Billboard.prototype.computeScreenSpacePosition returning wrong y position

[e-andersson]

The behavior of computeScreenSpacePosition seems to have changed recently (there seem to be a few new commits changing Billboard.js). It looks like the y screen coordinate is the inverse of what it used to be. According to the API docs, the y coordinate increases from top to bottom.

The attached Billboards sandcastle example prints the screen positions and the computed screen positions of the clicked billboard.

Am I possibly missing something here?
sandcastle_compute_screenposition.txt

[pjcozzi]

@bagnell any idea here? Is this related to 651a8b2#diff-97a34c40d0a290b6fab62f87f92bcc6f or dae0242#diff-97a34c40d0a290b6fab62f87f92bcc6f?

[emackey]

Git bisect says this went bad in 651a8b2.

[mramato]

Given this has been brought up several times this month, we should probably try to get to this before the next release.

[emackey]

Here's some test code. A small div should float in the middle of the Cesium logo, and not go to the top of the screen when the logo is at the bottom of the screen and vice-versa.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var billboards = scene.primitives.add(new Cesium.BillboardCollection());
var b = billboards.add({
    image : '../images/Cesium_Logo_overlay.png',
    position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883)
});

var floater = document.createElement('div');
floater.innerHTML = 'Test';
floater.style.cssText = 'position:absolute;border: 1px solid #aaa;background:black';
document.body.appendChild(floater);

var scratchPos = new Cesium.Cartesian2();
viewer.clock.onTick.addEventListener(function(clock) {
    var pos = b.computeScreenSpacePosition(scene, scratchPos);
    if (pos) {
        floater.style.display = 'block';
        floater.style.top = pos.y + 'px';
        floater.style.left = pos.x + 'px';
    } else {
        floater.style.display = 'none';
    }
});

[mramato]

So is this just a one-character change to negate y?

[emackey]

It's basically height - y, yes. We should probably check that the solution works when the canvas has a different number of CSS pixels from canvas pixels, unless we're explicitly giving the answer in canvas pixels with CSS-oriented coordinates. Also, several other classes besides Billboard offer a computeScreenSpacePosition, they should be tested too.

[hpinkos]

Possibly related: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/2LkqgI7LemA

He says Label.computeScreenSpacePosition works in 3D but not 2D

[pjcozzi]

@bagnell can you look at this in time for the release next week?