God rays for games

2025-03-02 20:12 UTC gpt-4o Open in ChatGPT ↗

Alright! Here’s an ELI5 (Explain Like I’m 5) version of the paper:

What is this paper about?

This paper is about making cool-looking light beams (like “God rays” you see in foggy forests or dusty rooms) appear in video games and graphics, but without making computers too slow.

Why is it hard to make these light beams?

Light doesn’t just hit objects—it bounces around in the air if there’s fog, dust, or smoke.
To show this in a game, computers check every point in the air to see how much light is bouncing, which takes a LOT of calculations.
This makes things slow, especially if you want the game to run smoothly.

What is the solution?

Instead of checking every tiny point in the air, the authors found a smart way to pick the important ones:

They noticed that light beams change smoothly along their direction but can change suddenly across them (because of shadows).
They use “epipolar lines” (a fancy way of saying they place samples along where the light rays go).
They skip unnecessary calculations and only check where light changes a lot (like at shadows).
They guess (interpolate) the rest, saving tons of time.

What’s the result?

The method makes realistic God rays in games way faster.
It works on graphics cards (GPUs), making it useful for real-time effects.
It looks better than old methods while also running faster.

TL;DR

This paper finds a smarter way to draw God rays by skipping unnecessary work. It makes games look great while keeping them fast and smooth.

Would you like me to explain any part in more detail? 😊

Great question! The authors use a clever trick to figure out where to do calculations:

1️⃣ They Follow Epipolar Lines

Light beams (God rays) move in straight lines from the light source.
Instead of checking everywhere, they check only along these lines.
These lines are called epipolar lines and help them organize the sampling in a way that matches how light behaves.

2️⃣ They Detect Where Shadows or Changes Happen

They check depth changes in the scene (using the depth buffer, which stores how far objects are from the camera).
If a depth value jumps suddenly, it means there’s an edge, like the start of a shadow.
Instead of wasting effort on smooth areas, they add extra samples near these edges.

3️⃣ They Use A Smart Search Instead of Checking Everything

Instead of checking every single pixel, they only search along the epipolar lines for sudden depth changes.
This lets them find important areas quickly without checking the entire screen.

4️⃣ They Only Calculate Light for Important Spots

They do full calculations (ray marching) only at these important spots.
Everywhere else, they guess (interpolate) based on nearby values, which is much faster.

🔥 TL;DR: How do they skip unnecessary calculations?

They follow the paths of light beams (epipolar lines) instead of randomly checking everywhere.
They detect where light suddenly changes (like shadows) by checking depth differences.
They only compute light where it’s needed and skip smooth areas by using interpolation.

This way, they get fast, realistic lighting effects without wasting time on unnecessary calculations! 🚀

Great questions! Let’s break it down step by step.

1️⃣ How Do They Select the Original Rays?

They don’t just pick random rays—instead, they organize them using epipolar lines. Here’s how:

Find the light source in the image
- They project the position of the light source onto the screen.
- If the light is outside the screen, they find the vanishing point where rays would converge.
Create epipolar lines
- They draw lines from the light source projection outward to the edges of the screen.
- Each of these lines represents a set of rays that travel from the camera into the scene.
Place initial samples along each epipolar line
- They start with a small number of samples (like 8-32) evenly spaced along each line.
- This gives an initial rough guess of how light behaves along each line.

2️⃣ How Do They Check the Depth Buffer Difference?

(How Does Each Ray Know About Its Neighbors?)

🔹 Step 1: Read Depth Along Epipolar Lines

The depth buffer stores how far objects are from the camera at each pixel.
They take the initial samples along each epipolar line and check their depth values.

🔹 Step 2: Find Sudden Changes (Depth Discontinuities)

They compare each sample’s depth to its neighbor along the epipolar line.

If the depth difference is big (above a threshold), it means the ray is hitting a new object, likely a shadow edge.

Example:

Sample 1: Depth = 5.0
Sample 2: Depth = 5.2 (small change → smooth surface)
Sample 3: Depth = 10.0 (BIG jump → edge of an object or shadow!)

If they detect a big jump, they insert extra samples before and after the change to capture the shadow transition more accurately.

🔹 Step 3: Efficiently Search for Discontinuities

Instead of checking every pixel, they only scan along each epipolar line (1D search).
They store depth values in a small 1D array (one per epipolar line).
They use a fast linear search to detect where depth changes suddenly.

🔹 Step 4: Mark Which Samples Need Full Computation

If a sample is near a depth change → do full ray marching.
If it’s in a smooth area → skip full calculation and interpolate from nearby results.

🚀 TL;DR

How do they select rays?
- They create epipolar lines from the light source to the screen edges.
- They place a few initial samples along each line.
How do they check depth differences?
- They read depth values along each epipolar line.
- They compare each sample’s depth to its neighbor along the same line.
- If there’s a big jump, they add more samples.
- This lets them detect shadow edges without checking every pixel.

This method saves a TON of processing power while still making shadows and light beams look super realistic! 🔥 🚀

explain what this hlsl does and write documentation:

void FlyingEmbers_float(float2 uv, float velocity, float width, float feather, out float alpha) { // [0, 1] -> [-1, 1] uv = uv * 2 - 1;

uv.x *= velocity + width;
uv.y *= width;

float2 v = float2(2 * velocity, 0);
float2 p = uv - float2(velocity, 0);
float r = width;

float a = dot(v, v);
float b = dot(v, p);
float c = dot(p, p) - r * r;
float d = b * b - a * c;

// edge fade
float h = saturate(-b / a);
float dist = length(p + h * v);
alpha = smoothstep (0, feather * width, width - dist);

if (d > 0.0)
{
    d = sqrt(d);

    float t0 = max(0, (-b - d) / a);
    float t1 = min(1, (-b + d) / a);

    alpha *= saturate(2 * (t1 - t0));
}

}

Here’s an explanation of what this HLSL (High-Level Shader Language) function does, along with detailed documentation.

