I am sorry for the delay in posts; I’ll try to keep it more regular, one post in a week or so.
So, yesterday we were discussing various lighting approaches at local IRC chat, and someone said that it was impossible to make a ps.2.0 single-pass lighting shader for 8 lights with diffuse, specular and attenuation, of course with normal map. Well, I told him he was actually wrong, and that I can prove it. Proving it turned out to be a very fun and challenging task. I am going to share the resulting shader and the lessons learned with you.
From the start I knew it was not going to be easy – ps.2.0 means that no arbitrary swizzles are available (which can in some cases restrict possible optimizations), and there is a limit of 64 arithmetic instructions. Add to that the fact that there is no instruction pairing as in ps.1.x (which is rather strange, as all hardware I know of is capable of pairing vector and scalar instructions).
I decided to compute lighting in world space, as I thought that passing all lighting data from VS to PS is going to consume too much interpolators. So I passed world-space view vector, world-space position and tangent to world space conversion matrix from VS to PS, and had PS transform normal to world space and compute everything else. This proved to be insufficient to reach target result, but I am going to show you the shader anyway, as it has some interesting ideas, some of which are still in the final shader.
Here is the shader: link. Ignore stuff like bindings.h, TRILINEAR_SAMPLER, etc. – these are to allow FX Composer bindings of parameters.
Interesting things to note:
There are several solutions. The first one is to fix specular power to some convenient value. For example, pow(a, 16) can be implemented as 4 muls (and in fact HLSL compiler does it automatically), and muls can be vectorized – so you can compute specular power for all 4 lights for 4 instructions, which is much better.
However, there is a better solution, which is described here: A Non-Integer Power Function on the Pixel Shader. Basically, you can approximate the function pow(x, N) with pow(max(A*x + B, 0), M). A and B can be tuned such that the maximum error is low enough for the approximation to be useable in practice, and M can be made very small, for example I use M=2, and there are no artifacts for N=18 (the results ARE different, and it can be seen by switching between formulas in realtime, but the difference is small and no one will be able to tell that you use an approximation). Alternatively, you can have your artists tune A and B directly.
The net effect is that instead of several instructions per light, we compute specular power in 2 instructions – mad_sat to compute A*x+B, and mul to compute the result (remember, M=2).
Okay, we have a bunch of cool optimizations here. Are we done? Sadly, no, we are not. The presented shader compiles into 75 instructions. The problem is that per-light cost is still too high. We have to compute a unnormalized light vector (1 sub), compute its squared length (1 dp3), normalize it (1 rsq + 1 mul), compute dot(N, L) for diffuse lighting (1 dp3), compute dot(R, L) for specular lighting (1 dp3), and combine diffuse lighting with specified colors (1 mad). This is 7*8=56 instructions, which leaves 8 instructions, and we need much more. What can we do?
Well, we can simplify our calculations and replace light colors with light intensities. This will strip 8 instructions, but add 2 instructions for multiplying NdotL vector by intensity, and 1 instruction for summing all diffuse components together (dp3), which is still not enough – we need to reserve instructions for other things (for example, normal transformation takes 3 instructions, specular computation is 4 instructions for 8 lights, attenuation is 4 instructions for 8 lights, and there are still several other things left).
Yesterday, I gave up and went to sleep. But today, after some thinking, I felt like a barrier in my head collapsed – I knew why it did not work out, and I knew the better solution.
See, it is true that we have a lot of overhead per light source. The problem is not in the fact that we need to do a lot of calculations – the problem is in that we are using computation power inefficiently.
Let’s look at the instruction counts I presented earlier. Yes, it’s true that we do 1 sub per light – but sub is capable of processing 4-float vectors, and we are using it to subtract 3-component vectors, so we lose some power here. The same stays true for everything else – all dot products and mul for example.
And the barrier that collapsed in my head had an inscription “You have the mighty dot product, use it”. As it turned out, there is not a lot of sense in treating GPU assembly differently from for example SSE instructions. We do not have dot product in SSE. Trying to compute 4 dot products at once in a straightforward way in SSE is subject to miserable failure – most likely FPU will be faster. But if you change the way your data is organized, and instead of laying it in AoS order:
v1.x v1.y v1.z (unused)
v2.x v2.y v2.z (unused)
v3.x v3.y v3.z (unused)
v4.x v4.y v4.z (unused)
lay it out in SoA order:
v1.x v2.x v3.x v4.x
v1.y v2.y v3.y v4.y
v1.z v2.z v3.z v4.z
And lo and behold, a lot of slow dot product instructions are now just 3 muls and 2 adds – simple and fast.
I decided to try the same thing with my shader code. It solved all problems like magic. The resulting code (link) while doing exactly the same calculations, now compiles in 54 instructions.
The reason is simple, of course. For example, where in the previous shader we computed squared lengths in 1 instruction per light, here we do it for 3 instructions for 4 lights, effectively using 25% less ALU. The new layout also made it possible to pass lights via interpolators (light data fits into 6 interpolators), which allowed to remove 1 sub instruction per light, and also 3 instructions for transforming normal into tangent space (at the expense of adding 1 expand instruction, of course).
