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Learn how to draw a 3D model using MonoGame, load models through the Content Management Pipeline, set up matrices, and understand the graphics rendering pipeline.
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Week 2 - Friday COMP 4290
Last time • What did we talk about last time? • Graphics rendering pipeline • Geometry Stage
Drawing a model • We're going to start by drawing a 3D model • Eventually, we'll go back and create our own primitives • Like other MonoGame content, the easiest way to manage it is to add it to your Content folder and load it through the Content Management Pipeline • MonoGame can load (some).fbx, .x, and .obj files • Note that getting just the right kind of files (with textures or not) is sometimes challenging
Loading a model • First, we declare a member variable to hold the model • Then we load the model in the LoadContent() method Model model; model = Content.Load<Model>("Ship");
Setting up the matrices • To draw anything in 3D, we need a world matrix, a view matrix and a projection matrix • Since you'll need these repeatedly, you could store them as members Matrix world = Matrix.CreateTranslation(new Vector3(0, 0, 0)); Matrix view = Matrix. CreateLookAt(new Vector3(0, 0, 7), new Vector3(0, 0, 0), Vector3.UnitY); Matrix projection = Matrix.CreatePerspectiveFieldOfView(0.9f, (float)GraphicsDevice.Viewport.Width / GraphicsDevice.Viewport.Height, 0.1f, 100.0f);
What do those matrices mean? • The world matrix controls how the model is translated, scaled, and rotated with respect to the global coordinate system • This code makes a matrix that moves the model 0 units in x, 0 units in y, and 0 units in z • In other words, it does nothing Matrix world = Matrix.CreateTranslation(new Vector3(0, 0, 0));
And the view matrix? • The view matrix sets up the orientation of the camera • The easiest way to do so is to give • Camera location • What the camera is pointed at • Which way is up • This camera is at (0, 0, 7), looking at the origin, with positive y as up Matrix view = Matrix.CreateLookAt(new Vector3(0, 0, 7), new Vector3(0, 0, 0), Vector3.UnitY);
And projection? • The projection matrix determines how the scene is projected into 2D • It can be specified with • Field of view in radians • Aspect ratio of screen (width / height) • Near plane location • Far plane location Matrix projection = Matrix.CreatePerspectiveFieldOfView( .9f, (float)GraphicsDevice.Viewport.Width / GraphicsDevice.Viewport.Height, 0.1f, 100f);
Drawing • Drawing the model is done by drawing all the individual meshes that make it up • Each mesh has a series of effects • Effects are used for texture mapping, visual appearance, and other things • They need to know the world, view, and projection matrices foreach(ModelMeshmesh inmodel.Meshes) { foreach(BasicEffecteffect inmesh.Effects) { effect.World = world; effect.View = view; effect.Projection = projection; } mesh.Draw(); }
Backface culling • I did not properly describe an important optimization done in the Geometry Stage: backface culling • Backface culling removes all polygons that are not facing toward the screen • A simple dot product is all that is needed • This step is done in hardware in MonoGame and OpenGL • You just have to turn it on • Beware: If you screw up your normals, polygons could vanish
Graphics rendering pipeline • For API design, practical top-down problem solving, and hardware design, and efficiency, rendering is described as a pipeline • This pipeline contains three conceptual stages:
Rasterizer Stage • The goal of the Rasterizer Stage is to take all the transformed geometric data and set colors for all the pixels in the screen space • Doing so is called: • Rasterization • Scan Conversion • Note that the word pixel is actually a portmanteau for "picture element"
More pipelines • As you should expect, the Rasterization Stage is also divided into a pipeline of several functional stages:
Triangle Setup • Data for each triangle is computed • This could include normals • This is boring anyway because fixed-operation (non-customizable) hardware does all the work
Triangle Traversal • Each pixel whose center is overlapped by a triangle must have a fragment generated for the part of the triangle that overlaps the pixel • The properties of this fragment are created by interpolating data from the vertices • Again, boring, fixed-operation hardware does this
Pixel Shading • This is where the magic happens • Given the data from the other stages, per-pixel shading (coloring) happens here • This stage is programmable, allowing for many different shading effects to be applied • Perhaps the most important effect is texturing or texture mapping
Texturing • Texturing is gluing a (usually) 2D image onto a polygon • To do so, we map texture coordinates onto polygon coordinates • Pixels in a texture are called texels • This is fully supported in hardware • Multiple textures can be applied in some cases
Merging • The final screen data containing the colors for each pixel is stored in the color buffer • The merging stage is responsible for merging the colors from each of the fragments from the pixel shading stage into a final color for a pixel • Deeply linked with merging is visibility: The final color of the pixel should be the one corresponding to a visible polygon (and not one behind it)
Z-buffer • To deal with the question of visibility, most modern systems use a Z-buffer or depth buffer • The Z-buffer keeps track of the z-values for each pixel on the screen • As a fragment is rendered, its color is put into the color buffer only if its z value is closer than the current value in the z-buffer (which is then updated) • This is called a depth test
Pros and cons of the Z-buffer • Pros • Polygons can usually be rendered in any order • Universal hardware support is available • Cons • Partially transparent objects must be rendered in back to front order (painter's algorithm) • Completely transparent values can mess up the z buffer unless they are checked • z-fighting can occur when two polygons have the same (or nearly the same) z values
More buffers • A stencil buffer can be used to record a rendered polygon • This stores the part of the screen covered by the polygon and can be used for special effects • Frame buffer is a general term for the set of all buffers • Different images can be rendered to an accumulation buffer and then averaged together to achieve special effects like blurring or antialiasing • A back buffer allows us to render off screen to avoid popping and tearing
Finals words on the pipeline • This pipeline is focused on interactive graphics • Micropolygon pipelines are usually used for film production • Predictive rendering applications usually use ray tracing renderers • The old model was the fixed-function pipeline which gave little control over the application of shading functions • The book focuses on programmable GPUs which allow all kinds of tricks to be done in hardware
Next time… • GPU architecture • Programmable shading
Reminders • Read Chapter 3 • Start on Assignment 1, due next Friday, September 13 by 11:59 • Keep working on Project 1, due Friday, September 27 by 11:59 • Amazon Alexa Developer meetup • Thursday, September 12 at 6 p.m. • Here at The Point • Hear about new technology • There might be pizza…
Internship opportunities • Want a Williams-Sonoma internship? • Visit http://wsisupplychain.weebly.com/ • Interested in coaching 7-18 year old kids in programming? • Consider working at theCoderSchool • For more information: • Visit https://www.thecoderschool.com/locations/westerville/ • Contact Kevin Choo at kevin@thecoderschool.com • Ask me!