What Does This Function Do?

The FlyingEmbers_float function computes an alpha (transparency) value for a visual effect that simulates flying embers (or particles) moving in a direction with a soft fade effect.

It transforms UV coordinates from [0,1] to [-1,1].
It modifies the coordinates based on velocity, width, and feathering.
It determines whether the pixel falls within a moving ember based on ray-circle intersection.
It applies a smooth fading effect at the ember’s edges.
It adjusts the transparency (alpha) using a quadratic equation for intersection testing.

Documented HLSL Code

Below is the function with detailed comments explaining each step:

/**
 * Computes the alpha transparency for a flying ember effect.
 * This function determines whether a pixel is inside a moving ember
 * and applies a soft fade effect at the edges.
 *
 * @param uv        - (float2) The UV coordinates of the fragment in range [0,1].
 * @param velocity  - (float) Speed at which the ember moves horizontally.
 * @param width     - (float) Radius of the ember.
 * @param feather   - (float) Controls how soft the ember fades at the edges.
 * @param alpha     - (out float) Output alpha transparency for this fragment.
 */
void FlyingEmbers_float(float2 uv, float velocity, float width, float feather, out float alpha)
{
    // Convert UV from [0,1] range to [-1,1] range
    uv = uv * 2 - 1;

    // Scale X by (velocity + width) and Y by width
    uv.x *= velocity + width;
    uv.y *= width;

    // Define the movement vector (direction of ember motion)
    float2 v = float2(2 * velocity, 0);

    // Compute the position of the ember relative to the moving direction
    float2 p = uv - float2(velocity, 0);

    // Radius of the ember
    float r = width;

    // Quadratic equation coefficients for ray-circle intersection
    float a = dot(v, v);       // v•v (squared magnitude of velocity vector)
    float b = dot(v, p);       // v•p (projection of position onto velocity)
    float c = dot(p, p) - r*r; // (distance squared - radius squared)
    float d = b * b - a * c;   // Discriminant of quadratic equation

    // **Edge fading effect**
    // Compute the shortest distance from the ember centerline
    float h = saturate(-b / a);
    float dist = length(p + h * v);

    // Use smoothstep to fade alpha at the ember's edges
    alpha = smoothstep(0, feather * width, width - dist);

    // **Handle ray-circle intersection**
    if (d > 0.0)  // If discriminant is positive, there's an intersection
    {
        d = sqrt(d); // Compute square root of the discriminant

        // Solve for t0 and t1 (the intersection points along the ray)
        float t0 = max(0, (-b - d) / a);
        float t1 = min(1, (-b + d) / a);

        // Modify alpha based on the segment of the ray inside the ember
        alpha *= saturate(2 * (t1 - t0));
    }
}

Breaking It Down

1️⃣ Transform UVs to [-1,1] Space

uv = uv * 2 - 1;

Converts UV coordinates from [0,1] to [-1,1].
This ensures that the ember is centered correctly.

2️⃣ Adjust Coordinates for Motion

uv.x *= velocity + width;
uv.y *= width;

X-coordinates are stretched based on velocity and ember width.
Y-coordinates are scaled by ember width.

3️⃣ Define the Movement of the Ember

float2 v = float2(2 * velocity, 0);
float2 p = uv - float2(velocity, 0);
float r = width;

v is the movement vector, showing the ember’s horizontal motion.
p is the relative position of the ember.
r is the ember radius.

4️⃣ Solve the Quadratic Equation for Ray-Circle Intersection

float a = dot(v, v);
float b = dot(v, p);
float c = dot(p, p) - r * r;
float d = b * b - a * c;

This is a standard ray-circle intersection calculation:
- a, b, c are the coefficients of the quadratic equation.
- d (discriminant) tells if there is an intersection.

5️⃣ Apply Edge Fading Effect

float h = saturate(-b / a);
float dist = length(p + h * v);
alpha = smoothstep (0, feather * width, width - dist);

Uses smoothstep to soften the ember’s edges based on feather.
dist measures how far the fragment is from the ember’s centerline.

6️⃣ Adjust Alpha Based on Ray-Circle Intersection

if (d > 0.0)
{
    d = sqrt(d);

    float t0 = max(0, (-b - d) / a);
    float t1 = min(1, (-b + d) / a);

    alpha *= saturate(2 * (t1 - t0));
}

If d > 0, the ray intersects the ember.
Computes t0 and t1 (intersection points along the ray).
Adjusts alpha to reflect how much of the ember is visible.

Summary

This function determines the transparency (alpha) of a flying ember at a given pixel.
It uses ray-circle intersection to figure out if the pixel is inside the ember.
It applies a smooth fade effect at the ember’s edges.
The function optimizes rendering by skipping expensive calculations for pixels that don’t need them.

Where Can This Be Used?

Particle effects for fire embers, dust, or sparks.
Stylized lighting effects in games and animations.
Procedural textures that require soft motion blur-like effects.

This should give you a complete understanding of what the function does and how it works. 🚀 Let me know if you need more clarifications! 😊