How to create a Three.js 3D line series with width

问题:

Is there a way to create a Three.js 3D line series with width and thickness?

Even though the Three.js line object supports linewidth, this attribute is not yet supported in all browsers on all platforms in WebGL.

Here's where you set linewidth in Three.js:

    var material = new THREE.LineBasicMaterial({
        color: 0xff0000,
        linewidth: 5
    });

The Three.js ribbon object - which had width - has recently been dropped.

The Three.js tube object generates 3D extrusions but - being Bezier-based - the lines do not pass through the control points.

Can anybody think of a method of drawing a line series (polylines, plotlines) in Three.js that has some sort of user definable 'bulk' such as width, thickness or radius?

This question may be a restating of this question: Extruding a graph in three.js.

Given that I do not think that there is a readily available method, I would be happy to participate in an effort to create a simple function that responds to this question.

But a response that points to an existing workable method would be cool...

As WestLangley suggests, one possible solution includes the polyline being of constant pixel width - as is currently available with the Three.js canvas renderer.

A comparison of the two renderers is shown here:

Canvas and WebGL Lines Compared via GitHub Pages

Canvas and WebGL Lines Compared via jsFiddle

A solution where you could specify linewidth and similar results occurred on both renderers would be very cool.

There are, however, other ways of thinking of 3D lines where lines have actual physical constructs. They cast shadows, they respond to events. These also need to be looked into.

Here are links to GitHub Pages with two demos of lines made up of multiple meshes:

Sphere and Cylinder Polylines

An 'expensive solution. Each joint is made up of a full sphere.

Cubes Polylines

My guess is that building either of these as smooth single meshes will be complex to problems to solve. So in the meantime here is a link to a partial visualization of 3D lines that are wide and have height:

3D Box Line on jsFiddle

The goal is have to code 'with a low level of complexity - in other words - for dummies'. Thus a 3D line should be as easy and as familiar as adding a sphere or cube. Geometry + material = mesh > scene. And the geometry should be quite economical in terms of creating vertices and faces.

The lines should have width and height. Up is always in the Y direction. The demo shows this. What the demo does not show is corners being mitred nicely...

回答1:

I cooked up a possible solution which I believe meets most of your requirements:

http://codepen.io/garciahurtado/pen/AGEsf?editors=001

The concept is fairly simple: render any arbitrary geometry in "wireframe mode", then apply a full screen GLSL shader to it to add thickness to the wireframe lines.

The shader is inspired by the blur shaders in the ThreeJS distro, which essentially copy the image a bunch of times along the horizontal and vertical axis. I automated that process and made the number of copies a user defined parameter, while ensuring that the copies were offset by 1 pixel.

I used a 3D cube mesh in my demo (with an ortho camera), but it should be trivial to convert it to a poly line.

The real meat and potatoes of this thing is in the custom shader (fragment shader portion):

    uniform sampler2D tDiffuse;
    uniform int edgeWidth;
    uniform int diagOffset;
    uniform float totalWidth;
    uniform float totalHeight;
    const int MAX_LINE_WIDTH = 30; // Needed due to weird limitations in GLSL around for loops
    varying vec2 vUv;

    void main() {
        int offset = int( floor(float(edgeWidth) / float(2) + 0.5) );
        vec4 color = vec4( 0.0, 0.0, 0.0, 0.0);

        // Horizontal copies of the wireframe first
        for (int i = 0; i < MAX_LINE_WIDTH; i++) {
            float uvFactor = (float(1) / totalWidth);
            float newUvX = vUv.x + float(i - offset) * uvFactor;
            float newUvY = vUv.y + (float(i - offset) * float(diagOffset) ) * uvFactor;  // only modifies vUv.y if diagOffset > 0
            color = max(color, texture2D( tDiffuse, vec2( newUvX,  newUvY  ) ));    
            // GLSL does not allow loop comparisons against dynamic variables. Workaround below
            if(i == edgeWidth) break; 
        }

        // Now we create the vertical copies
        for (int i = 0; i < MAX_LINE_WIDTH; i++) {
            float uvFactor = (float(1) / totalHeight);
            float newUvX = vUv.x + (float(i - offset) * float(-diagOffset) ) * uvFactor; // only modifies vUv.x if diagOffset > 0
            float newUvY = vUv.y + float(i - offset) * uvFactor;
            color = max(color, texture2D( tDiffuse, vec2( newUvX, newUvY ) ));  
            if(i == edgeWidth) break;
        }

        gl_FragColor = color;
    }

Pros:

• No need for additional geometry beyond the line vertices
• Line thickness is user definable
• A full screen shader should be relatively gentle on the GPU
• Can be implemented fully within the WebGL canvas

Cons:

• Line thickness is close to pixel perfect on horizontal and vertical edges, but slightly off on diagonal edges. This is due to the algorithm used and is a limitation of the solution. Having said that, for low line thickness and complex geometries, this is barely noticeable with the naked eye.
• The joints between lines will show gaps for large enough line thickness. You can play with the Codepen demo to see what I mean. I started to implement a solution to this by adding a second "diagonal pass", but it got a little hairy and I think this would only be an issue for higher line thicknesses (+8 pixels) or extreme line angles. If you are interested in this solution, you can look at the original source to see where I was going with it.
• Since this uses a full screen filter, you can only use the WebGL context for displaying objects of this thickness. Showing various line widths would require additional rendering passes.

回答2:

As a potential solution. You could take your 3d points, then use THREE.Vector3.project method to figure out screen-space coordinates. Then simply use canvas and it's lineTo and moveTo operations. Canvas 2d context does support variable line thickness.

var w = renderer.domElement.innerWidth;
var h = renderer.domElement.innerHeight;
vector.project(camera);
context2d.lineWidth = 3;
var x = (vector.x+1)*(w/2);
var y = h - (vector.y+1)*(h/2);
context2d.lineTo(x,y);

Also, i don't think you can use the same canvas for that, so it would have to be a layer (another canvas) above your gl rendering context canvas.

If you have infrequent camera changes - it is also possible to construct line out of polygons and update it's vertex positions based on camera transform. For orthographic camera this would work best as only rotations would require vertex position manipulation.

Lastly, you could disable canvas clearing and draw your lines several times with offset inside a circle or a box. After that you can re-enable clearing. This would require several extra draw operations, but it's probably the most scalable approach.

The reason lines don't work as you'd expect out of the box is due to how ANGLE works, it's used in Chrome and in Firefox to my knowledge, it emulates OpenGL via DirectX. Guys from ANGLE state that WebGL spec only requires support of line thickness up-to 1, so they do not see it as a bug and don't intend to "fix" it. Line thickness should work on non-windows OSs though, where ANGLE is not used.