Tiled terrain. User wants to build a structure. He presses build and for each tile there is an "allow" or "disallow" tile sprite added to the scene. FPS drops right away, since there are 600+ tiles added to the screen. Since map equals screen, there is no scrolling. I came to an idea to make an allow grid covering the whole map and mask the disallow fields.

Approach 1:

• Create allow and disallow grid textures.
• Draw a polygon on screen.
• Pass both textures to the fragment shader.
• Determine the position inside the polygon and use color from allowTexture if the fragment belongs to the allow field, disallow otherwise

Problem: How do I know if I'm on the field that isn't allowed if I cannot pass the matrix representing the map (enum FieldStatus[][] (Allow / Disallow)) to the shader? Therefore, inside the shader I don't know which fragments should be masked.

Approach 2:

• Create allow texture.
• Create an empty texture buffer same size as the allow texture
• Memset the pixels of the empty texture to desired color for each pixel that doesn't allow building.
• Draw a polygon on screen.
• Pass both textures to the fragment shader.
• Use texture2 color if alpha > 0, texture1 color otherwise.

Problem: I'm not sure what is the right way to manipulate pixels on a texture. Do I just make a buffer with width*height*4 size and memcpy the color[] to desired coordinates or is there anything else to it? Would I have to call glTexImage2D after every change to the texture?

Another problem with this approach is that it takes a lot more work to get a prettier effect since I'm manipulating the color pixels instead of just masking two textures.

varying vec2 TexCoordOut;

uniform sampler2D Texture1;
uniform sampler2D Texture2;

void main(void){
    vec4 allowColor = texture2D(Texture1, TexCoordOut);
    vec4 disallowColor = texture2D(Texture2, TexCoordOut);

if(disallowColor.a > 0){
    gl_FragColor= disallowColor;
}else{
    gl_FragColor= allowColor;
}}

I'm working with OpenGL on Windows. Any other suggestion is welcome.