Sync snow effect

uniform half intensity;

const float farthestSnowLayerWiggleSpeed = 5.8;

const float closestSnowLayerWiggleSpeed = 2.6;

/**

 * Generates snow flakes.

 *

 * @param screenAspectRatio the aspect ratio of the fragment where we will display the effect.

 * @param time the elapsed time.

 * @param snowGridSize the size of the grid, where each cell contains a snow flake.

 * @param layerNumber the layer of snow that we want to draw. Higher is farther from camera.

 * @param layerIndex the index of the current layer of snow that we want to draw. (Higher index

 *                      indicates that it's farther away from camera).

 * @param minLayerIndex the index of the minimum layer.

 * @param maxLayerIndex the index of the maximum layers.

 *

 * @returns Snow with the snow info.

 */

    in float screenAspectRatio,

    in float time,

    in vec2 snowGridSize,

    in float layerNumber

    in float layerIndex,

    in float minLayerIndex,

    in float maxLayerIndex

) {

    // Normalize the layer index. 0 is closest, 1 is farthest.

    half normalizedLayerIndex = map(layerIndex, minLayerIndex, maxLayerIndex, 0, 1);

    /* Grid. */

    // Increase the last number to make each layer more separate from the previous one.

    float depth = 1. + layerNumber * 0.35;

    float speedAdj = 1. + layerNumber * 0.15;

    float layerR = idGenerator(layerNumber);

    float depth = 0.65 + layerIndex * 0.41;

    float speedAdj = 1. + layerIndex * 0.15;

    float layerR = idGenerator(layerIndex);

    snowGridSize *= depth;

    time += layerR * 58.3;

    // Number of rows and columns (each one is a cell, a drop).

    gridUv.y += verticalGridPos;

    // Generate column id, to offset columns vertically (so snow flakes are not aligned).

    float columnId = idGenerator(floor(gridUv.x));

    gridUv.y += columnId * 2.6;

    // Have time affect the position of each column as well.

    gridUv.y += columnId * 2.6 + time * 0.09 * (1 - columnId);

    /* Cell. */

    // Get the cell ID based on the grid position. Value from 0 to 1.

    }

    /* Cell-id-based variations. */

    // Adjust time based on columnId.

    // Adjusts scale of each snow flake (higher is smaller).

    float scaleVariation = 2.0 + 2.7 * cellId;

    float opacityVariation = (1. - 0.9 * cellId);

    // 0 = snow flake invisible, 1 = snow flake visible.

    float visibilityFactor = smoothstep(

        cellIntensity,

        max(cellIntensity - (0.02 + 0.18 * intensity), 0.0),

        1 - intensity);

    // Adjust the size of each snow flake (higher is smaller) based on cell ID.

    float decreaseFactor = 2.0 + map(cellId, 0., 1., -0.1, 2.8) + 5. * (1 - visibilityFactor);

    // Adjust the opacity of the particle based on the cell id and distance from the camera.

    float farLayerFadeOut = map(normalizedLayerIndex, 0.7, 1, 1, 0.4);

    float closeLayerFadeOut = map(normalizedLayerIndex, 0, 0.2, 0.6, 1);

    float opacityVariation =

        (1. - 0.9 * cellId) *

        visibilityFactor *

        closeLayerFadeOut *

        farLayerFadeOut;

    /* Cell snow flake. */

    // Horizontal movement: Wiggle.

    float wiggleSpeed = 3.0;

    // Horizontal movement: Wiggle (Adjust the wiggle speed based on the distance).

    float wiggleSpeed = map(

        normalizedLayerIndex,

        0.2,

        0.7,

        closestSnowLayerWiggleSpeed,

        farthestSnowLayerWiggleSpeed);

    // Adjust wiggle based on layer number (0 = closer to screen => we want less movement).

    float wiggleAmp = 0.4 + 0.4 * smoothstep(0.5, 2.5, layerNumber);

    float wiggleAmp = 0.6 + 0.4 * smoothstep(0.5, 2.5, layerIndex);

    // Define the start based on the cell id.

    float horizontalStartAmp = 0.5;

    // Add the wiggle (equation decided by testing in Grapher).

    float horizontalWiggle = wiggle(uv.y + cellId * 2.1, wiggleSpeed * speedAdj);

    float horizontalWiggle = wiggle(

        // Current uv position.

        uv.y

        // Adjustment so the shape is not skewed.

        - cellUv.y / snowGridSize.y

        // variation based on cell ID.

