This demo shows how we can generate a full screen rendering area by using a simple triangle.
The triangle will be much larger than the viewport area but only a part of it will be visible as the rest will be clipped. This is a very efficient way of rendering what used to be called a "Screen Quad"
There is a good article here showing this and this demo is a good starting point for any project that requires rendering some form of buffer / texture to the screen.
- We first load the shaders which will be called when drawing our triangle.
- Next we need a VertexArrayObject to enable drawing. This will not contain any buffers or data but still needs to be bound.
- call glDrawArrays(GL_TRIANGLES,0,3); This calls the vertex shader 3 times, where gl_VertexID is used to determine which of the triangle vertices to generate and also the texture co-ordinates
- These are passed onto the fragment shader.
In this demo we will fill a buffer with values then bind to a texture to render to the screen.
- Space : reset the canvas to white
- a : toggle animation
- p : draw points mode
- l : draw lines mode
- Rendering a Screen-Covering Triangle in OpenGL (Rauwendaal) — why one clipped triangle beats a two-triangle quad.
- OpenGL Wiki — Built-in Variable (GLSL): gl_VertexID — the attributeless-rendering trick of generating positions from the vertex index.