DirectX Computational Graphics

What is Post-Processing?

In essence, post-processing involves rendering the scene into an intermediate texture (often called a framebuffer or render target) and then applying a series of shaders to this texture. This allows for effects that operate on the entire image rather than individual objects.

Common goals of post-processing include:

• Improving visual appeal
• Simulating complex optical phenomena
• Adding artistic styles
• Correcting or enhancing image quality

Key Post-Processing Effects

Here are some of the most widely used post-processing techniques:

1. Tone Mapping

Tone mapping is essential for handling High Dynamic Range (HDR) rendering. It compresses the wide range of luminance values in an HDR image into a lower range that can be displayed on standard High Dynamic Range (SDR) displays. This prevents blown-out highlights and crushed blacks, preserving detail across the entire luminance spectrum.

// Pseudocode for Tone Mapping Shader
float4 PS_ToneMap(float2 texCoord : TEXCOORD0) : SV_TARGET
{
    float4 color = TextureBuffer.Sample(SamplerState, texCoord);
    // Apply a tone mapping operator (e.g., Reinhard, ACES Filmic)
    color.rgb = ReinhardToneMap(color.rgb);
    // Apply gamma correction for display
    color.rgb = pow(color.rgb, 1.0f / 2.2f);
    return color;
}

2. Bloom

Bloom creates an effect of glowing light emanating from bright areas of the scene. This is achieved by blurring the bright pixels and then adding them back to the original image. It's commonly used to simulate light scattering from intense sources like lamps, explosions, or the sun.

// Pseudocode for Bloom Shader (simplified)
float4 PS_Bloom(float2 texCoord : TEXCOORD0) : SV_TARGET
{
    float4 originalColor = TextureBuffer.Sample(SamplerState, texCoord);
    float bloomIntensity = 0.0f;

    // If pixel is bright enough, calculate bloom contribution
    if (originalColor.r > BRIGHTNESS_THRESHOLD || originalColor.g > BRIGHTNESS_THRESHOLD || originalColor.b > BRIGHTNESS_THRESHOLD) {
        bloomIntensity = BloomTexture.Sample(SamplerState, texCoord).r; // Sample from blurred texture
    }

    return originalColor + bloomIntensity * BLOOM_AMOUNT;
}

3. Depth of Field (DoF)

Depth of Field simulates the optical effect where objects in focus appear sharp, while objects outside the focal plane are blurred. This is achieved by sampling the scene multiple times with varying offsets and blending the results based on depth information. It helps to guide the viewer's eye by emphasizing the subject.

// Pseudocode for Depth of Field Shader (simplified sampling)
float4 PS_DepthOfField(float2 texCoord : TEXCOORD0) : SV_TARGET
{
    float4 color = float4(0.0f, 0.0f, 0.0f, 1.0f);
    float totalWeight = 0.0f;
    float centerDepth = DepthBuffer.Sample(SamplerState, texCoord).r;

    // Sample around texCoord based on depth and blur kernel
    for (int i = -KERNEL_SIZE / 2; i <= KERNEL_SIZE / 2; ++i) {
        for (int j = -KERNEL_SIZE / 2; j <= KERNEL_SIZE / 2; ++j) {
            float2 offset = float2(i * PIXEL_SIZE, j * PIXEL_SIZE);
            float2 sampleCoord = texCoord + offset;
            float sampleDepth = DepthBuffer.Sample(SamplerState, sampleCoord).r;
            float weight = CalculateWeight(centerDepth, sampleDepth); // Based on distance from focal plane
            color += SceneTexture.Sample(SamplerState, sampleCoord) * weight;
            totalWeight += weight;
        }
    }
    return color / totalWeight;
}

4. Motion Blur

Motion blur simulates the effect of movement during a camera's exposure. It's achieved by accumulating multiple samples of a moving object over a short time period. This is often implemented using velocity buffers generated during the rendering pass.

// Pseudocode for Motion Blur Shader (using velocity buffer)
float4 PS_MotionBlur(float2 texCoord : TEXCOORD0) : SV_TARGET
{
    float4 color = SceneTexture.Sample(SamplerState, texCoord);
    float2 velocity = VelocityBuffer.Sample(SamplerState, texCoord).rg;

    // Accumulate samples along the velocity vector
    for (int i = 1; i < NUM_SAMPLES; ++i) {
        float2 sampleCoord = texCoord + velocity * (float)i * SAMPLE_STRENGTH;
        color += SceneTexture.Sample(SamplerState, sampleCoord);
    }
    return color / NUM_SAMPLES;
}

5. Anti-Aliasing (Temporal and Spatial)

While often integrated into the rendering pipeline, advanced anti-aliasing techniques like Temporal Anti-Aliasing (TAA) are frequently implemented as a post-process effect. TAA utilizes information from previous frames to smooth jagged edges, significantly improving image quality.

Implementation Considerations

Implementing post-processing effects in DirectX involves several key steps:

• Render Targets: Rendering the scene to one or more textures (render targets).
• Fullscreen Quad: Drawing a fullscreen quad in the scene.
• Shaders: Using pixel shaders to read from the rendered textures and apply the desired effects.
• Texture Blending: Chaining multiple post-processing passes by rendering the output of one effect to a texture that serves as the input for the next.
• Performance: Post-processing can be computationally expensive. Careful optimization, shader profiling, and judicious use of effects are crucial.

Advanced Techniques

Beyond these fundamental effects, DirectX enables more complex post-processing:

• Screen Space Ambient Occlusion (SSAO): Approximating ambient occlusion based on depth buffer information.
• Screen Space Reflections (SSR): Simulating reflections on surfaces using depth and normal data.
• Color Grading: Adjusting the color and tone of the image to achieve specific artistic moods.
• Vignette: Darkening the edges of the screen to focus attention on the center.