Forge Shader Grid

Shader Grid

When to use

• You need a ground grid or floor plane in a 3D scene
• You want procedural patterns (no texture) with anti-aliasing
• You need a debug visualization plane for positioning objects
• You want to combine multiple rendering techniques in one render pass
• You need to render objects with different vertex formats in the same pass

Core technique: anti-aliased procedural grid

The grid is rendered on a flat quad. The fragment shader computes grid lines procedurally from the world-space position using screen-space derivatives.

HLSL fragment shader pattern

/* Step 1: Scale world position to grid space */
float2 grid_uv = world_pos.xz / grid_spacing;

/* Step 2: Distance to nearest grid line (0 = on line, 0.5 = between) */
float2 dist = abs(frac(grid_uv - 0.5) - 0.5);

/* Step 3: Screen-space rate of change (pixel footprint in grid space) */
float2 fw = fwidth(grid_uv);

/* Step 4: Anti-aliased line mask (smooth edge, not hard step) */
float2 aa_line = 1.0 - smoothstep(line_width, line_width + fw, dist);

/* Step 5: Combine X and Z lines */
float grid = max(aa_line.x, aa_line.y);

/* Step 6a: Frequency-based fade (prevent moiré at low angles) */
/* When fwidth >= 0.5, a pixel spans more than half a grid cell —
 * the grid pattern exceeds the Nyquist limit and cannot be resolved. */
float max_fw = max(fw.x, fw.y);
grid *= 1.0 - smoothstep(0.3, 0.5, max_fw);

/* Step 6b: Distance fade (secondary limit for extreme distances) */
float cam_dist = length(world_pos - eye_pos.xyz);
float fade = 1.0 - smoothstep(fade_distance * 0.5, fade_distance, cam_dist);
grid *= fade;

/* Mix line and background colors */
float3 surface = lerp(bg_color.rgb, line_color.rgb, grid);

Why this works

• fwidth() uses the same derivative hardware as mip selection (Lesson 05)
• The transition width is always one pixel, so lines look crisp at any distance
• Frequency-based fade detects when grid cells become sub-pixel (Nyquist limit) and fades the grid regardless of viewing angle — this prevents moiré at low grazing angles where the distance fade alone is insufficient
• Distance fade provides a secondary limit for extreme distances

Multiple pipelines pattern

Different objects in a scene often need different pipeline configurations. You can switch pipelines within a single render pass:

/* Begin one render pass with color + depth targets */
SDL_GPURenderPass *pass = SDL_BeginGPURenderPass(cmd, &color, 1, &depth);

/* Pipeline 1: grid (position-only vertices, no sampler, CULL_NONE) */
SDL_BindGPUGraphicsPipeline(pass, grid_pipeline);
SDL_PushGPUVertexUniformData(cmd, 0, &grid_vert_uniforms, sizeof(...));
SDL_PushGPUFragmentUniformData(cmd, 0, &grid_frag_uniforms, sizeof(...));
/* bind grid VB/IB, draw */

/* Pipeline 2: model (pos+normal+uv vertices, sampler, CULL_BACK) */
SDL_BindGPUGraphicsPipeline(pass, model_pipeline);
/* push model uniforms, bind model VB/IB, draw */

SDL_EndGPURenderPass(pass);

Both pipelines share the same render targets. The depth buffer handles occlusion between them automatically.

Grid pipeline setup

/* Vertex format: position only (float3, 12 bytes) */
SDL_GPUVertexAttribute grid_attr;
SDL_zero(grid_attr);
grid_attr.location = 0;
grid_attr.format   = SDL_GPU_VERTEXELEMENTFORMAT_FLOAT3;
grid_attr.offset   = 0;

/* Pipeline: no backface culling (visible from both sides) */
pipeline_info.rasterizer_state.cull_mode = SDL_GPU_CULLMODE_NONE;

/* Fragment shader: 0 samplers, 1 uniform buffer */
/* Vertex shader: 0 samplers, 1 uniform buffer (VP matrix) */

Grid geometry

A simple quad on the XZ plane:

#define GRID_HALF_SIZE 50.0f

float vertices[] = {
    -GRID_HALF_SIZE, 0.0f, -GRID_HALF_SIZE,
     GRID_HALF_SIZE, 0.0f, -GRID_HALF_SIZE,
     GRID_HALF_SIZE, 0.0f,  GRID_HALF_SIZE,
    -GRID_HALF_SIZE, 0.0f,  GRID_HALF_SIZE,
};
Uint16 indices[] = { 0, 1, 2, 0, 2, 3 };

Grid fragment uniforms (96 bytes)

typedef struct GridFragUniforms {
    float line_color[4];    /* grid line color (RGBA, linear space)    */
    float bg_color[4];      /* background color (RGBA, linear space)   */
    float light_dir[4];     /* light direction (xyz, w unused)         */
    float eye_pos[4];       /* camera position (xyz, w unused)         */
    float grid_spacing;     /* world units between lines (e.g. 1.0)   */
    float line_width;       /* line thickness in grid space (e.g. 0.02)*/
    float fade_distance;    /* distance for fade-out (e.g. 40.0)      */
    float ambient;          /* ambient intensity [0..1]                */
    float shininess;        /* specular exponent                       */
    float specular_str;     /* specular intensity [0..1]               */
    float _pad0, _pad1;     /* pad to 16-byte alignment                */
} GridFragUniforms;

