Barely Functional Theories

Musings on science and game design by James Furness.



Project Tanks 1: Simple Fake-3D Wireframes


The 3D tank

With Probus finished I’ve started out building a new game to keep me busy at the weekends, “Project: Tanks”. I’m sticking with the HTML5 canvas platform again, but aiming at PC web browsers this time. I’m hoping to play around with supporting player made content in this one, so I set out on making a slower paced ‘twin stick’ shooter inspired by the Wii Tanks game I enjoyed playing with my brother years ago.

I had a basic engine running pretty quickly and filled it with programmer art using the vector drawing functions of the HTML5 canvas. I intended to update this ‘art’ later in development but to my surprise by applying a simple technique to turn the dull 2D shapes into ‘3D’ wireframes I was able to make the programmer art into the games full visual style. I was so happy with the results that I wanted to share the technique in this post.

Over the course of the article we are going to develop a drawing routine to take a Tank model from the humble beginnings shown on the left, to the glorious ‘3D’ seen on the right.

The drawing functions will be detailed in pseudocode along the way and the Javascript that powers the examples can be downloaded here for reference.


The Oblique Projection

The 3D effect will mimic a top-down oblique projection of our models in order to draw our 3D models on the canvas. This projection technique works by passing parallel rays through the 3D scene at an angle a little off vertical to project the 3D scene onto the 2D surface. For the top-down style we want we can achieve the projection without worrying about the precise mathematics, simply by drawing anything in the horizontal plane as normal whilst scaling the vertical coordinates by about 0.3.

The easiest way to get an feel for this projection is to see it in action. Take Nintendo’s Pokémon Fire Red for example,

The top down oblique projection in pokemon fire red.


Stacking Shapes

At its most basic, the effect will work by defining a 3D model as a series of 2D layers, expanding them vertically into volumes, then stacking on top of each other to form the final image. By the rules of the oblique projection we will leave the X and Y coordinates of each layer as they are, and draw the vertical shifts by moving along Y to 0.3 times the upper and lower Z coordinates, z_top and z_bottom respectively.

To make rotations easy we will define each layer in the model as a list of vertices, ordered anti-clockwise, relative to a central axis: (0,0). When we come to draw the layer we will first rotate it around this axis, then translate it to the model’s position in the world space, and finally draw it shifted in Y by 0.3 times z_top and z_bottom.

We will use this function to draw each layer in the model from lowest to highest to make a crude projection.

function draw(Model shape, Canvas g)
{
	Vector2D transformed;
	Vector2D start;

	for (Layer layer in shape)
	{
		g.start_stroke();
		g.start_fill();

		// Move the stroke to the first vertex
		start = layer.vertices[0].rotate(shape.rotation) 
		start += shape.position;
                
		// Shift down to the z_bottom position.
		start.y += layer.z_bottom*0.3;
		
		g.move_to(start);
		// Draw a line along the rest of the vertices
		for (int i = 1; i < layer.vertices.length; i++)
		{
			transformed = layer.vertices[i].rotate(shape.rotation);
			transformed += shape.position;
			transformed.y += layer.z_bottom*0.3;
			g.line_to(transformed);
		}

		// Close the shape with a line to the first vertex
		g.line_to(start);
		g.end_fill();
		g.end_stroke();

		// Repeat the procedure for the upper layer
		g.start_stroke();
		g.start_fill();

		start = layer.vertices[0].rotate(shape.rotation) 
		start += shape.position;
		// This time we shift to z_top
		start.y += layer.z_top*0.3;
		
		g.move_to(start);
		for (int i = 1; i < layer.vertices.length; i++)
		{
			transformed = layer.vertices[i].rotate(shape.rotation);
			transformed += shape.position;
			transformed.y += layer.z_top*0.3;
			g.line_to(transformed);
		}

		g.line_to(start);
		g.end_fill();
		g.end_stroke();
	}
}

Using this routine the tank has a (somewhat abstract) 3D feel!

It's worth noting that by adding an opaque fill to the layers we are departing a little from a true wireframe render. The end result is worth it, however, as the occlusion of the lower layers by the higher ones reduces the visual clutter and helps to sell the 3D feel. For this reason we'll stick with this filling strategy as we develop the method further.


Linking The Layers

By drawing each layer in ascending order we have created much of the 3D effect, but to make it convincing we need to draw the layers as volumes rather than a stack of planes. We will construct said volumes by drawing each layer at both z_top and z_bottom, as we did before, and joining the vertices between the layers to fill the vertical faces.

To correctly draw the vertical faces we must limit ourselves to only drawing convex shapes, i.e. those with internal angles all smaller than 180 degrees. Convex shapes are important for this drawing as they allow us to use the counter-clockwise ordering of the vertices to draw the vertical faces in the right order.

To achieve this generalisation of layers to volumes we will expand the drawing function to 7 phases:

  1. Create and store a full list of transformed vertices.
  2. Identify the left and right most transformed vertices, using the largest Y coordinate to tie-break.
  3. Draw the vertical faces by starting from the left most vertex shifted to z_top and connecting it down to its position at z_bottom.
  4. Sequentially connect onwards through the transformed vertex list (all shifted to z_bottom) until we reach the right most vertex identified before.
  5. Connect up to the right most vertex at z_top and close the shape back to the left most vertex at z_top.
  6. Draw vertical lines from z_top to z_bottom for all the vertices between the right the left extremes.
  7. Finally, draw the whole layer shape at z_top as before.

