A story in several parts. 1) how shader compilation is done in upcoming Unity 4.5; and 2) how it was developed. First one is probably interesting to Unity users; whereas second one for the ones curious on how we work and develop stuff.
Short summary: Unity 4.5 will have a “wow, many shaders, much fast” shader importing and better error reporting.
When you create a new shader file (.shader) in Unity or edit existing one, we launch a “shader importer”. Just like for any other changed asset. That shader importer does some parsing, and then compiles the whole shader into all platform backends we support.
Typically when you create a simple surface shader, it internally expands into 50 or so internal shader variants (classic “preprocessor driven uber-shader” approach). And typically there 7 or so platform backends to compile into (d3d9, d3d11, opengl, gles, gles3, d3d11_9x, flash – more if you have console licenses). This means, each time you change anything in the shader, a couple hundred shaders are being compiled. And all that assuming you have a fairly simple shader - if you throw in some multi_compile directives, you’ll be looking at thousands or tens of thousands shaders being compiled. Each. And. Every. Time.
Does it make sense to do that? Not really.
Like most of “why are we doing this?” situations, this one also evolved organically, and can be explained with “it sounded like a good idea at the time” and “it does not fix itself unless someone works on it”.
A long time ago, Unity only had one or two shader platform backends (opengl and d3d9). And the amount of shader variants people were doing was much lower. With time, we got both more backends, and more variants; and it became very apparent that someone needs to solve this problem.
In addition to the above, there were other problems with shader compilation, for example:
So we’re changing how shader importing works in Unity 4.5. The rest of this post will be mostly dumps of our internal wiki pages.
This was mostly a one-or-two person effort, and developed in several “sprints”. For this one we used our internal wiki for detailed task planning (Confluence “task lists”), but we could have just as well use Trello or something similar. Overall this was probably around two months of actual work – but spread out during much longer time. Initial sprint started in 2013 March, and landed in a “we think we can ship this tomorrow” state to 4.5 codebase just in time for 1st alpha build (2013 October). Minor tweaks and fixes were done during 4.5 alpha & beta period. Should ship anyday now, fingers crossed!
Surprisingly (or perhaps not), largest piece of work was around “how do you report errors in shaders?” area. Since now shader variants are imported only on demand, that means some errors can be discovered only “some time after initial import”. This is a by-design change, however - as the previous approach of “let’s compile all possible variants for all possible platforms” clearly does not scale in terms of iteration time. However, this “shader seemed like it did not have any errors, but whoops now it has” is clearly a potential downside. Oh well; as with almost everything there are upsides & downsides.
Most of development was done on a Unity 4.3-based branch, and after something was working we were sending off custom “4.3 + new shader importer” builds to the beta testing group. We were doing this before any 4.5 alpha even started to get early feedback. Perhaps the nicest feedback I ever got:
I’ve now used the build for about a week and I’m completely blown away with how it has changed how I work with shaders.
I can try out things way quicker.
I am no longer scared of making a typo in an include file.
These two combine into making me play around a LOT more when working.
Because of this I found out how to do fake HDR with filmic tonemapping [on my mobile target].
The thought of going back to regular beta without this [shader compiler] really scares me.
Anyhoo, here’s a dump of tasks from our wiki (all of them had little checkboxes that we’d tick off when done). As usual, “it basically works and is awesome!” was achieved after first week of work (1st sprint). What was left after that was “fix all the TODOs, do all the boring remaining work” etc.
2013 March Sprint:
2013 July Sprint:
2013 August Sprint:
2013 September Sprint:
Some more in shader compilation land will go into Unity 5.0 and 5.x. Outline of our another wiki page describing 5.x related work: