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Passing shader parameters from a scene node to the renderer

jbb · 2012-10-06T21:10:50

I commented on this post asking a further question but I think it would be better in it's own post and with the question expanded. I've done a few toy landscape render programs in the past but until now I always used a "traditional" OOP approach of making models, landsape segments, ui elements etc all implement an IDrawable interface something like this - class Model : public IDrawable { public: virtual void render(Device& device, Camera& camera); }; where the render function uses the d3d11 device to directly draw itsself. After reading the post above I entirely understand how it might be better to instead make drawable objects produce a Mesh to draw which can then be drawn separately by the rendering system. It simplifies the code, and separates concerns much better and potentially makes the code much more reusable without lots of unnecessary abstraction. Instead of directly drawing the things, the scene node items will instead put the data for a rendering operation onto a queue... Including mesh, camera and so on. Now the problem I have is this - Inside my Model class the constructor would previously obtained references to the shaders it used and the textures and so on, and when the render function is called, it knows which shaders it's using and it knows what parameters those shaders require and can fill in a constant buffer and set it directly. Nothing outside of the Model class need have any knowledge of the shaders it uses, or what data they require. Which is a good thing. Except that it doesn't fit with the new way of drawing things. I'm thinking I could create a structure for the parameters for each "class" of shader and pass that into the render function as opaque data to give to the shader but that seems ugly. I could create a constant buffer for each class of shader and get my geometry classes to fill that in when they create the render operations but that feels ugly too. How do people suggest that the part of code that wants to draw a model passes the shader parameters (and the material in general I guess) to the renderer in an elegant way? Although I want my code to be for D1D11 I''d like to keep an eye on doign things in a way that would make it easy to change in future so some form of abstraction is needed here even if it's only passing a handle to a constant buffer or something... Does this even make any sense? edit: To explain further, what makes this hard is that there is no real common structure. Each shader requires different numbers of vectors, values, textures etc. And the IRender interface shouldn't really have to know anything about the shader... I could pass a big map of name - value pairs for them all, but doing synamic memory allocation for a map in a renderer doesn't seem like a great idea

General and Gameplay Programming Programming

Started by jbb September 29, 2012 12:05 PM

9 comments, last by ATC 11 years, 6 months ago

ATC

551

October 06, 2012 09:10 PM

+1 for phantom... Very informative and well thought-out post, for which I must thank you...

I'm aware that there is indeed a penalty for virtual calls in both C# and C++. I know about V-tables and the likes, and how it is resolved by the CLR in C#. However, these "hierarchies" of mine are very shallow and the penalty is very small. So small, in fact, that is one of the least costly things the engine does each frame. The engine takes a brief fraction of a second to load up and then it's full speed ahead after the first frame; with everything JIT'ed and ready to rock.

Pretty much everything you say is correct and I'm with you on it. For instance, having to reflect on vertex data each frame would be very inefficient. Therefore when a new "Mesh" instance is created it pre-processes everything for the underlying renderer... when I call, say, Mesh10.CreateFromMeshData( ... ), the HLSL and GLSL input bindings are automatically resolved, stored in an easily-accessible array that can be bound to the device / input assembler immediately. The process of reflecting on vertex data and figuring out how to describe it to the underlying API and GPU happens only when a mesh is loaded/created; or if the application suddenly wants to change to another, completely different rendering API (e.g., a software renderer or something 3rd party).

Thank you again for your post; there is much wisdom in it. I will be over-looking some of our code during this coming week and will try to apply your advice on things!

Regards,

ATC

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