How would I get this to UV map correctly?

Question

Alright so I have my code to draw out a big landscape using C++ and DirectX. I had it textured with one texture and then needed to add more. I saw people doing it where they had 1 texture image and the image contained 2 textures. Thats what I made, it's a 256x128 image. My problem now is that since my terrain automatically generated the coordinates to UV map 1 texture now it is displaying both textures. I need to make it so when the height of the world is high enough it is 1 texture and everything under is another texture. My code for the UV coordinates,

Vertices[y * WIDTH * x].U = x / 1.28;

Vertices[y * WIDTH * x].V = y / 1.28;

those are my mapping coordinates, X is the current X value of the vertice it is drawing and the Y value is its current y position. The heightmap is 128x128 so I divided by 1.28 to make it so that each polygon had the texture UV mapped on it. The height is calculated as well since I am loading a heightmap and im trying to get it so when it is high enough it UV maps 1 half of the image and if it is the other it UV maps the other side of the image. Someone please help!

Answer 1

bool topTexture = height[x][y] > threshold;
float u = x / 1.28;
float v = y / 1.28;
Vertices[y * WIDTH * x].U = (u - (int)u) / 2 + (topTexture ? 0.5 : 0);
Vertices[y * WIDTH * x].V = (v - (int)v) / 2 + (topTexture ? 0.5 : 0);

You may want to blend the two textures at the threshold level. Then, you have to do it in the PixelShader.