Simple Flat Plane Tessellation Shader

Question

Part 1:

So I want to create a basic tessellation program that takes a plane of quads and transforms it into a more, well, detailed/tessellated plane of quads. Such as the picture below. How much it gets tessellated would depend on user controls, passed in by a uniform (initially). However I am so new to tessellation shaders that I can't even figure out how to do this.

How is this typically done? Surely you shouldn't actually draw the plane of quads prior to the shader program, since from my understanding quads won't get tessellated this way, instead the get tessellated into a way like the below picture:

I believe the answer could to be to draw a plane of points, and these points are then tessellated into more points, and these points are transformed into quads of the appropriate size in the geometry shader I think? Alternatively, instead of converting points into quads could I just draw quads between each four closest points (that would be much better)? Examples very much appreciated!

NOTE: Using GLSL > 4.0 & C only (No C++/Python)

Part 2:

After I get part 1 working, how would I make it so that certain quads are more tessellated than others, such as this?:

I want the parts closer to the camera to be more tessellated.

Part 3:

If I were able to get that far, the next part would be to alter the z-axis of points to make the plane into an interesting environment. This would be done by reading in a 2Dsampler, I know how to do that and all. However, if I am correct in Part 1 about using a plane of points then I need to do more than just alter the points that are converted into quads, because quads need to be sharing vertices essentially in order there to be no gaps between quads. How would that be done? Alternatively if we draw quads between points, with each point being the appriate height, then this wouldn't be a issue.

Answer 1

Part 1

Yes you're correct: generate a 'patch' as a simple grid of points, specify the tesselation levels as uniforms into the TCS (tesselation control shader) and generate the vertex data in the TES (tesselation evaluation shader).

Sounds complicated? Here's a nice tutorial I based my work on: http://antongerdelan.net/opengl/tessellation.html

Part 2

What you are talking about here is LOD (or level of detail). You would need to tesselate and render the higher polygon-count bottom-left corner of your mesh as a separate object.

Your suggested approach is correct: break the overall scene into 'chunks' and determine the LOD (ie the tesselation parameters) for each chunk separately, usually by some distance-to-camera algorithm.

Part 3

Another excellent tutorial which does exactly what you are after I believe: http://codeflow.org/entries/2010/nov/07/opengl-4-tessellation/

I used this approach to get a very highly detailed but memory and frame efficient terrain.

Hope this helps.