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    <p>The best way to do this is probably to split up the border into four trapezoidal quads, but since you explicitly mentioned you didn't want to go down that path, there are a couple of other options you could pursue.</p> <p>The next best method is probably to use the depth buffer: <a href="http://www.opengl.org/sdk/docs/man/xhtml/glEnable.xml" rel="nofollow"><code>glEnable(GL_DEPTH_TEST)</code></a>. Draw the inside quad first (with blending enabled). Then draw the outer quad, but using coordinates such that it's depth is greater than the first quad (it is "farther away"). If depth testing is enabled when the border quad is drawn, it will fail for the quad you already drew, since those pixels have a smaller depth value. Note that using this method, if you are writing to the depth buffer when rendering the things behind these quads, you may run into some issues with parts of the background being considered closer than the new quads. You can solve this for the inner quad by configuring the depth test to always succeed, even when the test fails: <a href="http://www.opengl.org/sdk/docs/man/xhtml/glDepthFunc.xml" rel="nofollow"><code>glDepthFunc(GL_ALWAYS)</code></a>. This won't help for the outer quad, however. In that case the next method might be helpful:</p> <p>You can use the stencil buffer in a similar manner to the depth buffer. When rendering the inside quad, you set the stencil value (or one bit of it at least) to a specific value:</p> <pre><code>glEnable(GL_STENCIL_TEST); glStencilFunc(GL_ALWAYS, 1, GLuint(-1); glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); </code></pre> <p>Then when you draw the outer quad, configure the stencil test to succeed only if a fragment does not have that value:</p> <pre><code>glStencilFunc(GL_NOT_EQUAL, 1, 1); </code></pre> <p>Essentially, this is the same thing the version above was doing with the depth buffer, but uses the stencil buffer instead. You may want to take a look at the documentation for <a href="https://www.opengl.org/sdk/docs/man/xhtml/glStencilFunc.xml" rel="nofollow">glStencilFunc</a> and <a href="http://www.opengl.org/sdk/docs/man4/xhtml/glStencilOp.xml" rel="nofollow">glStencilOp</a>.</p> <p>There may be other ways to achieve this behavior, but all the other solutions I can think of involve using custom shaders, which you indicated is not acceptable. Personally, I'd just split up the border into 4 quads. Sure, it's 3 more quads than the other methods, but it doesn't require any extra OpenGL features or state changes.</p>
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    1. COI'm not sure I understand, so the depth buffer will clip away the outer quads inner section for free? Is this what would happen using this method?
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    2. CO@paulm Yes, it is essentially free, unless you weren't using the depth buffer at all before, in which case creating a context which has a depth buffer will use approximately twice as much memory for the framebuffer, since each pixel requires (usually) 3 bytes for color data (4 with alpha) and (usually) 3 bytes for a 24-bit depth buffer (most common), although 16-bit depth buffers are also common.
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    3. COI figured this would still give the same bad results as now where the 2 quads are blended as this would be the same effect as glass?
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