ScratchCardNoiseShader.kt

/*
 * Copyright 2026 Nazarii Moshenskyi
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.graphics.RuntimeShader
import androidx.compose.foundation.Canvas
import androidx.compose.foundation.background
import androidx.compose.foundation.gestures.detectDragGestures
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.shape.RoundedCornerShape
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableIntStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.clip
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.graphics.BlendMode
import androidx.compose.ui.graphics.Brush
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.CompositingStrategy
import androidx.compose.ui.graphics.Path
import androidx.compose.ui.graphics.ShaderBrush
import androidx.compose.ui.graphics.StrokeCap
import androidx.compose.ui.graphics.StrokeJoin
import androidx.compose.ui.graphics.drawscope.Stroke
import androidx.compose.ui.graphics.graphicsLayer
import androidx.compose.ui.input.pointer.pointerInput
import androidx.compose.ui.semantics.clearAndSetSemantics
import androidx.compose.ui.semantics.contentDescription
import androidx.compose.ui.semantics.semantics
import androidx.compose.ui.semantics.stateDescription
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp
import com.moshenskyi.shaderssample.ui.theme.ShadersSampleTheme
import org.intellij.lang.annotations.Language

@Language("AGSL")
private const val GRAIN_SHADER = """
    uniform float2 resolution;
    
    const float2 SEED_VECTOR = float2(12.9897, 78.2323);
    const float HASH_SCALE = 43758.5453;
    
    // try to experiment with frequency and amplitude and see how it changes the pattern
    const float LINE_FREQUENCY = 1000.0;
    const float LINE_AMPLITUDE = 0.03;
    
    float generateGrain(float2 point) {
        // simple coordinate hash to convert 2D coord into pseudo-random scalar
        // without the seed vector symetric coordinates will produce diagonal stripes
        float hashedCoord = dot(point, SEED_VECTOR);
        
        // multiply by a large number to spread the fractional part
        float noiseWave = sin(hashedCoord) * HASH_SCALE;
        return fract(noiseWave); // get fractional part
    }
    
    float generateLine(float coord) {
        return sin(coord * LINE_FREQUENCY) * LINE_AMPLITUDE;
    }
    
    half4 main(float2 fragCoord) {
        float2 uv = fragCoord / resolution;
        
        // grey color
        float3 baseColor = float3(0.72, 0.72, 0.70);
        
        float grain = generateGrain(fragCoord);
        
        // I use uv.y to generate horizontal lines
        float lines = generateLine(uv.y);
        
        // raw noise is too bright, multiply it by 0.3
        float brightness = grain * 0.3 + lines;
        float3 color = baseColor + brightness;
        
        return half4(color, 1.0);
    }
"""

@Composable
fun ScratchCard(
    modifier: Modifier = Modifier,
    cardNumber: String = "4242 4242 4242 4242"
) {
    val cardBackground = remember {
        Brush.linearGradient(
            colors = listOf(
                Color(0xFF222831),
                Color(0xFF393E46),
                Color(0xFF1B1F24)
            )
        )
    }

    Box(
        modifier = modifier
            .clip(RoundedCornerShape(24.dp))
            .background(cardBackground)
            .padding(24.dp),
        contentAlignment = Alignment.Center
    ) {
        Text(
            text = cardNumber,
            modifier = Modifier.clearAndSetSemantics { },
            color = Color.White
        )

        ScratchOverlay(
            modifier = Modifier
                .fillMaxWidth()
                .height(50.dp)
        )
    }
}

@Composable
private fun ScratchOverlay(
    modifier: Modifier = Modifier
) {
    val shader = remember { RuntimeShader(GRAIN_SHADER) }
    val shaderBrush = remember { ShaderBrush(shader) }

    val strokeWidth = 30.dp
    val scratchPath = remember { Path() }
    var pathVersion by remember { mutableIntStateOf(0) }

    Canvas(
        modifier = modifier
            .graphicsLayer {
                compositingStrategy = CompositingStrategy.Offscreen
            }
            .semantics {
                contentDescription = "Scratch area"
                stateDescription = "Card number hidden"
            }
            .pointerInput(Unit) {
                detectDragGestures(
                    onDragStart = { offset: Offset ->
                        scratchPath.moveTo(offset.x, offset.y)
                        pathVersion++
                    },
                    onDrag = { change, _ ->
                        change.consume()
                        scratchPath.lineTo(change.position.x, change.position.y)
                        pathVersion++
                    }
                )
            }
    ) {
        pathVersion

        shader.setFloatUniform("resolution", size.width, size.height)
        drawRect(brush = shaderBrush)

        drawPath(
            path = scratchPath,
            color = Color.Transparent,
            style = Stroke(
                width = strokeWidth.toPx(),
                cap = StrokeCap.Round,
                join = StrokeJoin.Round,
            ),
            blendMode = BlendMode.Clear
        )
    }
}

@Preview
@Composable
fun ScratchCardPreview() {
    ShadersSampleTheme {
        Box(
            modifier = Modifier.fillMaxSize(),
            contentAlignment = Alignment.Center
        ) {
            ScratchCard(
                modifier = Modifier
                    .width(340.dp)
                    .height(210.dp)
            )
        }
    }
}