIGHT
        {
            for(int k=0; k<4; k++) // DEPTH
                new_memory[new_index++] = picture.memory[index++];//picture.memory[i + picture.height * (j + 4 * k)];
        }
        new_index += new_tex_height - picture.height;
    }


    glGenTextures(1, &png_texture);
    glBindTexture(GL_TEXTURE_2D, png_texture);

    glTexImage2D(GL_TEXTURE_2D, 0, 3, new_tex_width, new_tex_height, 0 , GL_RGBA, GL_UNSIGNED_BYTE, new_memory);

Since OpenGL-2 textures may be of arbitrary size. So your concerns about this have no longer any foundation. — datenwolf

Ylisar Ylisar · Accepted Answer · 2012-06-18T10:01:23

Non power of two textures has been supported since a good while back. However, creating textures atlases and rearranging textures still have a lot of merit, the way we do it is to simply use freeimage as they handle all of this for you and supports some of the compressed formats.

If you want to do it your way, and know that it's just a bitmap, then I'd do it more along the lines of ( not tested, and does not check inputs, but should give you an idea ):

void Blit( int xOffset, int yOffset, int targetW, int sourceW, int sourceH, unsigned char* source, unsigned char* target, unsigned int bpp )
{
    for( unsigned int i = 0; i < sourceH; ++i )
    {
        memcpy( target + bpp * ( targetW * ( yOffset + i ) + xOffset ), source + sourceW * i * bpp, sourceW * bpp );
    }
}

Basically, just take each row and memcpy it over.

Reorganize image/picture arrays in OpenGL to fit power of 2 textures size

1 Answers