Andor Salga

OBJModel obj;

void setup(){
  size(100, 100, P3D);
  obj = new OBJModel();
  obj.load("object.obj");
}

void draw(){
  obj.drawMode(POLYGON);
}

PShape obj;

void setup() {
  size(100, 100, P3D);
  obj = loadShape("object.obj");
}

void draw() {    
  shape(obj);
}

Shadows in WebGL Part 1 December 12, 2011

Posted by Andor Salga in FSOSS, Open Source, point cloud, webgl, XB PointStream.
4 comments

RUN ME!

In my last blog I wrote about an anaglyph demo I created for my FSOSS presentation in October. It was part of a series of delayed blogs which I only recently had time to write up. So, in this blog I’ll be proceeding with my next fun experiment: Shadows in WebGL.

Shadows are useful since they not only add realism, but can also provide additional visual cues in a scene. Having never implemented any type of shadows, I started by performing some preliminary research and found that there are numerous methods to achieve this effect. Some of the more common techniques include:

• vertex projection
• projected planar shadows
• shadow mapping
• shadow volumes

I chose vertex projection since it seemed very straightforward. After a few sketches, I got a fairly good grasp of the idea. Given the position for a light and vertex, the shadow cast (for that vertex) will appear at the line intersection between the slope created by those points and the x-intercept. If we had the following values:

• Light = [4, 4]
• Vertex = [1, 2]

Our shadow would be drawn at [-2, 0]. Note that the y component is zero and would be equal to zero for all other vertices since we’re concentrating on planar shadows.

At this point, I understood the problem well; I just needed a simple formula to get this result. If you run a search for “vertex projection” and “shadows” you’ll find a snippet of code on GameDev.net which provides the formula for calculating the x and z components of the shadow. But if you actually try it for the x component:

It doesn’t work.

When I ran into this, I had to take a step back to think about the problem and review my graphs. I was convinced that I could contrive a working formula that would be just as simple as the one above. So I conducted additional research until I eventually found the point-slope equation of a line.

Point-Slope Equation

The point-slope equation of a line is useful for determining a single point on the slope give the slope and another point on the line. This is exactly the scenario we have!

Where:
m – The slope. This is known since we have two given points on the line: the vertex and the light.

[x1, y1] – A known point on the line. In this case: the light.

[x, y] – Another point on the line which we’re trying to figure out: the shadow.

Since the final 3D shadow will lie on the xz-plane, the y components will always be zero. We can therefore remove that variable which gives us:

Now that the only unknown is x, we can start isolating it by dividing both sides by the slope:

Which gives us:

And after rearranging we get our new formula, but is it sound?

If we use the same values as above as a test:

It works!

I now had a way to get the x component for the shadow, but what about the z component? What I did so far was create a solution for shadows in 2 dimensions. But if you think about it, both components can be broken down into 2 2D problems. We just need to use the z components for the light and point to get the z component of the shadow.

Shader Shadows

The shader code is a bit verbose, but at the same time, very easy to understand:

void drawShadow(in vec3 l, in vec4 v){
  // Calculate slope.
  float slopeX = (l.y-v.y)/(l.x-v.x);
  float slopeZ = (l.y-v.y)/(l.z-v.z); 

  // Flatten by making all the y components the same.
  v.y = 0.0;
  v.x = l.x - (l.y / slopeX);
  v.z = l.z - (l.y / slopeZ);

  gl_Position = pMatrix * mVMatrix * v;
  frontColor = vec4(0.0, 0.0, 0.0, 1.0);
}

Double Trouble

The technique works, but its major issue is that objects need to be drawn twice. Since I’m using this technique for dense point clouds, it significantly affects performance. The graph below shows the crippling effects of rendering the shadow of a cloud consisting of 1.5 million points—performance is cut is half.

Fortunately, this problem isn’t difficult to address. Since detail is not an important property for shadows, we can simply render the object with a lower level of detail. I had already written a level of detail python script which evenly distributes a cloud between multiple files. This script was used to produce a sparse cloud—about 10% of the original.

Matrix Trick

It turns out that planar shadows can be alternatively rendered using a simple matrix.

void drawShadow(in vec3 l, in vec4 v){
  // Projected planar shadow matrix.
  mat4 sMatrix = mat4 ( l.y,  0.0,  0.0,  0.0, 
                       -l.x,  0.0, -l.z, -1.0,
                        0.0,  0.0,  l.y,  0.0,
                        0.0,  0.0,  0.0,  l.y);

  gl_Position = pMatrix * mVMatrix * sMatrix * v;
  frontColor = vec4(0.0, 0.0, 0.0, 1.0);
}

This method doesn’t offer any performance increase versus vertex projection, but the code is quite terse. More importantly, using a matrix opens up the potential for drawing shadows on arbitrary planes. This is done by modifying all the elements of the above matrix.

Future Work

Sometime in the future I’d like to experiment with implementing shadows for arbitrary planes. After that I can begin investigating other techniques such as shadow mapping and shadow volumes. Exciting! (:

Anaglyph Point Clouds November 18, 2011

Posted by Andor Salga in FSOSS, Open Source, point cloud, webgl, XB PointStream.
4 comments

See me in 3D!

A couple of weeks ago I gave a talk at FSOSS on XB PointStream. For my presentation I wanted to experiment and see what interesting demos I could put together using point clouds. I managed to get a few decent demos complete, but I didn’t have a chance to blog about them at the time. So I’ll be blogging about them piecemeal for the rest of the month.

The first demo I have is an anaglyph rendering. Anaglyphs are one way to give 2D images a depth component. The same object is rendered at two slightly different perspectives using two different colors. Typically red and cyan (blue+green) are used.

The user wears anaglyph glasses, which have filters for both colours. A common standard is to use a red filter for the left eye and a blue filter for the right eye. These filters ensure each eye only sees one of the superimposed perspectives. The mind them merges these two images into a single 3D object.

Method

There are many ways to achieve this effect. One method which involves creating two asymmetric frustums can be found here. However, you can also create the effect by simply rotating or translating the object. It isn’t as accurate, but it’s very easy to implement:

// ps is the instance of XB PointStream
// ctx is the WebGL context

ps.pushMatrix();
// Yaw camera slightly for a different perspective
cam.yaw(0.005);
// Create a lookAt matrix. Apply it to our model view matrix.
ps.multMatrix(M4x4.makeLookAt(cam.pos, V3.add(cam.pos, cam.dir), cam.up));   
 
// Render the object as cyan by using a colour mask.
ctx.colorMask(0,1,1,1);
ps.render(pointCloud);
ps.popMatrix();
  
// Preserve the colour buffer but clear the depth buffer
// so subsequent points are drawn over the previous points.
ctx.clear(ctx.DEPTH_BUFFER_BIT);

ps.pushMatrix();
// Restore the camera's position for the other perspective.
cam.yaw(-0.005);
ps.multMatrix(M4x4.makeLookAt(cam.pos, V3.add(cam.pos, cam.dir), cam.up));   

// Render the object as red by using a colour mask.
ctx.colorMask(1,0,0,1);
ps.render(pointCloud);
ps.popMatrix();

Future Work

I hacked together the demo just in time for my talk at FSOSS, but I was left wondering how much better the effect would look if I had created two separate frustums instead. For this I would need to expose a frustum() method for the library. I can’t see a reason not to add it considering this is a perfect use case, so filed!