Live long and Render (III)

I’m slowly moving forward with my Vulkan implementation. After several days of trial and error, I finally managed to render a simple triangle, which is a big deal for Vulkan. But I’m getting ahead of myself. Let me talk you about the journey first.

As mentioned in previous posts, the majority of the design decisions at the moment are how to introduce Vulkan’s concepts into Crimild and make sense of them. I talked about render devices and swapchains before and the next step was to start dealing with how to draw objects in the screen.

Shaders

Shaders have been part of Crimild for a long time, but the time has come to update them in order to support modern features. For the moment, the most important change I introduced in the Vulkan branch is that we can have multiple shader sources for each program. Besides the typical vertex/fragment shader pair, we can now specify geometry and compute shaders too. These are not implemented yet, but it’s a start.

Graphics Pipelines

Graphics pipelines define how objects are rendered in the screen, including everything from viewport size, vertex inputs, depth testing, color blending, etc.

Older graphics APIs like OpenGL define a graphics pipeline in a very strict fashion. Yes, it was possible to introduce some customization in the form of shaders here and there, but in the end everything was rendered in the same way.

Vulkan introduces the concept of highly customizable graphics pipelines. We can know specify things like rasterization options, depth/stencil settings, multisampling, etc in a single object and use it a way that’s really efficient. As usual with Vulkan, this means two things: on one hand, a great power. And, on the other, a very, very explicit amount of code to create the pipelines.

Custom graphics Pipelines are, of course, another new concept for Crimild and it wasn’t easy to reach a consensus about how to work with them (and, to be honest, I’m still second guessing some decisions).

Having one pipeline shared by every single renderable object doesn’t make any sense. But neither does the opposite, since I would end up having too many instances of the same pipeline for objects that are similar.

Associating pipelines with materials didn’t feel right either. Again, some materials may reuse the same pipelines.

In the end, I made up my mind and decided that pipelines are independent of both drawables and materials. Why? Because there may be times when we need to render objects disregarding their geometry (i.e. don’t care about normals or vertex colors) and/or material properties (like we’re rendering a shadow map).

What about linking pipelines and shaders? Well, that makes more sense, but it’s not enough. Pipelines handle much more information than shaders, like viewport sizes and blending, for example.

And that’s how the Pipeline class was born.

Render Passes, Attachments & Framebuffers

Render passes are already a very important (albeit experimental) feature of Crimild. And they don’t differ too much from Vulkan’s own render passes.

The most important difference is that in Vulkan the actual rendering is performed in sub-passes. Render passes only serve as a way to declare which resources (that is, attachments) are needed for the sub-passes to work. Then, you can declare a single render pass that performs deferred lighting on a scene by implementing multiple sub-passes, all working with the same shared attachments.

The use of sub-passes makes the render pass much more efficient, even if working with OpenGL Since attachments are shared, we only need to bind them once before executing all sub-passes. This is a change that I’m planning to make soon.

Render Graphs

Vulkan does not have a render graph API, although it is implemented internally by specifying sub-pass dependencies in each render pass. It is our job to correctly set those dependencies which might quickly become cumbersome for complex renderers.

I’m still trying to figure out the changes require to Crimild’s render graph API. Not only to support sub-passes, but also because I want it to become much more than just a bunch of passes and dependencies. I want to include things like scene culling, filtering (i.e. render only UI elements), commands and much more. My goal is to make the render graph a descriptor for how an entire frame should be drawn for each application, not only the scene.

I believe this will be extremely beneficial both for complex and simple applications. You don’t need to cull objects because it’s a simple app? Do you need post-processing only on the 3D scene? Do you want a different post-processing for the UI? Are you making a headless path tracer for generating images? All of those scenarios can be supported.

Like I said, I’m still working on this and I’m not planning on it to be ready any time soon.

Moving on.

Command Pools & Command Buffers

Almost there, I promise.

Here’re another two new concepts for Crimild: Command pools and Command Buffers.

Command buffers are used to store commands that will be later executed when a frame is actually rendered. This is probably the biggest difference between OpenGL and Vulkan. While the former works by setting the state machine immediately (in theory, some drivers may change that), Vulkan declares everything up front and defers most operations for (possible much) later use.

For example, when rendering a triangle we usually issue commands for clear the screen buffer, bind vertex and index data, define a viewport, etc. When everything is ready, we issue a draw command (aka, a “draw call”). A command buffer will record all of these commands sequentially.

Command buffers are created for given specific command pool, depending on their type. There may be many different pools for different purposes, like graphics or compute pools.

Wait. Doesn’t Crimild’s render queues work in the same fashion? What’s different? It’s true that I tried to achieve something like this in Crimild before in the form of render queues, yet they are of a much higher level. With render queues, visible objects are recorded (which may be done in separated threads) to be rendered later. But it’s only the renderable object the one that is saved, not the actual render commands. This requires that we compute what state changes are triggered every time we draw that object. This is clearly an overhead, specially if we consider the fact that the renderer triggers draw state changes and draw calls without actually checking if those are needed. I made this call on purpose in the past to ensure that any object can be rendered independently of what came before, always reseting states to default values before drawing.

By using command buffers, instead, we can avoid that overhead while keeping the safety net. For each renderable object, we record the list of state changes and draw calls needed to make it appear on the screen. Then, we can check which of those commands are redundant and discard them. And by the time the render process is triggered, we’ll have the minimum number of commands that are needed to draw all objects.

Obviously, recording commands is a costly operation. The challenge, then, will be to understand when to trigger the recording of render commands. After all, doing it every frame may end up causing more overhead than the one we’re trying to solve. But that’s another problem for my future self (I hate you too, future self!).

And then… Victory!

After all the hard work, the mighty Triangle shows up in the screen:

…Up Next!

Phew, that was a long post.

Now it’s time to make a pause. Think. Design.

There are many new concepts introduced into the engine and I want to do it right before moving on to other features like buffers and textures.

And yes, I think that the render graph is the most interesting feature I’ve ever made for Crimild… assuming it works 🙂

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