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RealMarkP

Unity How do YOU render?

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I'm trying to find an optimal method to rendering a given number of 'renderables' by optimizing state changes. Ordering state changes is not hard, I'm just having trouble putting together a design that is easy to use. Here are my two designs I have come up with (Neither of which make me 100% happy): Design 1: I define a 'Renderable' as a class that holds pointers to vertex buffers, index buffers, a shader (Cg in this case), and a technique. My 'RenderList' is a list of these Renderables that are ordered in a linked list fashion. I order my Renderables by sorting them when I add them to the list. They are sorted by (Render Target, Transparency, Shader, Technique). For each Renderable in the list that gets sent to the Renderer, the Renderable is polled for new information. New parameters are set in the shader and vertex data is drawn. Pros: Linked list is easy to push, insert and pop from. Cons: Each Rendrable must know about the state of the world in order to set the shader parameters properly - ie. Transformation matrices, textures, etc. Design 2: As above, A Renderable class is defined. It only contains a pointer to a vertex and index buffer, nothing else. A RenderTree is now used as a collection of these Renderables. It groups the Renderables by (Render Target, Transparency, Shader, Technique, and Parameter) changes. It basically contains a stack of states that get pushed and popped off the stack (ie. you only set the world view and projection matrices once). This mimics the way OpenGL does it's transformation stack. This is similar to a scenegraph, I believe. Pros: This is a bit more efficient then the above in that it does not poll renderables for their states. Each renderable does not need to know the world state in order to set the shader parameters (like world view projection matrices). Cons: Relies heavily on the user of my code to set up her own render list. Does not automate code (maybe it's a pro?). Anyway, I have both designs on the table. I would like to hear what the GameDev community uses as a rendering mechanism. Perhaps, if anyone has experience with some of the more known engines out there, they can throw in their two cents with rigards to how they render objects (OGRE, Irrlicht, SlimDX, etc?). Thanks for the help.

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Here is the design for my (unfinished) graphics engine.

Goals:
* minimize and coalesce API-dependent code into a few key areas
* generality (works for different projection types, 2D/3D, shaders)
* built-in support for commonly used features (shadows, dynamic environment maps, reflections)

Classes:
* GraphicsObject - sort of like a scene graph node but only for transformations. Each object has a pointer to a GraphicsShape and a list of child graphic objects, plus a transformation (position, orientation, scale). Transformations are accumulated as one recurses down the tree. It also contains a bounding volume for fast hierarchical culling.
* GraphicsShape - generic shape superclass which contains information common to all types of shapes (bounding volume, material, object-space transformation). Uses the visitor pattern with the ShapeDrawer class to avoid RTTI when drawing.
* Material - contains a shader, list of texture units (shader name/index/texture), and a list of named shader attributes for the material (of any type: float, vector, matrix).
* ShapeDrawer - interface for shape drawing. has a draw*(shape) method for each type of shape (ex: sphere, cylinder, static mesh, articulated mesh, heightfield). Most of the API-dependent code is here.
* VisibilityCuller - abstract type which contains all GraphicsObjects being drawn and support queries to determine which objects are in the view of a given camera. Probably uses an octree internally to speed queries.
* Renderer - class which does the organizational work when rendering. For each camera/viewport (including shadow maps) it uses the VisibilityCuller to determine which objects to draw. It then sorts the list by transparency, then material (they can be shared between shapes), then shader. It then iterates through the list drawing all opaque objects (with a ShapeDrawer) making minimal state changes. Finally, it sorts all transparent objects back-to-front and draws them in order. The Renderer also handles pushing/popping transformation matrices.

This is probably quite a bit more complicated than your designs but I think it is the best I've come up with yet.

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I have a separate Spacial Partitioning system in my code, which I will leave separate from the rendering system. I also used to have a Material class that worked as you did, but I ended up dumping it for something else a while ago (now I'm starting to think I should have kept it). Your Material class seems very generic - it holds a shader and it's state. This seems like a decent design and I might end up resurrecting my Material class to match your description.

Quote:

built-in support for commonly used features (shadows, dynamic environment maps, reflections)


How exactly do you achieve this? Do you expect a certain shader type to interact with when drawing shadows/reflections/environment maps?

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Quote:
Original post by RealMarkP
Quote:

built-in support for commonly used features (shadows, dynamic environment maps, reflections)


How exactly do you achieve this? Do you expect a certain shader type to interact with when drawing shadows/reflections/environment maps?


Each material object also has 2 names (of the variables in GLSL) for the shadow and environment maps, and boolean flags indicating whether or not the shader needs them. The renderer takes care of rendering shadow maps for each light source and therefore the shader only has to perform depth comparisons. The assumption is that whatever is creating the material object will know whether or not it accepts shadows.

Of course this is just the "plan." None of this is actually fully implemented yet.

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