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Programming

DirectX 10 Game Programming : Shaders and Effects - Vertex Shaders

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7/9/2013 3:31:54 AM

Vertex shaders are the part of the pipeline where you are given control of every vertex that gets processed by the system. In previous versions of Direct3D, you had the option of using the fixed function pipeline, which had a built-in set of functionality that it used when processing vertices. Now with the latest Direct3D, you must do all the processing yourself. To that end, you’ll need to write at least a simple vertex shader.

Vertex shaders are one of three shaders that can exist within an effect file. As objects are sent to be drawn, their vertices are sent to your vertex shader. If you don’t want to do any additional processing to the vertices, you can pass them along to the pixel shader to be drawn. In most cases though, you’ll at least want to apply a world or projection transform so the vertices are placed in the proper space to be rendered.

In the following example vertex shader, the incoming vertex position is multiplied by the projection matrix before being sent to the pixel shader. You’ll also notice that the vertex color is left alone and just passed along.

////////////////////////////////////////////////
// Vertex Shader - Main Function
///////////////////////////////////////////////
PS_INPUT VS(float4 Pos : POSITION, float4 Color : COLOR)
{
    PS_INPUT psInput;

    // Pass through both the position and the color
    psInput.Pos = mul( Pos, Projection );
    psInput.Color = Color;

    return psInput;
}

Using vertex shaders, you have a lot of power to manipulate the vertices past just doing a simple transform. The vertex can be translated along any of the axes, its color changed, or any of its other properties manipulated. In the next section you’ll be shown how the vertices can be made to animate within a vertex shader.

Grid Animation in the Shader

Just because you want to move around the vertices within an object doesn’t mean that you have to do it in your application code. If the movement can be defined into either a set pattern or the movement value can be passed in, all the vertex animation can be done in the shader.

In the following example, the calling program sends in a time value using the TimeStep variable in addition to the vertices. The time value gives the vertex shader knowledge of time passing so that the vertices can be changed each frame.

As each vertex from this object is sent through the vertex shader, a new height position is calculated using the TimeStep variable and the built-in sine function. This results in a wave motion being applied to the grid.

// constant buffer of external variables
cbuffer Variables
{
    matrix Projection;
    matrix World;
    float TimeStep;
};

////////////////////////////////////////////////
// Vertex Shader - Main Function
///////////////////////////////////////////////
PS_INPUT VS(float4 Pos : POSITION, float4 Color : COLOR)
{
    PS_INPUT psInput;

    float4 newPosition;

    newPosition = Pos;

    // generate a new height value based on the time
    newPosition.y = sin((newPosition.x * TimeStep) + (newPosition.z / 3.0f)) *
5.0f;

    // Pass through both the position and the color
    psInput.Pos = mul(newPosition, Projection);

    psInput.Color = Color;

    return psInput;
}

					  

Timing the Animation

As I mentioned before, the animation in the vertex shader is controlled by an updating time value called TimeStep. TimeStep is created and accessed just like the other variables you’ve sent to the shader. First an ID3D10EffectScalarVariable is declared to use as a binding between the application and shader code.

ID3D10EffectScalarVariable* pTimeVariable = NULL;

Next, the pTimeVariable is bound to the TimeStep shader variable.

pTimeVariable = modelObject->pEffect->GetVariableByName("TimeStep")->AsScalar();

					  

Finally, the pTimeVariable is used to update the time value every frame.

// The time is set in the shader using the SetFloat function
pTimeVariable->SetFloat((float)currentTime);

The currentTime value is just a float value that contains the amount of time elapsed between frames.

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