如何使用three.js在运行时绘制线段 [英] How to draw a line segment at run time using three.js

问题描述

我是Three.js的新手，我正在尝试实现Microsoft Paint中用于绘制线段的技术。我正在尝试获取onMouseDown上一个点的坐标，然后用onMouseMove扩展一行直到onMouseDown。请帮忙！

I am new to Three.js and I'm trying to implement the technique used in Microsoft Paint for drawing a line segment. I'm trying to get coordinates of a point onMouseDown then extend a line with onMouseMove until onMouseDown. Please help!

推荐答案

three.js主要用于3D绘图。如果你想复制像绘画这样的2D绘图应用程序，那么使用2D画布可能会更容易： canvas.getContext（2d）; 。

three.js is mainly for drawing in 3D. If you want to copy a 2D drawing application like paint then using the 2D canvas will probably be easier: canvas.getContext("2d");.

我会假设您确实要绘制三个.js。在这种情况下，我将 此示例 放在一起。请按照以下步骤操作：

I will assume that you do want to draw in three.js. In which case I have put together this example. Follow these steps:

1. 单击页面上的任意位置并拖动鼠标以绘制线条。该线在z平面上绘制。


2. 单击形状按钮，注意一个形状是如何接近而另一个形状更接近，这是因为一个形状位于z平面上方，另一个位于z平面上方。


3. 单击旋转按钮，这将使相机缩小并围绕轴旋转。请注意，当您通过z平面时，所有绘图都在该平面上。

查看代码，主要部分是：

Have a look at the code, the main parts are:

您需要将鼠标单击坐标投影到平面上。这是通过此功能完成的：

You need to project the mouse click coordinates onto the plane. This is done with this function:

function get3dPointZAxis(event)
{
    var vector = new THREE.Vector3(
                ( event.clientX / window.innerWidth ) * 2 - 1,
                - ( event.clientY / window.innerHeight ) * 2 + 1,
                0.5 );
    projector.unprojectVector( vector, camera );
    var dir = vector.sub( camera.position ).normalize();
    var distance = - camera.position.z / dir.z;
    var pos = camera.position.clone().add( dir.multiplyScalar( distance ) );    
    return pos;
}

然后从上一个点开始画线：

Then draw the line from previous to this point:

geometry.vertices.push(lastPoint);
geometry.vertices.push(pos);
var line = new THREE.Line(geometry, material);
scene.add(line);

请注意，当您在旋转时接近通过z平面时，投影到Z非常为了防止这种情况发生以下情况，我们将完成以下检查：

Note that when you get close to passing through the z plane while rotating, the projection to Z is very far off and you go out of bounds, to prevent this the following check is done:

if( Math.abs(lastPoint.x - pos.x) < 500 && Math.abs(lastPoint.y - pos.y) < 500 && Math.abs(lastPoint.z - pos.z) < 500 )

作为参考，我找到了投影鼠标坐标的信息这里（SO答案）和这里（three.js样本）。

For reference, I found information for projecting the mouse coordinate here (SO answer) and here (three.js sample).

更新

要从mousedown位置绘制一条线到mouseup位置，请参阅 此演示 即可。代码更改只是在鼠标向上的点之间进行绘制。

To draw a line from the mousedown position to the mouseup position see this demo. The code changes are to instead just do the draw between points on mouse up.

function stopDraw(event)
{
     if( lastPoint )
    {
        var pos = get3dPointZAxis(event);
        var material = new THREE.LineBasicMaterial({
            color: 0x0000ff
        });
        var geometry = new THREE.Geometry();
        if( Math.abs(lastPoint.x - pos.x) < 2000 && Math.abs(lastPoint.y - pos.y) < 2000 && Math.abs(lastPoint.z - pos.z) < 2000 )
        {
            geometry.vertices.push(lastPoint);
            geometry.vertices.push(pos);

            var line = new THREE.Line(geometry, material);
            scene.add(line);
            lastPoint = pos;        
        }
        else
        {
            console.debug(lastPoint.x.toString() + ':' + lastPoint.y.toString() + ':' + lastPoint.z.toString()  + ':' + 
                        pos.x.toString() + ':' + pos.y.toString()  + ':' + pos.z.toString());
        }
    }
}      

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