HTML5 Canvas Threshold filter Image Tutorial

To apply filter to an Konva.Image, we have to cache it first with cache() function. Then apply filter with filters() function.

To apply a threshold effect to an image with Konva, we can use the Konva.Filters.Threshold. The threshold filter converts the image into a black and white image where all pixels above the threshold value become white and all pixels below become black.

Instructions: Slide the control to change threshold value.

For all available filters go to Filters Documentation.

Vanilla

import Konva from 'konva';

const stage = new Konva.Stage({
  container: 'container',
  width: window.innerWidth,
  height: window.innerHeight,
});

const layer = new Konva.Layer();
stage.add(layer);

const imageObj = new Image();
imageObj.onload = () => {
  const image = new Konva.Image({
    x: 50,
    y: 50,
    image: imageObj,
    draggable: true,
  });

  layer.add(image);

  image.cache();
  image.filters([Konva.Filters.Threshold]);
  image.threshold(0.5);

  // create slider
  const container = document.createElement('div');
  container.style.position = 'absolute';
  container.style.top = '20px';
  container.style.left = '20px';
  
  const text = document.createElement('span');
  text.textContent = 'Threshold: ';
  container.appendChild(text);
  
  const slider = document.createElement('input');
  slider.type = 'range';
  slider.min = '0';
  slider.max = '1';
  slider.step = '0.01';
  slider.value = image.threshold();
  slider.style.width = '200px';
  
  slider.addEventListener('input', (e) => {
    const value = parseFloat(e.target.value);
    image.threshold(value);
    layer.batchDraw();
  });
  
  container.appendChild(slider);
  document.body.appendChild(container);
};
imageObj.src = '/images/lion.png';

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React

import { Stage, Layer, Image } from 'react-konva';
import { useState, useEffect } from 'react';

const App = () => {
  const [image, setImage] = useState(null);
  const [threshold, setThreshold] = useState(0.5);

  useEffect(() => {
    const imageObj = new Image();
    imageObj.onload = () => {
      setImage(imageObj);
    };
    imageObj.src = '/images/lion.png';
  }, []);

  return (
    <>
      <div style={{ position: 'absolute', top: '20px', left: '20px' }}>
        <label>Threshold: </label>
        <input
          type="range"
          min="0"
          max="1"
          step="0.01"
          value={threshold}
          onChange={(e) => setThreshold(parseFloat(e.target.value))}
          style={{ width: '200px' }}
        />
      </div>
      <Stage width={window.innerWidth} height={window.innerHeight}>
        <Layer>
          {image && (
            <Image
              x={50}
              y={50}
              image={image}
              draggable
              filters={[Konva.Filters.Threshold]}
              threshold={threshold}
            />
          )}
        </Layer>
      </Stage>
    </>
  );
};

export default App;

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Vue

<template>
  <div>
    <div style="position: absolute; top: 20px; left: 20px">
      <label>Threshold: </label>
      <input
        type="range"
        min="0"
        max="1"
        step="0.01"
        :value="threshold"
        @input="(e) => threshold = parseFloat(e.target.value)"
        style="width: 200px"
      />
    </div>
    <v-stage :config="stageSize">
      <v-layer>
        <v-image
          v-if="image"
          :config="{
            x: 50,
            y: 50,
            image: image,
            draggable: true,
            filters: [Konva.Filters.Threshold],
            threshold: threshold,
          }"
        />
      </v-layer>
    </v-stage>
  </div>
</template>

<script setup>
import { ref, onMounted } from 'vue';

const stageSize = {
  width: window.innerWidth,
  height: window.innerHeight,
};

const image = ref(null);
const threshold = ref(0.5);

onMounted(() => {
  const imageObj = new Image();
  imageObj.onload = () => {
    image.value = imageObj;
  };
  imageObj.src = '/images/lion.png';
});
</script>

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