        + cellId * 2.1,

        wiggleSpeed * speedAdj);

    // Add the start and wiggle and make that when we are closer to the edge, we don't wiggle much

    // (so the drop doesn't go outside it's cell).

    vec2 snowFlakeShape = vec2(1., 1.2);

    vec2 snowFlakePos = vec2(snowFlakePosUncorrected / cellAspectRatio, cellUv.y);

    snowFlakePos -= vec2(0., uv.y - 0.5) * cellId;

    snowFlakePos *= snowFlakeShape * scaleVariation;

    vec2 snowFlakePosR = 1.016 * abs(rot45 * (snowFlakePos + (cellId * 2. - 1.) * vec2(0.050)));

    snowFlakePos *= snowFlakeShape * decreaseFactor;

    vec2 snowFlakeShapeVariation = vec2(0.055) * // max variation

        vec2((cellId * 2. - 1.), // random A based on cell ID

            (fract((cellId + 0.03521) * 34.21) * 2. - 1.)); // random B based on cell ID

    vec2 snowFlakePosR = 1.016 * abs(rot45 * (snowFlakePos + snowFlakeShapeVariation));

    snowFlakePos = abs(snowFlakePos);

    // Create the snowFlake mask.

    float flakeMask = smoothstep(

uniform shader foreground;

uniform shader background;

uniform shader accumulatedSnow;

uniform shader noise;

uniform float2 uvOffsetFgd;

uniform float2 uvScaleFgd;

uniform float2 uvOffsetBgd;

#include "shaders/utils.agsl"

#include "shaders/snow.agsl"

/* Constants that can be modified. */

// Snow tint.

const vec3 snowColor = vec3(0.9);

// Glass tint.

const vec3 glassTint = vec3(0.8); // gray

const vec4 snowColor = vec4(1., 1., 1., 0.95);

// Background tint

const vec4 bgdTint = vec4(0.8, 0.8, 0.8, 0.07);

// snow opacity (how visible it is).

const float snowOpacity = 1.4;

// how frosted the glass is.

const float frostedGlassIntensity = 0.07;

// Indices of the different snow layers.

const float farthestSnowLayerIndex = 9;

const float midSnowLayerIndex = 3;

const float closestSnowLayerIndex = 0;

vec4 main(float2 fragCoord) {

    float2 uv = fragCoord / screenSize;

    float2 uvAdjusted = vec2(uv.x, uv.y / screenAspectRatio);

    /**

     * The effect is consisted of 2 image textures (foreground and background) + 10 layers of

     * snow + 1 layer of snow accumulation. Below describes the rendering order (back to front):

     * 1. Background

     * 2. Background snow layers (from farthest layer to mid layer)

     * 3. Foreground

     * 4. Snow accumulation layer (on subject)

     * 5. Foreground snow layers (from mid layer to closest layer)

     */

    // Adjusts the UVs to have the expected rect of the image.

    float2 adjustedUvForeground = fragCoord * uvScaleFgd + uvOffsetFgd;

    vec4 colorForeground = foreground.eval(adjustedUvForeground);

    vec4 colorBackground = background.eval(fragCoord * uvScaleBgd + uvOffsetBgd);

    vec4 color = vec4(0., 0., 0., 1.);

    // 1. Draw background.

    vec4 color = colorBackground;

    // Add some slight tint to the frosted glass.

    // Add slight tint to the background.

    color.rgb = normalBlendNotPremultiplied(color.rgb, bgdTint.rgb, bgdTint.a);

    // Get color of the background texture.

    color.rgb = mix(colorBackground.rgb, glassTint, frostedGlassIntensity);

    for (half i = 9.; i > 2.; i--) {

        // Generate snow behind the subject.

        // Normalized layer index.

        half idx = (i - 2.) / (9. - 2.);

    // 2. Generate snow layers behind the subject.

    for (float i = farthestSnowLayerIndex; i > midSnowLayerIndex; i--) {

        Snow snow = generateSnow(

            uv,

            screenAspectRatio,

              time * mix(1.25, 5., idx),

              /* Grid size = */ vec2(mix(3.0, 6.0, idx), mix(1.0, 3.0, idx)),

              /* layer number = */ i);

        color.rgb = mix(color.rgb, snowColor, snowOpacity * snow.flakeMask);

            time,

            // TODO: adjust grid size based on aspect ratio.

            /* Grid size = */ vec2(7., 1.5),

            /* layer number = */ i,

            closestSnowLayerIndex,

            farthestSnowLayerIndex);

        color.rgb =

            normalBlendNotPremultiplied(color.rgb, snowColor.rgb, snowColor.a * snow.flakeMask);

    }

    // Add the foreground. Any effect from here will be in front of the subject.

    // 3. Add the foreground layer. Any effect from here will be in front of the subject.

    color.rgb = normalBlend(color.rgb, colorForeground.rgb, colorForeground.a);

    // Add accumulated snow.

    vec2 accSnow = accumulatedSnow.eval(adjustedUvForeground).rg;

    float snowLayer = smoothstep(0.2, 0.8, accSnow.r);

    float snowTexture = smoothstep(0.2, 0.7, accSnow.g);

    color.rgb = mix(color.rgb, vec3(0.95), 0.98 * snowLayer * (0.05 + 0.95 * snowTexture));

    // 4. Add accumulated snow layer.

    // Load noise texture to give "fluffy-ness" to the snow. Displace the sampling of the noise.

    vec3 cloudsNoise = noise.eval(uvAdjusted * 7000 + vec2(fragCoord.y, -fragCoord.x)).rgb;

    // Add dither to give texture to the snow and ruffle the edges.

    float dither = abs(triangleNoise(fragCoord * 0.01));

    for (half i = 2.; i >= 0.; i--) {

        // Generate snow behind the subject.

