CODEWITHSUNDEEP
opengl
Alpha Centauri A B
Alpha Centauri A B

Reputation: 143

Lines clip when overlapping meshs

I am rendering lines at the same position as a mesh and when the lines are right on top of the mesh I get an effect where the lines start to clip and parts of them disappear, especially when moving around. What I eventually have to do is something like the screenshot below, move the lines off the mesh a little bit in order to get rid of that effect. Does anyone have any ideas on how I can render the lines so that they are right on the mesh without parts of it disappearing?

enter image description here

Upvotes: 0

Views: 281

Answers (1)

Andon M. Coleman
Andon M. Coleman

Reputation: 43329

First of all, are these lines really lines? Or are they the mesh drawn a second time with glPolygonMode (GL_FRONT_AND_BACK, GL_LINES)? In either case, the problem is that the filled polygons and your lines are generating (roughly) the same depths and passing/failing the depth test is not working like you want.

The solution is to add a very slight depth offset to one of the primitives, and the easiest way to do this is to use glPolygonOffset (...). But it is worth mentioning that this only works for filled primitives (e.g. GL_POLYGON, GL_QUADS, GL_TRIANGLES) and not GL_LINES or GL_POINTS.

If it is the same mesh drawn twice, on the second pass you can do something like this:

glPolygonOffset (-0.1f, -1.0f);
glEnable        (GL_POLYGON_OFFSET_LINE);

   ... Draw Mesh Second Time

glDisable       (GL_POLYGON_OFFSET_LINE);

Otherwise, you will need to apply the depth offset when you draw the (solid) mesh the first time:

glPolygonOffset (0.1f, 1.0f);
glEnable        (GL_POLYGON_OFFSET_FILL);

  ... Draw Mesh

glDisable       (GL_POLYGON_OFFSET_FILL);

... Draw Lines

You may need to tweak the values used for glPolygonOffset (...), it depends on a number of factors including depth buffer precision and the implementation. Generally the second parameter is going to be more important, it is a constant offset applied to the depth. The first factor varies according to the change in depth (e.g. angle).

Upvotes: 2

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