Creating shaders based on various object transforms

Before we start discussing the topic, here is a small background about shaders and how data is sent from C++ to GPU.

Shader: Shaders are special programs that reside on the GPU to create custom effects which are not possible/hard to recreate on the CPU side. There are two main types of shaders. They are the vertex and the fragment/pixel shader. As their name suggests, the vertex shader acts on each vertex of a triangle and the fragment shader acts on each fragment that is encompassed by that triangle.

The main output from vertex shader is the position of a vertex with respect to the window coordinates. We can also output any other data we like from vertex shader with the color being the most common. All the outputs from vertex shaders are then passed onto fragment shaders where they are interpolated between the vertices.

Fragment shaders run for each fragment inside the triangle and output a color which is then displayed at the location that is output by the vertex shader. There are many other types of shaders such as geometry shaders, tesselation shaders, and mesh shaders, but we will only use vertex and fragment shaders for now.

When we send data from C++ the primary information that we submit is the vertex data. Apart from this, we can also send data using constant buffers. Constant buffers contain data which is constant over a frame or draw call. In our engine, we have two constant buffers, one is the constant frame buffer, and other is the constant draw call buffer which looks like the following.

struct sPerFrame
{
Math::cMatrix_transformation g_transform_worldToCamera;
Math::cMatrix_transformation g_transform_cameraToProjected;

Math::sVector g_CameraPositionInWorld;
float padding0;

float g_elapsedSecondCount_systemTime = 0.0f;
float g_elapsedSecondCount_simulationTime = 0.0f;
float padding1[2]; // For float4 alignment
};

struct sPerDrawCall
{
Math::cMatrix_transformation g_transform_localToWorld;
Math::cMatrix_transformation g_transform_localToProjected;
};

To use the above constant buffers, we need to declare them in shaders as shown below. Shaders for Direct3D are written in HLSL while those for OpenGL are written in GLSL. Since we will be working only with Direct3D, we will use HLSL.

cbuffer g_constantBuffer_perFrame : register( b0 )
{
float4x4 g_transform_worldToCamera;
float4x4 g_transform_cameraToProjected;

float3 g_CameraPositionInWorld;
float g_padding0;

float g_elapsedSecondCount_systemTime;
float g_elapsedSecondCount_simulationTime;
// For float4 alignment
float2 g_padding1;
};

cbuffer g_constantBuffer_perDrawCall : register( b2 )
{
float4x4 g_transform_localToWorld;
float4x4 g_transform_localToProjected;
};

We use the above matrices in our shaders to create effects based on the position of the object relative to the world or camera. Since we also output more data than just position from the vertex shader, I created a struct which contains all the data that I pass from vertex shader and which can be easily accessed in the fragment shader. The struct can be modified as necessary to include additional data.

struct VS_OUTPUT
{
float4 o_vertexPosition_projected : SV_POSITION;
float4 o_vertexPosition_local : TEXCOORD1;
float4 o_vertexColor_projected : COLOR;
float4 o_vertexColor_local : TEXCOORD2;
};
  1. Creating a shader that is independent of its position in the world.

To create a shader that has effects which are independent of world position, we need to use the local position of the fragment. This is what I am doing in my fragment shader

o_color = float4(floor(sin(i_VSInput.o_vertexPosition_local.x) / cos(i_VSInput.o_vertexPosition_local.x)),floor(sin(i_VSInput.o_vertexPosition_local.y) / cos(i_VSInput.o_vertexPosition_local.y)),floor(sin(i_VSInput.o_vertexPosition_local.z) / cos(i_VSInput.o_vertexPosition_local.z)), 1.0)* i_VSInput.o_vertexPosition_local;

This produces the following output:

2. Creating effect through which object can move through: The fragment shader output code is similar to the above except instead of using the local vertex position, we use the world position of the vertex.

3. Creating a grow and shrink effect: Until now, we were only modifying the fragment shader, but to create a grow and shrink effect, we need to change the positions of vertices. We do it by creating a scaling matrix. We then modify the scaling matrix value based on the time and multiply it to local position. The rest of the transformations remain the same.

float s = (sin(g_elapsedSecondCount_simulationTime) + 0.5) + 1;
float4x4 scale= {
s,0,0,0,
0,s,0,0,
0,0,s,0,
0,0,0,1,
};
// Transform the local vertex into world space
float4 vertexPosition_world;
{
float4 vertexPosition_local = float4( i_vertexPosition_local, 1.0 );
vertexPosition_local = mul(scale, vertexPosition_local);
vertexPosition_world = mul(g_transform_localToWorld, vertexPosition_local);}

Even though this method works, it might not be the most optimized. So, instead of matrix multiplication, we can use scalar multiplication to get the same results.

float s = (sin(g_elapsedSecondCount_simulationTime) + 0.5) + 1;
// Transform the local vertex into world space
float4 vertexPosition_world;
{
// This will be done in a future assignment.
// For now, however, local space is treated as if it is world space.
float3 scaledLocalPosition = i_vertexPosition_local * s;
float4 vertexPosition_local = float4(scaledLocalPosition, 1.0 );
vertexPosition_world = mul(g_transform_localToWorld, vertexPosition_local);
}

4. Changing the effect on an object based on its proximity to the camera: To find the distance between the object and camera, we find the length between the camera position and world position of the vertex. I am then lerping between the current vertex color to red based on a clamped value of the distance to the far plane of the camera.

const float4 color = {1,0,0,1};
const float distance = length((g_CameraPositionInWorld - (vertexPosition_world).xyz));
output.o_vertexColor_projected = lerp(color, i_vertexColor_local, saturate(distance/100));

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