GLSL and Instances

Textures to Displace Instances

While the previous example demonstrates how we can think of displacement purely in terms of mathematics, it can sometimes be helpful to do this with a texture instead. Just like we can use height maps for displacing a mesh, we can use a similar principle for manipulating instances directly. In the most recent builds of TouchDesinger there are alternative ways to approach this set of manipulations — this set of examples, however, still stands as a reference for how to take advantage of instance manipulation in the vertex stage.

Here a texture is used to represent the change in height for our instances. In order to correctly sample a pixel value from this image we’ll use a helper function CoordsFromTex(). This function will return an ivec2 with a set of x and y coordinates that we’ll use to sample the texture. Here’s our helper function:

ivec2 CoordsFromTex(vec2 size, int id){
    float xCoord = mod(float(id), size.x);
    float yCoord = fract( (float(id) / (size.x * size.y)) ) * size.y;
    return ivec2(int(xCoord), int(yCoord));
}

This function takes a vec2 that describes the dimensions of our texture (the number of pixels in x and y), and the instance ID. From those two values we can derive the pixel coordinate where we want to sample. These values will be used with texelFetch() — a function that provides pixel accurate results, prefered over the alternative texture() function which uses noramlized coordinates. In the following three lines we’ll use this function to look up our texel, retrieve the value from the red channel, scale this by a uniform u_disp_scale, and finally add it to our new instance position:

ivec2 xyLookup  = CoordsFromTex(u_instance_pos, TDInstanceID());
float translate_y = texelFetch(s_disp, xyLookup, 0).r * u_disp_scale;
vec3 new_p = vec3(P.x, P.y + translate_y, P.z); 

This new result is used in our TDDeform() function. Try feeding the null TOP null_disp another 100 x 100 pixel image to see this at play.

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Tested in TouchDesigner099 2020.23680

Updated 06.03.20
Matthew Ragan
Zoe Sandoval