Exploring the Canvas with Full Element Panning, Minimap Included

Currently, I am working on incorporating a mini map onto my canvas that mirrors what is displayed on the main canvas. The main canvas includes zoom and pan functions. I have created a rectangular shape for the minimap to display the content of the canvas. However, when I pan or zoom in on the main canvas, the same actions are applied to the minimap as well.

Below is the draw method code which renders both the canvas and the minimap:

    redraw();
    context.save();
    context.translate(canvas.width / 2, canvas.height / 2);
    canvas.style.border = "red 1px solid";
    object.mPosition = new Vector(0,0);
    object.draw(context);
    context.restore();
    requestAnimationFrame(draw);
    context.save();
    context.beginPath();
    context.lineWidth = 3;
    context.strokeStyle="black";
    context.scale(0.25,0.25);
    context.rect(0,0,canvas.width,canvas.height);
    context.fillStyle="white";
    context.fillRect(0,0,canvas.width,canvas.height);
    context.stroke();
    context.clip();
    context.translate(canvas.width / 2, canvas.height / 2);
    object.draw(context);
    context.closePath();
    context.restore();

The following code demonstrates how panning is implemented:

function OnMouseMove(event) {
    var mousePosition = new Vector(event.clientX, event.clientY);
    if(leftMouseButton) {
        context.translate(mousePosition.getX() - previousMousePosition.getX(), mousePosition.getY() - previousMousePosition.getY());
    }
    previousMousePosition = mousePosition;
}

I am seeking advice on how to restrict the panning behavior to only affect the main canvas and its contents, without applying it to the minimap. Is there a way to achieve this?

Answer №1

Method 1

  • Reset all transformations to default (ctx.setTransform(1,0,0,1,0,0);)
  • Display mini-map at coordinates (0,0)
  • Apply translation
  • Render items

Keep track of the translation in separate variables (x/y) and use them for translating.

Method 2

  • Draw the mini-map once

  • Set canvas as its own CSS background:

    canvas.style.background = "url(" + canvas.toDataURL() + ")";

  • Clear and render items on each update without worrying about the mini-map (repeat first two steps if needed).

Method 3

  • Keep track of x/y values for current translation using variables
  • Clear and render mini-map after initially translating canvas by the negative of x/y (to position at (0,0)). ctx.translate(-x, -y);
  • Reapply translation and render items

Method 4

  • Store current translation values in x/y
  • Display mini-map at (0,0)
  • Render all items offset by x/y

This method requires more code as each item needs to be manually translated. While not recommended due to availability of translate function, it is included here for completeness.

Method 5

  • Render all items onto a large off-screen canvas that can accommodate them
  • Render mini-map
  • Overlay item canvas (as an image) with translation offset or just utilize x/y in this case

This technique could be combined with #2, but may consume more memory. Use discretion based on total canvas size.

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