Grass Geometry shader:

Grass Geometry shader:  Unity Version 2022.3

HLSL:
I’m a non-graphics programmer, learning graphics programming and I am attempting to prototype a scene with grass.  My first attempt to get something usable was a
a ShaderGraph Shader wrapped over some simple geometry Plaines then instanced in the scene 100’s of times.  It looked nice but the fps was horrible, something like 10fps - just for grass.

 

Geometry shader:
The internet told me I would need to use a geometry shader, so I used all my knowledge (Google), found some good examples, and started writing my own. Here is how it looked right at the start just getting a triangle to appear on each vertice of a mesh:

Pseudorandom Number Generation (PRNG):
The next thing is to increase the blade count per triangle.  For this I need a Pseudorandom Number Generation (PRNG), I’m using the following line of code for it: 

"float4 rando = float4(frac(sin(i * 43758.5453 * normal.x)), frac(sin(i * 143316.6325 * normal.y)), 0.0, 0.0);"

Note:  “i” is the count for each blade iteration on each triangle:

Just to explain for anyone new like me.  Frac, is the number after the decimal, and sin (Sin cos tan..) is great because it produces a number between 1 and -1, so this can be used to refactor large numbers easily to create a random number when normalised.  The diagram below shows how I get a point inside a triangle (A to B to C).

The function rando is used to get a random number between 0 & 1 based off the first vertices normal as a seed:

• Blue = (1 / (A to B) ) * rando.
• Red = ( (A to C) - Blue) * rando.

Then you can create a point by adding the vectors as shown above to get a new P1.

Known Bugs: 

• My triangulation code is flawed because it assumes a right triangle, unintentionally, but it’s good enough for now and works at around 80% accuracy.

• The randomisation is neutralised if the mesh plane is completely flat with uniform triangles because the UVs are all the same, so the number I use to start the randomisation is either 1 or 0:   Therefore, the multiplier is either producing 0 = nothing, or 1 the same number as previous.  I temporarily solved this by making sure the mesh’s vertices in Max are not equal to  float4(0.0, 0.0, 0,0, 0,0).

• The starting blade foreach Triangle is stuck at the first vertice, I think this is a starting seed issue, easily fixed, but I’m going to leave it for now as I can add more blades and the randomness is almost ok.

Finaly:
Around 90fps give or take:

Colour options:

 

If you are trying to do this yourself here are things you will need to know;  How to get a random point in a triangle using multiplication. This is for the new point of each blade.  And how to create random numbers, using as little GPU power as possible.

 

Thanks.