Bug: Execution interrupted while drawing

[espruino-discuss2]

Posted at 2023-08-10 by Frickolas

First, I should mention that I'm pretty new to JS, so forgive me if this is a rookie mistake. I'm trying to make a custom clock for my bangle.js 2, and it involves inverting the colors of portions of the screen. I wrote a function that iterates over the pixels in a given circle and inverts them:

// swaps background and foreground colors inside any given circle
function invertCircle(x, y, rad) {
  rad = Math.round(rad);
  sw = g.getWidth() - 1;
  bg = g.getBgColor();
  fg = g.getColor();
  
  // iterate over every onscreen pixel within the radius
  let iEnd = Math.min(g.getHeight() - 1, y + rad);
  for (let i = Math.max(0, y - rad); i <= iEnd; i ++) {
    
    let chord = Math.round(Math.sqrt(Math.pow(rad, 2) - Math.pow(Math.abs(i - y), 2)));
    let jEnd = Math.min(sw, x + chord);
    for (let j = Math.max(0, x - chord); j <= jEnd; j ++) {
      
      // invert pixel color
      if (g.getPixel(j, i) == bg) {
        g.setPixel(j, i, fg);
      } else {
        g.setPixel(j, i, bg);
      }
    }
  }
}

// draw test image
g.clear();
g.setFont("Vector", 16);
g.drawLine(0, g.getHeight() / 2, g.getWidth(), g.getHeight() / 2);
g.drawString("Test\nImage", g.getWidth() / 2, g.getHeight() / 2 - 15);

invertCircle(g.getWidth() / 2, g.getHeight() / 2, 30);

When inverting larger areas, execution gets halted a few hundred milliseconds into the drawing process.

Execution Interrupted
at line 1 col 319
...(j,i)==bg){g.setPixel(j,i,fg);}else{g.setPixel(j,i,bg);}}}
                                  ^
in function "invertCircle" called from line 1 col 53
invertCircle(g.getWidth() / 2, g.getHeight() / 2, 30);
                                                    ^
>echo(1)
=undefined
>

I'm guessing that this has something to do with hogging the CPU for too long, but I'm not really sure. Any advice on how to avoid halting execution and / or make this more efficient would be appreciated.

[espruino-discuss2]

Posted at 2023-08-11 by @gfwilliams

Yes, I think likely what's happening is the IDE uploads the code and then doesn't get a response after a second or so, so it issues a Ctrl-C to break out of it.

You could do:

setTimeout(function() {
  invertCircle(g.getWidth() / 2, g.getHeight() / 2, 30);
}, 1000);

to run the command after a delay so the IDE can upload the code fine and see a response, but then the code happens after.

BUT: iterating over every pixel is always going to be really slow I'm afraid - it's probably not something you want to be doing if there's any way to avoid it. What I'd suggest is actually making two images - one with what you want to draw and one with the circle, and XORing the actual binary data of the image together.

ga = Graphics.createArrayBuffer(176,176,1,{msb:true});
gb = Graphics.createArrayBuffer(176,176,1,{msb:true});
var ba = new Uint32Array(ga.buffer);
var bb = new Uint32Array(gb.buffer);

ga.setFont("Vector:80").drawString("Hello");
gb.fillCircle(40,40,40);

// define this once so JIT only runs once
function xor(a,i) { "jit"
  return a ^ bb[i];
}

function update() {
  var t = getTime();
  E.mapInPlace(ba, ba, xor);
  g.drawImage(ga); // draws with current FG and BG colors
  print("took",getTime()-t,"sec");
}
update();

There are other ways to do it (eg compiled code) but this way a relatively straightforward.

It still takes ~0.4sec to do the entire screen, but if you only wanted part of the screen done then it'd be a bit faster.

I was considering adding different draw styles (add/xor/or/etc) to the Graphics lib which would really help with this, but I'm afraid that isn't implemented yet.

---

Posted at 2023-08-11 by stweedo

I was trying to implement something similar for a stop watch app except mine was a rectangle, not a circle, so I was able to use g.setClipRect.

Maybe there could also be a g.setClipCircle?

---

Posted at 2023-08-14 by @gfwilliams

That feels like it's quite a rare thing to want to do - I could imagine maybe having a 'mask' image where you set the areas you want to write to though?

...but personally I'd have thought that extending drawImages would be better - so we could allow each image to have a 'blend mode', much like you might blend layers in photoshop/Gimp.

In the case above, you could have an XOR mode, but you could also do AND/OR or even just using an image to set the transparency, which would do what you want too

---

Posted at 2023-08-14 by stweedo

Yeah, I was using setClipRect as a mask essentially and layering the draws to create the animation.

const SCREEN_WIDTH = g.getWidth();
const SCREEN_HEIGHT = g.getHeight();
const BOX_HEIGHT = 50;
const NUMBER_SIZE = 30;
const ANIMATION_DURATION = 3000;

const BOX_DIMENSIONS = {
  left: 0,
  top: SCREEN_HEIGHT / 2 - BOX_HEIGHT / 2,
  width: SCREEN_WIDTH,
  height: BOX_HEIGHT
};

let numberValue = 0;
let startTime = 0;
let animationInterval;

function setRect(method) {
  g[method](BOX_DIMENSIONS.left, BOX_DIMENSIONS.top, BOX_DIMENSIONS.width, BOX_DIMENSIONS.top + BOX_DIMENSIONS.height);
}

function drawNumber(yPosition, color) {
  g.setColor(color);
  g.setFont("Vector", NUMBER_SIZE);
  g.drawString(numberValue.toString(), SCREEN_WIDTH / 2 - g.stringWidth(numberValue.toString()) / 2, yPosition);
}

function animateNumber() {
  const now = Date.now();
  const progress = Math.min(1, (now - startTime) / ANIMATION_DURATION);
  g.clear();

  const currentY = (SCREEN_HEIGHT - NUMBER_SIZE) * progress;

  drawNumber(currentY, g.theme.fg);

  setRect('fillRect');
  setRect('setClipRect');

  drawNumber(currentY, g.theme.bg);

  g.setClipRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
  
  g.flip();

  if (progress < 1) {
    animationInterval = setTimeout(animateNumber, 1000 / 60);
  } else {
    numberValue = Math.floor(Math.random() * 100);
    startTime = Date.now();
    animationInterval = setTimeout(animateNumber, 500);
  }
}

startTime = Date.now();
animateNumber();

But it sounds like extending drawImages like you said could be way more versatile.