Subject

I am creating a terrain map with triangle strips and I would like to make the bodies of the triangles black, but have their outlines be colored.

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Problem

The solution appears to be to draw the triangles twice, once in solid black, using GL_FILL and once in color using GL_LINES.

The problem with this approach is that, since I am using depth testing and the vertices are at the same position each time, my color lines end up blending with the black lines

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Attempted Solution

According to the OpenGL docs, and other sources, it looks like I should be able to utilize the glPolygonOffset() function to give polygons that are drawn with GL_LINES "precedence" in the depth buffer by offsetting their comparison value by the argument to the function. (so I would think 1.0 would be sufficient)

This, however, does not appear to be working for me.

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My Code

I have placed glEnable(GL_POLYGON_OFFSET_LINE); in my init function.

Here is the code in my draw loop where I do the actual rendering. col is the array that hods the real colors, col2 is filled with black.

Please excuse the mess.

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glPolygonOffset(1.0,0.0); glDeleteBuffers(1,&vertex_buffer); vertex_buffer = createBuffer(GL_ARRAY_BUFFER, vert, sizeof(GLfloat)*(buffer_size), GL_STATIC_DRAW); attributeBind(vertex_buffer, 0, 3); glDeleteBuffers(1,&color_buffer); color_buffer = createBuffer(GL_ARRAY_BUFFER, col2, sizeof(GLfloat)*(buffer_size), GL_STATIC_DRAW); attributeBind(color_buffer, 1, 3); glPolygonMode(GL_FRONT_AND_BACK,GL_FILL); glDrawArrays(GL_TRIANGLE_STRIP,0,s); glDeleteBuffers(1,&color_buffer); color_buffer = createBuffer(GL_ARRAY_BUFFER, col, sizeof(GLfloat)*(buffer_size), GL_STATIC_DRAW); attributeBind(color_buffer, 1, 3); glPolygonMode(GL_FRONT_AND_BACK,GL_LINE); glDrawArrays(GL_TRIANGLE_STRIP,0,s); 

I'm not sure if I need to call glPolygonOffset() each frame or not.

These are the attributeBind() and createbuffer() functions:

void attributeBind(GLuint buffer, int index, int points) { glBindBuffer(GL_ARRAY_BUFFER, buffer); glVertexAttribPointer( index, // position or color points, // how many dimensions? GL_FLOAT, // type GL_FALSE, // normalized? 0, // stride (void*)0 // array buffer offset ); } GLuint createBuffer(GLenum target, const void *buffer_data, GLsizei buffer_size, GLenum usageHint) { GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer(target, buffer); glBufferData(target, buffer_size, buffer_data, usageHint); return buffer; }