    // Get the accumulated snow buffer. r contains its mask, g contains some random noise.

    vec2 accSnow = accumulatedSnow.eval(adjustedUvForeground).rg;

    // Sharpen the mask of the accumulated snow, but not in excess.

    float accSnowMask = smoothstep(0.1, 0.9, /* mask= */ accSnow.r);

    // Makes the edges of the snow layer accumulation rougher.

    accSnowMask = map(accSnowMask, 1. - cloudsNoise.b - 0.3 * dither, 1., 0., 1.);

    // Load snow texture and dither. Make it have gray-ish values.

    float accSnowTexture = smoothstep(0.2, 0.7, /* noise= */ accSnow.g) * 0.7;

    accSnowTexture = map(accSnowTexture, dither - 1, 1, 0, 1);

    // Adjust snow texture coverage/shape.

    accSnowTexture = map(accSnowTexture, 0.67, 0.8, 0, 1);

    accSnowMask = map(accSnowMask, 0., 1., 0., 1.- 0.6 * accSnowTexture - 0.35 * dither);

    color.rgb = normalBlendNotPremultiplied(color.rgb, snowColor.rgb, snowColor.a * accSnowMask);

    // 5. Generate snow in front of the subject.

    for (float i = midSnowLayerIndex; i >= closestSnowLayerIndex; i--) {

        Snow snow = generateSnow(

            uv,

            screenAspectRatio,

              time * 1.25,

              /* Grid size = */ vec2(i + 1., 1.4),

              /* layer number = */ i);

        color.rgb = mix(color.rgb, snowColor, snowOpacity * snow.flakeMask);

            time,

            // TODO: adjust grid size based on aspect ratio

            /* Grid size = */ vec2(7., 1.5),

            /* layer number = */ i,

            closestSnowLayerIndex,

            farthestSnowLayerIndex);

        color.rgb =

            normalBlendNotPremultiplied(color.rgb, snowColor.rgb, snowColor.a * snow.flakeMask);

    }

    /* Debug snow */

    // resets color.

    // color.rgb *= 0.;

    // color.rgb += snow.flakeMask;

    // if (snow.cellUv.x > 0.49 || snow.cellUv.y > 0.49) color.r = 1.0;

    return color;

}

    return b * (1. - o) + o;

}

/*

 * This is the normal blend mode in which the foreground is painted on top of the background based

 * on the foreground opacity.

 *

 * @param b the background color.

 * @param f the foreground color.

 * @param o the mask or the foreground alpha.

 *

 * Note: this blending function expects the foreground to NOT have premultiplied alpha.

 */

vec3 normalBlendNotPremultiplied(vec3 b, vec3 f, float o) {

    return mix(b, f, o);

}

/** Math Utils */

// function created on Grapher (equation decided by testing in Grapher).

float wiggle(float time, float wiggleSpeed) {

            "background",

            BitmapShader(snowConfig.background, Shader.TileMode.MIRROR, Shader.TileMode.MIRROR)

        )

        snowConfig.shader.setInputBuffer(

            "noise",

            BitmapShader(snowConfig.noiseTexture, Shader.TileMode.REPEAT, Shader.TileMode.REPEAT)

        )

    }

    private fun prepareColorGrading() {

    val accumulatedSnowShader: RuntimeShader,

    /** The color grading shader. */

    val colorGradingShader: RuntimeShader,

    /**

     * The noise texture, which will be used to add fluffiness to the snow flakes. The texture is

     * expected to be tileable, and at least 16-bit per channel for render quality.

     */

    val noiseTexture: Bitmap,

    /** The main lut (color grading) for the effect. */

    val lut: Bitmap?,

    /** A bitmap containing the foreground of the image. */

        accumulatedSnowShader =

            GraphicsUtils.loadShader(context.assets, ACCUMULATED_SNOW_SHADER_PATH),

        colorGradingShader = GraphicsUtils.loadShader(context.assets, COLOR_GRADING_SHADER_PATH),

        noiseTexture = GraphicsUtils.loadTexture(context.assets, NOISE_TEXTURE_PATH)

                ?: throw RuntimeException("Noise texture is missing."),

        lut = GraphicsUtils.loadTexture(context.assets, LOOKUP_TABLE_TEXTURE_PATH),

        foreground,

        background,

        private const val SHADER_PATH = "shaders/snow_effect.agsl"

        private const val ACCUMULATED_SNOW_SHADER_PATH = "shaders/snow_accumulation.agsl"

        private const val COLOR_GRADING_SHADER_PATH = "shaders/color_grading_lut.agsl"

        private const val NOISE_TEXTURE_PATH = "textures/clouds.png"

        private const val LOOKUP_TABLE_TEXTURE_PATH = "textures/lut_rain_and_fog.png"

        private const val BLUR_RADIUS = 20f

        private const val DEFAULT_INTENSITY = 1f