HLSL vertex shader

Question

I've been studying shaders in HLSL for an XNA project (so no DX10-DX11) but almost all resouces I found were tutorial of effects where the most part of the work was done in the pixel shader. For istance in lights the vertex shader is used only to serve to the pixel one normals and other things like that. I'd like to make some effect based on the vertex shader rather than the pixel one, like deformation for istance. Could someone suggest me a book or a website? Even the bare effect name would be useful since than I could google it.

I don't think you'll have much luck finding material about this kind of effect, many deformation effect are done in the PS stage, usually the vertex shader is only used to project the model into the 2D plane. — Matan Shahar
@ISun Morphing/blending boned models, etc. is done entirely in the vertex shader in DX9 / XNA. The pixel shader will only let you fill polygons that come out of the VS, so deformation isn't really possible there (beyond bump maps, etc. that can be achieved via lighting and other pixel-level techniques.) — 3Dave

3Dave 3Dave · Accepted Answer · 2014-01-30T17:39:14

A lot of lighting, etc. is done in the pixel shader because the resulting image quality will be much better.

Imagine a sphere that is created by subdividing a cube or icosahedron. If lighting calculations are done in the vertex shader, the resulting values will be interpolated between face edges, which can lead to a flat or faceted appearance.

Things like blending and morphing are done in the vertex shader because that's where you can manipulate the vertices.

For example:

matrix World;
matrix View;
matrix Projection;
float WindStrength;
float3 WindDirection;


VertexPositionColor VS(VertexPositionColor input)
{
    VertexPositionColor output;

    matrix wvp = mul(mul(World,View),Projection);
    float3 worldPosition = mul(World,input.Position);

    worldPosition += WindDirection * WindStrength * worldPosition.y;

    output.Position = mul(mul(View,Projection),worldPositioninput); 
    output.Color = input.Color;

    return output;
}

(Pseudo-ish code since I'm writing this in the SO post editor.)

In this case, I'm offsetting vertices that are "high" on the Y axis with a wind direction and strength. If I use this when rendering grass, for instance, the tops of the blades will lean in the direction of the wind, while the vertices that are closer to the ground (ideally with a Y of zero) will not move at all. The math here should be tweaked a bit to take into account really tall things that would cause unacceptable large changes, and the wind should not be uniformly applied to all blades, but it should be clear that here the vertex shader is modifying the mesh in a non-uniform way to get an interesting effect.

No matter the effect you are trying to achieve - morphing, billboards (so the item you're drawing always faces the camera), etc., you're going to wind up passing some parameters into the VS that are then selectively applied to vertices as they pass through the pipeline.

A fairly trivial example would be "inflating" a model into a sphere, based on some parameter.

Pseudocode again,

matrix World;
matrix View;
matrix Projection;
float LerpFactor;

VertexShader(VertexPositionColor input)

float3 normal = normalize(input.Position);
float3 position = lerp(input.Position,normal,LerpFactor);

matrix wvp = mul(mul(World,View),Projection);

float3 outputVector = mul(wvp,position);
....

By stepping the uniform LerpFactor from 0 to 1 across a number of frames, your mesh (ideally a convex polyhedron) will gradually morph from its original shape to a sphere. Of course, you could include more explicit morph targets in your vertex declaration and morph between two model shapes, collapse it to a less complex version of a model, open the lid on a box (or completely unfold it), etc. The possibilites are endless.

For more information, this page has some sample code on generating and using morph targets on the GPU.

If you need some good search terms, look for "xna bones," "blendweight" and "morph targets."

HLSL vertex shader

1 Answers