Question

I have a texture onto which I render 16 drawings. The texture is 1024×1024 in size and it’s divided into 4×4 “slots”, each 256×256 pixels.

Before I render a new drawing into a slot, I want to clear it so that the old drawing is erased and the slot is totally transparent (alpha=0).

Is there a way to do it with OpenGL or need I just access the texture pixels directly in memory and clear them with memset oslt?

Answer 1

I imagine you’d just update the current texture normally:

std::vector<unsigned char> emptyPixels(1024*1024*4, 0); // Assuming RGBA / GL_UNSIGNED_BYTE
glBindTexture(GL_TEXTURE_2D, yourTextureId);
glTexSubImage2D(GL_TEXTURE_2D,
                0,
                0,
                0,
                1024,
                1024,
                GL_RGBA,
                GL_UNSIGNED_BYTE,
                emptyPixels.data());  // Or &emptyPixels[0] if you're stuck with C++03

Even though you’re replacing every pixel, glTexSubImage2D is faster than recreating a new texture.