如何使用 React Konva Image 将画布重新绘制到自身上？

Posted 2023-03-25

【中文标题】如何使用 React Konva Image 将画布重新绘制到自身上？

【英文标题】：How to draw canvas back onto itself with React Konva Image?

【发布时间】：2020-01-29 07:19:21

【问题描述】：

我正在尝试在react-konva 中实现this Stack Overflow questions' top answer，但卡在这行代码上：

  ctx.drawImage(canvas,
    blurredRect.x, blurredRect.y, blurredRect.width, blurredRect.height,
    blurredRect.x, blurredRect.y, blurredRect.width, blurredRect.height
  );

我们如何创建带有 9 个参数的 Image 组件？ 我们将哪个canvas 元素添加到第二个Image 组件以及如何添加？

// draft code
import useImage from 'use-image';

const [image] = useImage(url, "anonymous");

<Stage width=width height=height>
  <Layer>
    <Image image=image width=width height=height></Image>
    // second Image here with the blur?
    <Rect fill="rgba(255,255,255,0.2)" x=80 y=80 width=200 height=200 />
  </Layer>
</Stage>

【参考方案1】：

您不需要使用初始问题中的<Rect> 组件，因为您只需创建画布元素并将其用于<Image> 组件。

有很多方法可以做到这一点。这是如何使用钩子制作这样的画布的示例：

import React,  Component  from "react";
import  render  from "react-dom";
import  Stage, Layer, Image  from "react-konva";
import useImage from "use-image";

const useCanvas = () => 
  const [image] = useImage(
    "https://upload.wikimedia.org/wikipedia/commons/5/55/John_William_Waterhouse_A_Mermaid.jpg"
  );
  const [canvas, setCanvas] = React.useState(null);

  React.useEffect(() => 
    // do this only when image is loaded
    if (!image) 
      return;
    
    var blurredRect = 
      x: 80,
      y: 80,
      height: 200,
      width: 200,
      spread: 10
    ;
    const canvas = document.createElement("canvas");
    var ctx = canvas.getContext("2d");

    canvas.width = image.width / 2;
    canvas.height = image.height / 2;
    // first pass draw everything
    ctx.drawImage(image, 0, 0, canvas.width, canvas.height);
    // next drawings will be blurred
    ctx.filter = "blur(" + blurredRect.spread + "px)";
    // draw the canvas over itself, cropping to our required rect
    ctx.drawImage(
      canvas,
      blurredRect.x,
      blurredRect.y,
      blurredRect.width,
      blurredRect.height,
      blurredRect.x,
      blurredRect.y,
      blurredRect.width,
      blurredRect.height
    );
    // draw the coloring (white-ish) layer, without blur
    ctx.filter = "none"; // remove filter
    ctx.fillStyle = "rgba(255,255,255,0.2)";
    ctx.fillRect(
      blurredRect.x,
      blurredRect.y,
      blurredRect.width,
      blurredRect.height
    );

    setCanvas(canvas);
  , [image]);

  return canvas;
;

const App = () => 
  const canvas = useCanvas();
  return (
    <Stage width=window.innerWidth height=window.innerHeight>
      <Layer>
        <Image image=canvas />
      </Layer>
    </Stage>
  );
;

render(<App />, document.getElementById("root"));

https://codesandbox.io/s/react-konva-canvas-for-image-ypfmj