Limit the draw area of a polygon in OpenGL based off of UV?

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  OddlyOrdinary polycounter lvl 9

  Feb 2013

  This was probably too specific of an idea but in case someone finds themselves in this situation or finds this thread from a search I thought I'd post how I fixed the issue.
  
  What I ended up doing was generating an additional UV set at runtime, I'd look at the meshes and generate a list of polygons that were over the boundaries. So if a polygon had some vertices outside the boundary and some inside I would add it to a list.
  
  Then I would find the closest two UV values for that polygon and build a scalar to warp the other points by.
  Say we originally have a triangle with three UV values
  [-.5, -.2, .3] I'd chose the -2, and .3
  
  In the second UV set the .3 would be 1.0 and the -.2 would be -.66. The -.5 would be -1.66. The scalar being .33=(1/(.3 * 10)).
  
  Then in the shader I clamped the textcoord.x values of the second uv set to 0 or 1, this generates an immediate cutoff at the boundary, without being a harsh curve and also takes into account the different sizes of each polygon.

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  CrazyButcher polycounter lvl 18

  Feb 2013

  You can also clamp textures to live only within 0-1 by changing the texture's sampler state:
  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER)
  and you specify the border color, as "black")
  
  edit: actually as you are using shaders, why not just "not" sample the texture when uvs are outside of 0,1, though probably didn't fully understand what you were after

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