Refreshing a Canvas Image

Plugin builders
1

I’ve been working on a custom plugin for image processing, specifically for removing certain colors from an image. The plugin uses a canvas to load and manipulate the image. Unfortunately, I’ve hit a roadblock and I’m hoping someone in the community might have some insights.

Here’s a brief overview of what the plugin does:

  1. An image is loaded onto a canvas.
  2. The user clicks on a color in the image.
  3. The plugin identifies the clicked color and makes it transparent, using either a flood fill algorithm or a global color replacement, based on the user’s choice.

The problem I’m facing is that the changes to the image don’t seem to be reflected on the canvas immediately after the color removal operation. The image data is indeed being modified (I’ve confirmed this by exporting the modified image data to an exposed state), but the canvas still displays the original, unmodified image.

Here’s the relevant part of my code:

function removeColor(event, context, canvas, tolerance, isolatedTransparency, addTransparency) {
    console.log('removeColor called');
    const x = event.offsetX;
    const y = event.offsetY;
    const clickedColor = context.getImageData(x, y, 1, 1).data;
    const imageData = context.getImageData(0, 0, canvas.width, canvas.height);

    console.log('Clicked color:', clickedColor);
    
    if (addTransparency) {
        if (isolatedTransparency) {
            floodFill(x, y, clickedColor, imageData, tolerance);
        } else {
            replaceColorWithTransparency(imageData, clickedColor, tolerance);
        }
    }

    // After modifying the image data
    context.putImageData(imageData, 0, 0);
    
    // Convert the modified image data to a data URL
    var dataURL = canvas.toDataURL();

    // Publish the data URL to the "exportedImage" state
    instance.publishState('exportedImage', dataURL);
    console.log('Modified color:', context.getImageData(x, y, 1, 1).data);
}

I’ve tried clearing the canvas with context.clearRect(0, 0, canvas.width, canvas.height) before using context.putImageData(imageData, 0, 0), but that didn’t seem to make a difference.

I’m wondering if there’s some sort of caching going on that’s causing the canvas to display the old image data. Or perhaps there’s something I’m missing about how the canvas or the putImageData method works.

Has anyone encountered a similar issue or have any ideas on what might be going wrong? Any help would be greatly appreciated!

2

Gotta ask the browser to redraw.

Try adding this: context.draw(); in your code:

    // After modifying the image data
    context.putImageData(imageData, 0, 0);

   // Force redraw on the browser
    context.draw()

Hopefully this works out for you.

Also, sidenote, try to stay away from var. Use let, and const.

3

Thanks for your help!

I tried your suggestion by adding context.draw() after context.putImageData(imageData, 0, 0); but it doesn’t look like the context.draw() method exists in the HTML canvas API. I did find a context.drawImage() method, so I tried redrawing the image onto the canvas after putting the image data, like this:

// After modifying the image data
context.putImageData(imageData, 0, 0);

// Redraw the image onto the canvas
context.drawImage(img, 0, 0);

Unfortunately, this didn’t seem to solve the problem. The image on the canvas still isn’t updating to reflect the changes made to the image data.

As for your suggestion to use let and const instead of var, I completely agree. Old habits and all. I’ll get it updated.

I’m still stuck with the original issue, so if you or anyone else has any other suggestions, I would greatly appreciate it. Thanks again for your help!

4

Have you tried this on a separate dev setup? Like codepen, or locally?

5

Try instance.canvas.empty(); at the beginning of update?

6

It turns out in my img.onload function I needed to clear the image before loading a new one. Thanks everyone for your help. It gave me some ideas. Here is how I solved it:

    // Clear any previous image from the background div
    while (bgDiv.firstChild) {
        bgDiv.firstChild.remove();
    }