Forge Linear Fog

Linear Fog Skill

Adds distance fog to any SDL3 GPU scene. Derived from Lesson 20.

When to use

• Adding atmospheric perspective to a 3D scene
• Fading distant objects into a background color
• Simulating haze, mist, or ground fog
• Hiding the far-plane clipping boundary

Fog uniform block (append to existing fragment cbuffer)

/* Append these fields to your existing fragment uniform buffer.
 * Total addition: 32 bytes (2 x float4 worth). */
float4 fog_color;     /* Must match the clear color (rgb)    */
float  fog_start;     /* Linear mode: distance where fog begins */
float  fog_end;       /* Linear mode: fully fogged distance     */
float  fog_density;   /* Exp/Exp2 modes: density coefficient    */
uint   fog_mode;      /* 0 = linear, 1 = exp, 2 = exp-squared  */

Matching C struct fields:

float fog_color[4];
float fog_start;
float fog_end;
float fog_density;
Uint32 fog_mode;

HLSL fog calculation (paste after lighting)

/* Compute Euclidean distance from fragment to camera. */
float dist = length(eye_pos.xyz - input.world_pos);

float fog_factor = 1.0;
if (fog_mode == 0)
{
    /* Linear */
    fog_factor = saturate((fog_end - dist) / (fog_end - fog_start));
}
else if (fog_mode == 1)
{
    /* Exponential */
    fog_factor = saturate(exp(-fog_density * dist));
}
else
{
    /* Exponential squared */
    float f = fog_density * dist;
    fog_factor = saturate(exp(-(f * f)));
}

/* Blend: fog_factor=1 → lit color, fog_factor=0 → fog color */
float3 final_color = lerp(fog_color.rgb, lit_color, fog_factor);

Typical parameter values

Key rules

1. Fog color = clear color. Always set the framebuffer clear color to the same RGB as the fog color. Otherwise there will be a visible seam at the horizon.

2. Apply fog to ALL shaders. Every fragment shader in the scene (objects, grid, terrain, particles) must apply the same fog with the same parameters. Inconsistent fog breaks the atmospheric illusion.

3. Use Euclidean distance, not Z depth. Compute length(eye - frag_pos) in world space. Using only the Z component causes fog to shift when the camera rotates.

4. Fog goes after lighting. Compute the full Blinn-Phong (or PBR) result first, then blend it toward the fog color. Applying fog to individual lighting terms produces incorrect results.

5. Clamp the fog factor. Use saturate() to keep the factor in [0, 1]. Without clamping, objects beyond the fog end distance can produce negative values that invert the fog.

Common mistakes

• Fog color doesn't match clear color — creates a visible horizon line between fogged geometry and the background.
• Only one shader has fog — the grid floor appears to float on top of the fog while objects fade correctly (or vice versa).
• Using Z depth instead of distance — fog shifts when the camera rotates because fragments at the same world position get different fog values.
• Applying fog before lighting — the ambient/diffuse/specular terms get fogged individually, producing darker or brighter results than expected.
• Forgetting to pass fog_mode to the shader — defaults to 0 but toggling modes at runtime requires updating the uniform each frame.

Reference

• Lesson 20: lessons/gpu/20-linear-fog/
• Fragment shader: lessons/gpu/20-linear-fog/shaders/fog.frag.hlsl
• Grid shader: lessons/gpu/20-linear-fog/shaders/grid_fog.frag.hlsl