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Windows Phone 7 : Drawing with Vertices and Matrices - A New Approach to Drawing

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Three-dimensional rendering introduces a variety of new challenges: 3D graphics are rendered as models rather than as simple bitmaps, our brains need to shift into a different gear to keep track of movement in and out of the screen, and we have an entirely different way of telling XNA where we want to draw things on the screen.

Let's start by discussing some of the features of the 3D rendering environment.

1. Matrix-Based Positioning

As we prepare to render 3D graphics, we leave this approach behind for the time being. Instead, we use a system based around matrices.

Matrices allow us to encode a series of movements and transformations into a compact structure that can then be applied to the graphics that we want to draw. Most of the calculations required to do this are conveniently wrapped up in handy XNA functions, so we don't need to get too involved in their inner workings.

Just like anything else, it might take a little time to become accustomed to thinking with matrix transformations, but once you do you will find them a very useful tool. In fact, you might ultimately decide that you prefer them to XNA's sprite rendering approach.

2. Abstract Coordinate System

When we render in 3D, XNAgenerally uses an abstract coordinate system rather than a pixel-based coordinate system like the one used for sprite rendering, meaning that we are not concerned with pixels. Although this might sound like a disadvantage at first, freeing ourselves from pixel coordinates actually turns out to be rather useful.

When we initialize XNA, we can tell it the dimensions of the screen and the coordinate system will scale to match. Moving a graphic object a certain distance to the right, therefore, moves the same distance regardless of the back buffer size. As a result, should we decide to use a smaller buffer to increase performance, none of the rendering code needs to change (as it did when we were rendering sprites).

NOTE

After all the time we have spent with sprites, getting to grips with the 3D coordinate system requires a slight twist of the brain. First of all, the coordinate (0, 0) is generally right in the center of the screen rather than in the top-left corner. Second, movement along the positive y axis will travel up the screen, as opposed to down for sprites. It can be a nuisance having to keep these two conflicting coordinate systems in your brain, but once you are in the frame of mind for one system over the other, it should be easy to remember which way is up.

Becausewe are now using a 3D graphical environment, we actually need to add a third element to our coordinates. The coordinate values we have looked at in the past have been in the form of (x, y), representing the specified distances along the x and y axes. 3D coordinates are in the form (x, y, z), providing values in the z axis as well as the x and y axes. The z axis represents movement into or out of the screen—literally the third dimension. Positive values on the z axis result in movement toward the player, negative values result in movement into the screen.

3. Drawing Primitives

When it comes to drawing graphics, XNA is actually not able to draw anything more complex than triangles. This might at first seem very restrictive, but in fact, it is not as you will see when we start to use it in some example projects.

The reason we can create more complex scenes is partly because much more complex shapes can be created by putting lots of triangles together (for example, a rectangle is just two triangles joined along their long edge) and partly because we can put graphic images onto the triangles.

When we are drawing, we refer to each triangle as a surface. The points that form the triangle are called vertices. Figure 1 shows two triangular surfaces created using four vertices. Two of the vertices are shared between the triangles.

Figure 1. Vertices and surfaces used in XNA rendering

The vertices themselves are not actually displayed by XNA, just the surfaces that they define; the vertices are shown in Figure 1 just to clarify what they are.

The only primitives available other than triangles are lines.

4. Textures

Just as we used Texture2D objects to provide graphics for our sprites, so we can use them to fill the triangles that we are rendering. We have a lot of flexibility to use textures within our applications—much more so than we had with sprites. We can take small rectangular sections just as we did with sprites, or we can stretch textures in a variety of different ways across the shapes that we draw.

5. XNA is a State Engine

Whereas, with sprite rendering, each individual call to draw graphics provided all the information needed for drawing to take place, the approach for 3D rendering is slightly different. XNA maintains lots of state values for things such as which texture is currently being used for rendering, whether transparency is enabled, whether lighting is switched on, and so on.

In order for our rendering to appear as we expect, each of these states must be set prior to the rendering call. Once a state has been set, it will stay with its value until we decide to change it again.

NOTE

All this actually applies to sprite rendering, too, except that the sprite engine always sets the state values according to the parameters passed to the SpriteBatch.DrawDrawString methods.
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