Two arrow heads for polyline arrow material

[pjcozzi]

Run the polyline Sandcastle example, switch to Columbus view, and zoom in on the polyline with the arrow material. On AMD at least, I see two arrows:

[raviagrwl420]

I am unable to reproduce this bug is there something I'm doing wrong?
I think it may have something to do with AMD as you said..

[bagnell]

@macoda The bug is in the latest version in master.

@pjcozzi I was able to reproduce the bug on NVidia. It happens when the line is drawn in multiple frustums and the line is clipped to the near plane of the farther frustum.

[raviagrwl420]

Thanks @bagnell I was able to reproduce the bug. It is present not just in the columbus view but also in the 3D Globe view.

[nmschulte-aviture]

It appears this bug still exists. This is from the Materials Sandcastle example.

The distance to the line matters, as does the angle:

And, I was able to get it to render more than two arrow heads at extreme angles:

This is on Intel hardware (GNU/Linux, Chromium v43), though it seems this is something in the material [shader] and not hardware specific.

OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Haswell Mobile 
OpenGL core profile version string: 3.3 (Core Profile) Mesa 10.5.8

[hpinkos]

@bagnell this happens in 3D and columbus view

Here's an example in 3D:

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

var polylines = scene.primitives.add(new Cesium.PolylineCollection());
var polyline = polylines.add({
    positions : Cesium.PolylinePipeline.generateCartesianArc({
        positions : Cesium.Cartesian3.fromDegreesArray([-110.0, 42.0,
                                                        -85.0, 36.0,
                                                        -100.0, 25.0,
                                                        -77.0, 12.0])
    }),
    width : 10.0,
    material: Cesium.Material.fromType('PolylineArrow')
});

viewer.camera.setView({
    destination: new Cesium.Cartesian3(1404234.8527806934, -6079977.723369969, 1316874.3368459751),
    orientation: {
        heading: 5.474598641469072,
        pitch: -0.3385580768913532,
        roll: 6.282147026410673
    }
});

[bagnell]

The problem is clipping the line segments to the near planes of the multifrustum. For example, if the near plane of the multifrustum intersects a line segment and vertices of the line segment have the x texture coordinates 0.0 and 1.0, the vertex with x texture coordinate is 1.0 could be clipped to the near plane. This would draw the arrow head at the near plane in one frustum and again in the closer frustum.

The solution would be to add extra texture coordinates for the adjacent vertices and interpolate at the clipped point. Another idea is to have all of the vertex data in the batch table and add indices to look up the vertex data at the current vertex and the adjacent vertices.

[hpinkos]

Also reported here: https://groups.google.com/forum/#!topic/cesium-dev/IsF-ljI1IE0

[ggetz]

Related forum thread: https://groups.google.com/forum/#!searchin/cesium-dev/line$20clipping%7Csort:date/cesium-dev/VmvDX4nU_bM/2hAIMzW_BAAJ

[ggetz]

Looks like this was mostly resolved with log depth as I don't see multiple arrowheads, but there are some artifacts on the arrowhead itself at extreme angles:

[hpinkos]

No, I definitely saw two arrow heads last week sometime

[mramato]

Log depth reduces the likelyhood of multiple frustums, which makes the bug less command, but there will always be cases with multiple frustums (especially when log depth off) so the root cause of the issue is still there.