Vertex shader

By 21/10/2018 February 16th, 2019 Shaders

A Vertex Shader lets you create different effects by moving your objects’ vertices in the scene, such as: waves, distortion around force fields (such as black holes), melting, flags movement, grass movement if the player steps on it and way more!

I’ll cover those effects (and others) one by one in future tutorials (stay tuned!); now it’s time to cover the basis.


Shaders work all in a similar way and are made mostly by two sections, a “Vertex Shader” and a “Pixel Shader“.
Sections are mostly called Shaders too, which could be misleading and confusing…but that’s it.

The Vertex Shader/Program/Section comes first and calculates where your objects will be drawn. It places your objects in the world, calculating and operating each vertex individually.

The Pixel Shader operates on each pixel individually, for this reason it’s mostly used to apply textures and colors.
We’ve already seen a Pixel Shader in my previous posts, which is the “Surface Shader” (or a “Fragment Shader“).

 

Quick Post Map:


Vertex Shader

To tell our compiler that we have a Vertex Shader you must write #pragma vertex functionName between the CPROGRAM and ENDCG keywords. This way it will compile the method/function and know that we need a Vertex Shader.

After this, you have to declare a method, which is different depending if you want to use a Fragment Shader or a Surface Shader. In both cases, the Vertex Shader gets its information by the Mesh, then initialize each vertex and passes it to the Pixel Shader.

Documentation about: HLSL Snippets (#pragma).

Vertex and Fragment Shaders

Documentation about: Vertex and Fragment Shaders examples, Shader Semantics.

Vertex and Surface Shaders

Documentation about: Surface Shader examples (there are examples with vertex shaders in the middle of the article).

I’ll write the next part of the tutorial using a Surface Shader as Pixel Shader, for this reason my Vertex Shader will be of type “void” and so on, as you saw previously. Be aware that the process is the same in a Fragment Shader, you just need to change the declaration of the functions.

Basic Surface Shader

This is a basic Surface Shader, where I will modify the vert function.

You can apply this material to a simple sphere (and also placing a directional light), having this as result:


Move Vertices Horizontally

Now that we know where we can move our vertices, it’s time to start experimenting.

We know our object position thanks to the parameter inout appdata_full v in the “vert” method.

In fact, according to Unity’s Documentation (link here), the appdata_full provides position, tangent, normal, color and four texture coordinates.

You can move your objects modifying the vertex variable. In this case we want to move our object horizontally, so we’ll only change the x component.


Important notes

Colliders and other components

As you can see, this Vertex Shader (and in generall, all of them) only changes where the vertices of your objects are placed/drawn. Their collider, shadow and so on won’t change, so please be aware of this when making your game.

Vertices density

You also need to care about vertices density. More vertices you have and more times the Vertex Shader will run. Look at the next gif for an example:

P.S. More vertices means also less optimization… so, care!

That’s the line of code that moves the cubes: v.vertex.x += sin(_Time.x * 70 + v.vertex.y*2) * .7;

Vertices input value

Vertices are passed into the shader and they’re transformed from object space into ?clip space?, which is what determines where the vertices are on screen (they’re not screen coordinates by the way).

If you need the position of your vertices in worldSpace, you need to apply different transformations. One example is the following: float3 worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;.

Read more here: Built-in shader variables, Built-in shader helper functions, Surface Shaders (go to: Input structure).


Passing information to the Pixel Shader

You can apply any math operation to your vertices and you can pass this information to your Pixel Shader. In my case I’m multiplying the vertices based on their Y component, creating a simple “waving” effect.
I’m telling then the Surface Shader how much I’ve multiplied each vertex, so it can fade color to white or blue accordingly.

To pass info between shaders (in this case between Vertex-Surface) you need to modify their input and output variables. The struct Input is what we’re looking for.


Extrude Mesh

What if you want to “expand/extrude” your object? You can’t add a fixed amount (for example v.vertex.x += 2 ), because this way you’ll translate all of the vertices to the right.

You actually need to add this value based on the direction of the vertex/face, which is called “normal”. appdata_full already contains a normal variable, so we can use this.

Again, here’s Unity’s Documentation about: Providing vertex data to vertex programs.

Here’s another experiment I did, lerping the base model color with blue (and yellow comes along) depending on how much I’ve extruded the mesh.


Displacement Texture

With Shaders it’s super useful to use Textures to get information.

To sample a Texture in a vertex program we must use the method tex2Dlod (documentation here). What we want to achieve is: expand our object based on the white value taken from our Displacement Texture.

That’s the result:

That’s the texture that I’ll use (I’ve already shared it in the dissolve tutorial and it’s a simple cloud texture):

febucci dissolve shader graph hlsl unity cloud texture

In the Vertex Shader we can then calculate our “expand value” and then pass it to the Surface Shader, having this as result:


Shader Graph

If you want to create a Vertex Shader using Shader Graph, it’s super simple!

The concepts here are the same, you only need to use the “Position Node” and connect it to the Master Node. Then you can calculate your position in both Clip (Object), World, Normal and View Space! Unity will do everything for you under the hood, as written previously.

That’s the first shader (object moved horizontally) written in this tutorial, converted in Shader Graph:

Documentation: Position Node.