Key API calls (in order)

1. SDL_CreateGPUGraphicsPipeline — create grid pipeline (position-only vertex format, CULL_NONE, 1 vertex uniform, 1 fragment uniform, 0 samplers)
2. SDL_CreateGPUBuffer + SDL_CreateGPUTransferBuffer — upload grid quad vertices and indices
3. SDL_BeginGPURenderPass — begin pass with color + depth targets
4. SDL_BindGPUGraphicsPipeline(pass, grid_pipeline) — bind grid pipeline
5. SDL_PushGPUVertexUniformData(cmd, 0, &vp_matrix, ...) — push view-projection matrix
6. SDL_PushGPUFragmentUniformData(cmd, 0, &grid_frag_uniforms, ...) — push grid parameters
7. SDL_BindGPUVertexBuffers / SDL_BindGPUIndexBuffer — bind grid geometry
8. SDL_DrawGPUIndexedPrimitives — draw grid quad
9. SDL_BindGPUGraphicsPipeline(pass, model_pipeline) — switch to model pipeline (if needed)
10. SDL_EndGPURenderPass

Ready-to-use template

Minimal vertex shader (grid.vert.hlsl)

cbuffer VertUniforms : register(b0, space1)
{
    float4x4 vp_matrix;
};

struct VSInput  { float3 pos : TEXCOORD0; };
struct VSOutput { float4 clip_pos : SV_Position; float3 world_pos : TEXCOORD0; };

VSOutput main(VSInput input)
{
    VSOutput output;
    output.world_pos = input.pos;
    output.clip_pos  = mul(vp_matrix, float4(input.pos, 1.0));
    return output;
}

Minimal fragment shader (grid.frag.hlsl)

cbuffer FragUniforms : register(b0, space3)
{
    float4 line_color;
    float4 bg_color;
    float4 eye_pos;
    float  grid_spacing;
    float  line_width;
    float  fade_distance;
    float  _pad0;
};

float4 main(float4 clip_pos : SV_Position, float3 world_pos : TEXCOORD0) : SV_Target
{
    float2 grid_uv = world_pos.xz / grid_spacing;
    float2 dist    = abs(frac(grid_uv - 0.5) - 0.5);
    float2 fw      = fwidth(grid_uv);
    float2 aa_line = 1.0 - smoothstep(line_width, line_width + fw, dist);
    float  grid    = max(aa_line.x, aa_line.y);

    /* Frequency-based fade: prevent moire at low grazing angles */
    float max_fw = max(fw.x, fw.y);
    grid *= 1.0 - smoothstep(0.3, 0.5, max_fw);

    float cam_dist = length(world_pos - eye_pos.xyz);
    float fade     = 1.0 - smoothstep(fade_distance * 0.5, fade_distance, cam_dist);
    grid *= fade;

    float3 surface = lerp(bg_color.rgb, line_color.rgb, grid);
    return float4(surface, 1.0);
}

Minimal C setup

/* Grid geometry — flat quad on XZ plane */
#define GRID_HALF_SIZE 50.0f
float grid_verts[] = {
    -GRID_HALF_SIZE, 0.0f, -GRID_HALF_SIZE,
     GRID_HALF_SIZE, 0.0f, -GRID_HALF_SIZE,
     GRID_HALF_SIZE, 0.0f,  GRID_HALF_SIZE,
    -GRID_HALF_SIZE, 0.0f,  GRID_HALF_SIZE,
};
Uint16 grid_indices[] = { 0, 1, 2, 0, 2, 3 };

/* Grid fragment uniforms */
typedef struct GridFragUniforms {
    float line_color[4];
    float bg_color[4];
    float eye_pos[4];
    float grid_spacing;
    float line_width;
    float fade_distance;
    float _pad0;
} GridFragUniforms;

/* Pipeline switch in render pass */
SDL_BindGPUGraphicsPipeline(pass, grid_pipeline);
SDL_PushGPUVertexUniformData(cmd, 0, &vp_matrix, sizeof(vp_matrix));
SDL_PushGPUFragmentUniformData(cmd, 0, &grid_frag, sizeof(grid_frag));
/* bind VB/IB, draw indexed */
SDL_BindGPUGraphicsPipeline(pass, model_pipeline);
/* push model uniforms, bind model VB/IB, draw */

Common mistakes

1. Forgetting fwidth() — without it, lines alias badly at distance
2. No frequency-based fade — distance fade alone is not enough; at low grazing angles, grid cells become sub-pixel before the distance fade kicks in. Use max(fwidth(grid_uv).x, fwidth(grid_uv).y) with smoothstep to fade when pixels span more than ~30–50% of a grid cell (the Nyquist limit)
3. Using frac() directly — frac(grid_uv) puts the discontinuity at the grid line; frac(grid_uv - 0.5) - 0.5 centers the smooth region on the line, which is what you want
4. Wrong color space — with SDR_LINEAR swapchain, all colors must be in linear space (not sRGB). Convert hex colors: (value/255)^2.2
5. Missing depth write on grid — the grid must write to the depth buffer for correct occlusion with other objects
6. Culling the grid — use CULL_NONE so the grid is visible from below too

Reference

• Lesson: GPU Lesson 12 — Shader Grid
• Math: screen-space derivatives (Lesson 05), Blinn-Phong (Lesson 10)
• Concept: fwidth() = abs(ddx()) + abs(ddy()) — pixel footprint