Tips for aligning items in the center of konvajs

Question

Tips for aligning items in the center of konvajs

I am currently developing a meme-editor using React and incorporating KonvaJS to create a Canvas-like functionality. My challenge lies in centering the items within the canvas stage, as there seems to be a property overriding my styling attempts. Here is a snippet of the return statement in my React component:

<div className="mycanvas">
    <Stage width={500} height={500} className="stage">
        <Layer>
            <Image image={image} className="meme" />
            {textFields.map((text) => (
                <Text
                    text={text}
                    draggable
                    fontFamily="Impact"
                    fontSize="30"
                    stroke="white"
                />
            ))}
        </Layer>
    </Stage>
</div>

And this is how the final output appears.

https://i.sstatic.net/XVHf1.png

To illustrate where the image should be centered, I have set the background of the wrapper to blue.

I have experimented with applying CSS to classes such as "mycanvas," "stage," and "meme," as well as "konvajs-content" (which appeared in my inspector). Despite trying properties like align-items: center and margin: auto, conventional CSS methods don't seem to work effectively in this scenario. It appears to be an issue related to the overall styling of KonvaJS components, but unfortunately, I haven't been able to find a solution on platforms like Stack Overflow or in the Konva documentation.

css reactjs konvajs

Answer 1

Answer №1

In this scenario, CSS alone is not sufficient to achieve the desired outcome. When an image is placed onto a canvas using specific height and width coordinates (x and y), the pixels of the image blend into the rasterized canvas. Essentially, the image loses its independent identity outside of the canvas.

To center the image within the canvas, some mathematical calculations are necessary to determine the correct x and y coordinates that will position the image at the center of the canvas.

Interactive Demo

For instance, if your canvas is 500px tall and the image's height is 350px, you should set the y position to 75px (i.e., (500 - 350) / 2).

The demonstration code below exemplifies how to replicate the behavior of CSS object-fit: contain. This functionality adjusts the image to fill the canvas in one direction while centering it in the other direction.

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

function Example() {
  const w = window.innerWidth;
  const h = window.innerHeight;
  const src = "https://konvajs.org/assets/yoda.jpg";

  const [image, setImage] = useState(null);
  const [pos, setPos] = useState({ x: 0, y: 0 });

  useEffect(() => {
    const image = new window.Image();
    image.src = src;
    image.addEventListener("load", handleLoad);

    function handleLoad(event) {
      const image = event.currentTarget;
      /* after the image is loaded, you can get it's dimensions */
      const imgNaturalWidth = image.width;
      const imgNaturalHeight = image.height;

      /* 
        calculate the horizontal and vertical ratio of the 
        image dimensions versus the canvas dimensions
      */
      const hRatio = w / imgNaturalWidth;
      const vRatio = h / imgNaturalHeight;

      /*
        To mimic CSS Object-Fit behaviour like "contain," 
        select the smaller of the horizontal and vertical ratios.
        
        For a "cover" effect, use Math.max instead.
      */
      const ratio = Math.min(hRatio, vRatio);
      /* 
        Scale the image to fit the canvas 
      */
      image.width = imgNaturalWidth * ratio;
      image.height = imgNaturalHeight * ratio;

      /* 
        Calculate offsets for centering the image inside the canvas
      */
      const xOffset = (w - image.width) / 2;
      const yOffset = (h - image.height) / 2;

      setPos({
        x: xOffset,
        y: yOffset
      });
      setImage(image);
    }

    return () => {
      image.removeEventListener("load", handleLoad);
    };
  }, [src, h, w]);

  return (
    <Stage width={w} height={h} style={{ background: "black" }}>
      <Layer>
        <Image x={pos.x} y={pos.y} image={image} />
        <Text
          text="I am centered"
          fontFamily="Impact"
          fontSize={50}
          stroke="white"
          strokeWidth={1}
          x={pos.x}
          y={pos.y}
        />
      </Layer>
    </Stage>
  );
}

Answer 2

In this scenario, CSS alone is not sufficient to achieve the desired outcome. When an image is placed onto a canvas using specific height and width coordinates (x and y), the pixels of the image blend into the rasterized canvas. Essentially, the image loses its independent identity outside of the canvas.

To center the image within the canvas, some mathematical calculations are necessary to determine the correct x and y coordinates that will position the image at the center of the canvas.

Interactive Demo

For instance, if your canvas is 500px tall and the image's height is 350px, you should set the y position to 75px (i.e., (500 - 350) / 2).

The demonstration code below exemplifies how to replicate the behavior of CSS object-fit: contain. This functionality adjusts the image to fill the canvas in one direction while centering it in the other direction.

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

function Example() {
  const w = window.innerWidth;
  const h = window.innerHeight;
  const src = "https://konvajs.org/assets/yoda.jpg";

  const [image, setImage] = useState(null);
  const [pos, setPos] = useState({ x: 0, y: 0 });

  useEffect(() => {
    const image = new window.Image();
    image.src = src;
    image.addEventListener("load", handleLoad);

    function handleLoad(event) {
      const image = event.currentTarget;
      /* after the image is loaded, you can get it's dimensions */
      const imgNaturalWidth = image.width;
      const imgNaturalHeight = image.height;

      /* 
        calculate the horizontal and vertical ratio of the 
        image dimensions versus the canvas dimensions
      */
      const hRatio = w / imgNaturalWidth;
      const vRatio = h / imgNaturalHeight;

      /*
        To mimic CSS Object-Fit behaviour like "contain," 
        select the smaller of the horizontal and vertical ratios.
        
        For a "cover" effect, use Math.max instead.
      */
      const ratio = Math.min(hRatio, vRatio);
      /* 
        Scale the image to fit the canvas 
      */
      image.width = imgNaturalWidth * ratio;
      image.height = imgNaturalHeight * ratio;

      /* 
        Calculate offsets for centering the image inside the canvas
      */
      const xOffset = (w - image.width) / 2;
      const yOffset = (h - image.height) / 2;

      setPos({
        x: xOffset,
        y: yOffset
      });
      setImage(image);
    }

    return () => {
      image.removeEventListener("load", handleLoad);
    };
  }, [src, h, w]);

  return (
    <Stage width={w} height={h} style={{ background: "black" }}>
      <Layer>
        <Image x={pos.x} y={pos.y} image={image} />
        <Text
          text="I am centered"
          fontFamily="Impact"
          fontSize={50}
          stroke="white"
          strokeWidth={1}
          x={pos.x}
          y={pos.y}
        />
      </Layer>
    </Stage>
  );
}

Tips for aligning items in the center of konvajs

Answer №1

Interactive Demo

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