Geometry in DirectX Rendering

Geometry is the fundamental building block of any 3D scene rendered by DirectX. It defines the shapes and structures that are visible to the user. Understanding how geometry is represented, processed, and rendered is crucial for creating efficient and visually appealing graphics.

Core Concepts

In DirectX, geometry is primarily defined by a collection of vertices. These vertices, in turn, form primitives such as points, lines, and triangles. Triangles are the most common primitive used in modern graphics due to their simplicity and ability to approximate any complex surface.

Vertices

Each vertex is a data structure that typically contains:

Position: Its coordinates in 3D space (x, y, z, and sometimes w for homogeneous coordinates).
Normal: A vector indicating the surface's orientation at that point, used for lighting calculations.
Texture Coordinates: Values (u, v) used to map textures onto the surface.
Color: An optional color value for the vertex.
Other Attributes: Tangents, bitangents, bone weights for animation, etc.

These attributes are defined in structures and passed to the graphics pipeline. The specific vertex layout is crucial and must match between the application code and the vertex shader.

Primitives

Vertices are grouped to form geometric primitives:

Points: A single vertex.
Lines: Two vertices forming a line segment.
Triangles: Three vertices forming a triangular surface.
Line Strips: A sequence of connected line segments.
Triangle Strips: A sequence of connected triangles that share vertices, offering efficiency.

The graphics hardware renders these primitives using a rasterization process, converting them into pixels on the screen.

Note: DirectX supports various primitive types, but triangles and triangle strips are by far the most dominant for complex 3D models.

Geometry Processing Pipeline

Once defined, geometry goes through a series of stages within the graphics pipeline:

Input Assembler (IA): Reads vertex and index data from memory and organizes it into primitives.
Vertex Shader (VS): Processes each vertex individually, transforming its position from model space to screen space and preparing other vertex attributes for downstream stages.
Geometry Shader (GS) (Optional): Can create or destroy primitives, useful for effects like particle generation or generating extra geometry.
Rasterizer (RS): Determines which pixels on the screen are covered by the primitives.
Pixel Shader (PS): Processes each pixel (or fragment) that results from rasterization, determining its final color.

Simplified Geometry Flow

Data Representation and Buffers

Geometry data is typically stored in memory and uploaded to the graphics card as buffers:

Vertex Buffer: Contains the vertex data (position, UVs, normals, etc.).
Index Buffer: Contains a sequence of indices that reference vertices in the vertex buffer. Using an index buffer allows for reusing vertices and reducing memory bandwidth, especially for models with shared vertices.


struct Vertex
{
    XMFLOAT3 position;
    XMFLOAT2 uv;
    XMFLOAT3 normal;
};

// ... in rendering code ...

// Define vertex data
Vertex vertices[] =
{
    { XMFLOAT3(-1.0f, -1.0f, 0.0f), XMFLOAT2(0.0f, 0.0f), XMFLOAT3(0.0f, 0.0f, -1.0f)},
    { XMFLOAT3( 1.0f, -1.0f, 0.0f), XMFLOAT2(1.0f, 0.0f), XMFLOAT3(0.0f, 0.0f, -1.0f)},
    { XMFLOAT3( 0.0f,  1.0f, 0.0f), XMFLOAT2(0.5f, 1.0f), XMFLOAT3(0.0f, 0.0f, -1.0f)}
};

// Define index data for a single triangle
unsigned short indices[] =
{
    0, 1, 2
};

// Create and bind vertex and index buffers to the GPU...

Transformations

Geometry is manipulated using transformation matrices:

Model Transform: Positions, rotates, and scales an object in its local space.
View Transform: Positions and orients the camera in the world.
Projection Transform: Defines the camera's perspective (e.g., perspective or orthographic projection) and maps the 3D scene onto a 2D viewport.

These transformations are typically applied in the vertex shader to convert vertex positions from model space to clip space.