Hidden scatterplot instance throws an error when shown interactively

[plankter]

I could reproduce the issue #17. The problem occurs when plot is created, but hidden (in my case i created multiple plots in different Vuetify tabs). Probably this is due to canvas.getBoundingClientRect() returning (0, 0) somewhere in regl-scatterplot code in such cases.

I created a small demo to show this behavior. Second canvas is hidden, and then should be displayed when one makes a selection on the first canvas. But instead it'll throw errors like the following when mouse is over second canvas:

Uncaught TypeError: Cannot read property '0' of null
    at Module.transformMat4 (vec4.js:529)
    at getScatterGlPos (index.js:201)
    at raycast (index.js:206)
    at mouseMoveHandler (index.js:372)

Sorry if the demo is a bit awkward.

https://github.com/plankter/regl-scatterplot-two-instances

[flekschas]

Thanks for providing a demo. I'll take a look.

By the way, is this essentially the same as #17?

[plankter]

Yes, i think it is the same issue.

[flekschas]

I am not sure why you decided to use .hidden but I don't think it should be used in this scenario: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/hidden

This is quite different from using the CSS property display to control the visibility of an element. The hidden property applies to all presentation modes and should not be used to hide content that is meant to be directly accessible to the user.

Inappropriate use cases include:

• Hiding panels in a tabbed dialog box
• Hiding content in one presentation while intending it to be visible in others

When I replace canvas2.hidden = true; with canvas2.style.opacity = 0;, everything works as expected.

I'll try to find out why but I also suspect it's because the canvas element is 0x0 pixels in size.

[flekschas]

It seems to be related to the fact that canvas element does not have a size. Everything works also fine when I use canvas2.style.visibility = 'hidden'.

It appears as if canvas.hiddenputs the canvas element into a not-normal state. Do you know what the functional difference is to visually just hiding the canvas element?

[plankter]

I was trying to replicate behavior of Vue's v-show conditional rendering (https://vuejs.org/v2/guide/conditional.html#v-show) and also Vuetify uses the same approach in their Tabs component (https://vuetifyjs.com/en/components/tabs/).

In my application i have multiple tabs with separate regl-scatterplot instances. When user makes a selection on one plot, other plots are updated as well (i.e. subsetting tSNE vs PCA plots) because of cross-filtering. I discovered that when both plots are visible, then everything works properly, but if i use tabs or v-show directive, then plots, although they were initialized, are updated, but any interaction with them throws the mentioned exception.

Plots have zero width and height when hidden due to getBoundingClientRect() method, i suppose. Maybe predefined size can be used instead? I mean the size defined in createScatterplot method.

UPDATE: you are right about hidden property, but according to Vue docs:

The difference is that an element with v-show will always be rendered and remain in the DOM; v-show only toggles the display CSS property of the element.

and i still have this problem with v-show directive.

[plankter]

Here is a short screencast demonstrating the issue. As you can see, the first plot (with cell background image) is updated properly according to cell mask after i select cells on the PCA plot, but any mouse movement (zoom/panning) is throwing an error (can be seen on console panel):

https://drive.google.com/file/d/1HfXmnEAiiVFT61rKoUMlgBZiQzCIq2Yo/view

[flekschas]

Okay, the issue is that you need to call scatterplot.set({ width, height }). Why? The canvas resized after you do canvas.hidden = false;. regl-scatterplot doesn't recognize resize events on its own, that's the responsibility of the wrapper application.

E.g.:

canvas2.hidden = false;
window.requestAnimationFrame(() => {
  const { width, height } = canvas2.getBoundingClientRect();
  // Second scatterplot
  scatterplots[1].set({ width, height });
});

Besides, another simple fix is to manually set a proper width and height before hiding the canvas.

E.g., the following will work fine:

window.requestAnimationFrame(() => {
  const { width, height } = canvas2.getBoundingClientRect();
  canvas2.width = width * window.devicePixelRatio;
  canvas2.height = height * window.devicePixelRatio;
  canvas2.hidden = true;
});

The error was caused by dom-2d-camera because the camera's bounds weren't updated and, for example, for zooming one would divide by height, which was still 0 and therefore resulted in a NaN.

I'll update the README.md to make it more obvious that the wrapping application needs to handle the resizing.

[flekschas]

Let me know if this solves your issue.

[plankter]

Thank you for the suggestion, it really worked. I found out that Vuetify has a very useful directive v-intersect (https://vuetifyjs.com/en/directives/intersect/) that is using Intersection Observer browser API. I just added v-intersect="onIntersect" to canvas element with a method:

onIntersect(entries, observer, isIntersecting) {
  if (isIntersecting) {
    const canvas = this.$refs.canvas as Element;
    const { width, height } = canvas.getBoundingClientRect();
    this.scatterplot.set({ width, height });
  }
}

And issue is gone! Thanks again for your time and efforts.

[flekschas]

Awesome! I am glad it's working now. I added a note to the README.md to give people a hint.