How to change single texel in OpenGL texture

Question

Hi !

I want to change the some texels in a OpenGL texture for a given location. Can anyone help me with this pls ?

This is the functionnality that I want,

void ChangeTexelColor(int x, int y, GLuint id, int texW, int texH, GLenum format)
{
   //What is here ?     
}

This will use to maintain the minimap of my game (if anyone have a better idea of maintaining a dynamic map-texture-). Btw, this must done fast. Thanks.

Answer 1

OpenGL has the glTexSubImage2D function, which is exactly for your purpose.

Here's a functions that changes the color of one texel:

void changeTexelColor(GLuint id, GLint x, GLint y, uint8_t r, uint8_t g, uint8_t b, uint8_t a) {
    uint8_t data[4];
    data[0] = r;
    data[1] = g;
    data[2] = b;
    data[3] = a;
    glBindTexture(GL_TEXTURE_2D, id);
    glTexSubImage2D(GL_TEXTURE_2D,
                    0,
                    x,
                    y,
                    1,
                    1,
                    GL_RGBA,
                    GL_UNSIGNED_BYTE,
                    data);
}

Answer 2

Performance-wise, you may be better to store the map locally as your own array and draw it to the screen as a set of untextured quads.

Rendering primitives is heavily optimised, especially compared to creating or modifying textures.