Modifying a texture on a mesh at given world coordinate

Question

Im making an editor in which I want to build a terrain map. I want to use the mouse to increase/decrease terrain altitude to create mountains and lakes.

Technically I have a heightmap I want to modify at a certain texcoord that I pick out with my mouse. To do this I first go from screen coordinates to world position - I have done that. The next step, going from world position to picking the right texture coordinate puzzles me though. How do I do that?

Answer 1

If you are using a simple hightmap, that you use as a displacement map in lets say the y direction. The base mesh lays in the xz plain (y=0).

You can discard the y coordinate from world coordinate that you have calculated and you get the point on the base mesh. From there you can map it to texture space the way, you map your texture.

I would not implement it that way. I would render the scene to a framebuffer and instead of rendering a texture the the mesh, colorcode the texture coordinate onto the mesh. If i click somewhere in screen space, i can simple read the pixel value from the framebuffer and get the texture coordinate directly. The rendering to the framebuffer should be very inexpensive anyway.

Answer 2

Assuming your terrain is a simple rectangle you first calculate the vector between the mouse world position and the origin of your terrain. (The vertex of your terrain quad where the top left corner of your height map is mapped to). E.g. mouse (50,25) - origin(-100,-100) = (150,125).
Now divide the x and y coordinates by the world space width and height of your terrain quad.
150 / 200 = 0.75 and 125 / 200 = 0.625. This gives you the texture coordinates, if you need them as pixel coordinates instead simply multiply with the size of your texture.