This is summarised more precisely as the pseudo-code below,

function draw_volume(Model shape, Canvas g)
{
	for (Layer layer in shape)
	{
		int n_verts = layer.vertices.length;
		List<Vector2D> transformed;

		// Create the transformed list, maintaining the ordering		
		for (int i = 0; i < n_verts; i++)
		{
			transformed.push(layer.vertices[i].rotate(shape.rotation) + shape.position;
		}

		// Identify the left and right most transformed vertices
		Vector2D left(+MAX_NUMBER, 0);
		Vector2D right(-MAX_NUMBER, 0);
		int left_index;
		int right_index;

		for (var i = 0; i < n_verts; i++)
		{
			if (transformed[i].x < left.x or (transformed[i].x == left.x and transformed[i].y > left.y))
			{
				left = transformed[i];
				left_index = i;
			}
			if (transformed[i].x > right.x or (transformed[i].x == right.x and transformed[i].y > left.y))
			{
				right = transformed[i];
				right_index = i;
			}
		}

		// Draw the vertical faces
		g.start_stroke();
		g.start_fill();

		// Start at the left most vertex.
		int i = (left_index+1)%n_verts;
		g.move_to(transformed[i]+shape.z_top);
		g.line_to(transformed[i]+shape.z_bottom);
		
		// Run along the bottom of the panels.
		while (i != right_index+1)
		{
			g.line_to(transformed[i]+shape.z_bottom);
			// Move through the list rolling around to the start if needed.
			i = (i+1)%n_verts;
		}

		// Connect to the top
		g.line_to(transformed[right_index]+shape.z_top);
		// And back to the top right to close the shape
		g.line_to(transformed[left_index]+shape.z_top);
		g.end_fill();
		g.end_stroke();

		// Draw the layer shape at the top.
		g.start_stroke();
		g.start_fill();
		g.move_to(transformed[0]+shape.z_top);
		for (int i = 1; i < n_verts; i++)
		{
			g.line_to(transformed[i]+shape.z_top);
		}
		g.line_to(transformed[0]+shape.z_top);
		g.end_fill();
		g.end_stroke();
	}
}

This function is very close to the final method and produces some nice results, but there remains a problem that we need to fix: volumes that share a Z level can produce strange results. You can see this in the example tank below when then turret points downwards, the barrel of the cannon should emerge from the turret, instead it appears to be below it.


The Sorting Problem

What we have so far above works nicely when the model is a simple ‘totem pole’ style arrangement of layers stacked on top of each other, but If the model has side-by-side layers then we must be more careful.

We can clearly see what is wrong when this bulldozer faces down,

The problem lies in the order that we draw the layers. When the dozer is facing upwards we want to draw the scoop first so the main body is drawn over the top of it. When the dozer is facing downwards however, we want to draw the scoop last so it is on top.

Unfortunately it's not possible to solve this layer ordering problem in the general case and no matter how we sort things we can always make arrangements that don't play nicely. These three rectangles are a good example,

The problem with the painter's algorithm

To properly handle all cases we would need to implement a depth-buffer to control what is drawn on top of what on the pixel scale. That is beyond the scope of this post (and Project: Tanks in general) however, and instead we must be happy with a pretty good solution of sorting the layers into the best order we can before we draw the model. This style of sorting the layers then drawing them in ascending order is known as the painter's algorithm and whilst we know it will fail for some arrangements we can limit the number of problem cases by designing our models sensibly.

Note: It has been pointed out to me by reddit user /u/NeverComments (and others) that using the more capable WebGL framework instead of (mis)using the canvas would give a depth buffer that would completely and generally solve this problem without resorting to ugly model dependent logic. I am currently looking into this and will write a follow up article when I have it working. Thanks all for the constructive feedback!

Sorting the layers of the tank and bulldozer based on the direction the model is facing gives us the final effect we were looking for,

Whilst it would be nice to have a general purpose sorting routine experience suggests it is better to define the sorting on a model by model than to try to make a catch all algorithm. When we start to draw multiple models moving around each other we will find the layering problem rises again between the models, but for this sorting the models by ascending Y position seems to give a good result if all the models are of roughly the same size and height.


More Complex Models

This is all I've needed for Project: Tanks so far but it's easy to see how the technique could be generalised for more complex results. We could allow for sloped faces by defining independent top and bottom vertices and how they connect to give pyramidal structures and bevels. Additionally, we could add the means to draw non-convex shapes through a decomposition into convex sub-shapes and not stroking the internal edges.

In fact, we are very close to having a full 3D renderer for general models. If we defined our models as triangle lists (and maybe used a more refined perspective projection method), we would have a fully 3D 'painter's algorithm' renderer. At this point however, it feels like a the simplicity that was so appealing has been lost, and it might be time to consider an proper 3D library.


Final Thoughts

So there we have it, the basic 2D tank has been developed into a much more interesting '3D wireframe' model and as well as having a strong aesthetic identity the technique developed remains reasonably simple with a relatively low cost when drawing simple models. What's more, its easy for an 'artistically disadvantaged' programmer such as myself to make appealing graphics just by stacking basic shapes.

The strong wireframe aesthetic is nice for some projects, but it is clearly not right in most settings. The ideas at the core of the method are not doomed to a retro oblivion however, and the same technique of shifting by a scaled Z coordinate can be used with layers of bitmaps to give a more general 'pixel art' effect. A great example is given by like 100 bears in a post that formed the inspiration for this work and is definitely worth a read.

Right, with all that rendering set up it's time for me to make the rest of the game!


The early days of Project: Tanks.


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