Apart from the SoA data layout, which effectively is the reason why the new shader is so much smaller, there is only one trick - instead of normalizing each vector, we correct dot product results. This saves a couple of instructions for the entire shader.
The old shader did not fit into the instruction limit, the new one does, and it has 10 spare instructions. There is a bunch of things you could do with it. For example, you can implement parallax mapping – 10 instructions should be enough for several parallax steps. Note that one interpolator can be freed (view vector can be stored in COLOR interpolator at the expense of 1 additional instruction (expand from [0,1] to [-1,1])), so you can implement shadow mapping (for example, make first light source a directional one – this is straightforward, you just have to modify vertex shader to supply correct tangent-space direction, and place 0 in light_radius_inv to disable attenuation – and add shadows for it).
There is also space for some small tweaks – i.e. disable specular for triangles with dot(N, L) < 0, make wrap-around lighting, add ambient lighting, have colored specular (use a per-light color instead of the global one), add specular attenuation, etc.
Note, that a couple of instructions can still be saved if you do not want light colors, only intensities (read above).
So, this was a pleasant experience, and I am glad that I decided to write this shader. Sadly, all articles I’ve read about shader optimizations (save for one, “Bump My Shiny Metal” by Andrew Aksyonoff in ShaderX 4) are about common and trivial stuff – vectorize your computations, inspect compiler output... I hope that this post was more interesting. Read more!
Sunday, October 28, 2007
I am sorry for the delay in posts; I’ll try to keep it more regular, one post in a week or so.
Saturday, October 6, 2007
Today we’ll talk about render state management in D3D10 a little.
While designing D3D10, a number of decisions were made to improve runtime efficiency (to reduce batch cost, basically). It’s no secret that D3D9 runtime is not exactly lean & mean – there is a lot of magic going on behind the scenes, a lot of validation, caching, patching…
For example, D3D9 has the ability to set any render or sampler state at any time. However, hardware does not really work that way. The states are separated into groups, and you can only set the whole group at a time (of course, the exact structure of groups is hardware dependent). What this means for runtime/driver is that SetRenderState/SetSamplerState often do not actually set anything. Instead, they modify a so called shadow state – they update the changed states in a local shadow copy, and mark the respective groups as dirty. Then, when you call DrawIndexedPrimitive, the changed state groups are being set, and the dirty flags – cleared.
Also there could be some cross-state checking going on, I don’t know for sure.
So, D3D10 designers decided to replace hundreds of states by 4 simple state objects. Behold, ID3D10BlendState, ID3D10DepthStencilState, ID3D10RasterizerState and ID3D10SamplerState. These are immutable driver state objects – you create them once, you can’t modify them, and driver knows about them, which means it can do smart things (for example, store a chunk of push buffer that sets the respective state inside each state object) and thus optimize state setting. Also all possible validation is made at creation time only.
Sounds cool, right? Yeah, it did first time I’ve read about state objects. Except…
Problem #1. State objects are relatively expensive to construct. Which means 10-100 thousand CPU cycles on my Core 2 Duo. You know, given that we have a user mode driver, given that the smartest thing I can think of here is to 1. hash the state to check if the same state has already been created (it is actually done, you can check it by creating the same state object several times), 2. If the hash lookup failed, validate the state (perform some sanity checks), 3. construct a chunk with push buffer commands, 4. store it in allocated memory. And that should be performed by user mode code. Something is horribly wrong here, I swear.
Note, that even creating the already existing state object (hash state, hash lookup, compare actual state values, remember?) takes 10 thousand cycles. The caching should be performed in D3D10 runtime (the pointer returned is exactly the same - i.e. for input layouts, the caching is performed by the driver, as the pointer to the InputLayout is different, but the underlying driver object is the same; for state objects, pointers to runtime objects are equal), so this means that computing a hash of a 40-byte description object, doing a table lookup, and then comparing two 40-byte objects to test for equality in case of hash collision takes 10 thousand cycles.
Problem #2. You can’t create more than 4096 state objects. Which means that you can’t just forget about it and create a state for each object, even for static ones. Well, you can try, but one day it’ll fail.
Problem #3. The separation of states into groups is outrageous. I did not tell one small thing – not all states are actually immutable. There are two things you can change without constructing a new state object (they act as parameters for state object setting functions). Those are… stencil reference value and blend factor. Everything else is immutable, for example, depth bias and slope scaled depth bias.
How many of you have used blend factor with pixel shader (let’s say ps.2.0)-capable hardware?
How many of you have used a constantly changing stencil reference value?
I’d like to do things like progressively loading textures. What do I mean? Let’s load our texture from smaller mip levels to larger ones. Let’s interpolate MinLOD parameter from N to N-1 once N-1-th mip level is completely loaded – let’s do it over some fixed time. This way there will be no mip level popping, but instead we’ll see gradually improving quality as new levels are loaded – trilinear filtering will do proper interpolation for us. That’s easy, right? No. Not in D3D10.
Yes, I know I could cache render states inside objects, and perform some lifetime management (LRU?..). Though this won’t help in case of constantly changing parameters.
Yes, I know I could separate render states into groups how I like it best, have a 4096-entry hash table, and do lookups in it. And this is actually what I am doing now.
But it does not make me happy.