CODEWITHSUNDEEP
glslwebglfragment-shadercoordinate-transformation
Nicolas
Nicolas

Reputation: 2281

world position to screen position in fragment shader glsl

I have a WebGL renderer and I want to transform random world coordinates to screen coordinates in the fragment shaders.

That will allow me to compare the "computed screen coordinates" to the current fragment screen coordinates.

I have all the information needed to do so I believe, just not sure which is the right way to do so.

  ...
  // available information:

  // uCanvasWidth (float)
  // uCanvasHeight (float)
  // modelMatrix (mat4)
  // modelViewMatrix (mat4)
  // projectionMatrix (mat4)
  vec4 someWorldCoordinates = vec4(1., 2., 3., 1.);
  // map it to screen coordinates?
  vec4 screenCoordinates = ???

  // compare to current fragment location
  if(screenCoordinates.x > gl_FragCoord.x){
    // do stuff
  }
  ...

Upvotes: 2

Views: 3817

Answers (1)

LJᛃ
LJᛃ

Reputation: 8123

vec4 worldSpace = vec4(1., 2., 3., 1.);
// get homogeneous clip space coordinates
vec4 clipSpace = projectionMatrix * ( modelViewMatrix * worldCoords );
// apply perspective divide to get normalized device coordinates
vec3 ndc = clipSpace.xyz / clipSpace.w;
// do viewport transform
vec2 screenSpace = (ndc.xy * .5 + .5) * vec2(uCanvasWidth, uCanvasHeight);
screenSpace.y = uCanvasHeight - screenSpace.y;

Note that your modelView matrix likely contains the transforms of the model you're rendering. I would assume that the points specified in in shader are thought to be already "final" world space(as in not relative to the model you're rendering), so you might want to use just the view matrix here(preferrably a premultiplied viewProjection).

Upvotes: 2

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