Ultra Shadows with Huge FPS in The Elder Scrolls V: Skyrim

[Badsah]

Game defaults me to max settings (Ultra High Settings). Game is smooth outdoors like 50-60 fps constant, maybe 45 in intense areas or fights. Cities (depending on size) can range from 30-45 fps. Inside dungeons, sewers and anywhere indoors basically my fps has gone as low as 30, by staring at a simple wall with rocks. Now if I turn down shadow from ultra to high my FPS never hits below 50-60 ever, indoors or out, except in big cities where it seems to be at around 45-50. Is this normal? Why does changing just ultra shadows to high boost FPS so greatly? And how come on Ultra shadows sometimes indoors it runs at 60 fps?

[Irene-I]

It is normal currently, the shadow rendering is being done on the CPU instead of using the faster processing capabilities of the GPU since the game was designed on the XBOX and then ported to the PC -- as a result even the highest end CPUs become over worked trying to process the shadows on Ultra setting resulting in very poor performance even though the GPU is barely being used.

[DougWison]

People have analyzed CPU vs. GPU usage in the game and discovered that with the shadows on Ultra the GPU usage for Skyrim dropped significantly while the CPU load remained the same (100% of two cores) when the framerates tanked in shadowed areas. Ultra shadows cause a CPU bottleneck which reduces framerates because the CPU has slowed down its communication with the GPU due to it being swamped trying to render the shadows. If Skyrim were GPU-rendering the shadows, the GPU load would not DROP in shadowed areas. That is very simple logic.

[Ashley Parker]

Let's assume the CPU is doing this, and is enchanted with pixie dust that enables it to execute at a stable rate of one instruction per clock cycle with no memory bandwidth or latency issues despite all the other stuff going on, and the shadow-calculating program is an absolute piece of genius that requires an average of 12 instructions per shadow sample, including everything needed to set up the shadow map render, geometry processing and so on. Some computers have been observed running the game at 60 FPS at least occasionally, even with 4096 by 4096 shadow maps.

[Shae]

I don't know how many times I have to keep correcting this but I assure you that shadow rendering is not done on the CPU. Every AMD and nvidia GPU has supported hardware shadow mapping since 2001 (Geforce 3/Radeon 8500). The GPUs in both the Xbox 360 and the Playstation 3 support hardware shadow mapping. If shadows were being done on the CPU, you would not be able to play the game at all. You'd be measuring your speed in frames per minute rather than frames per second. Even the best Core i7 CPU cannot render multiple 4096x4096 or 8096x8096 depth buffers in real time. The increase in CPU usage is almost certainly a bug. A runaway thread or a geometry leak or some kind of race condition.

[Wyl]

Shadow mapping works by creating a special image called a depth buffer from the perspective of every light source. Anything the light source cannot see but the camera can see (the camera is what you see on your screen) is considered shadowed. Every light source has its own depth buffer (or shadow map, if you will). The shadow map size doesn't refer to the number of samples per second; it refers to the resolution of the texture holding the depth buffer. Higher shadow map sizes mean the precision is higher as far as determining which pixels are shadowed. In order to do shadow mapping on the CPU, the CPU would have to render a depth buffer for EVERY light source on the scene. It's simply not possible to do that in real time on a CPU.

[Bubboy]

No need to explain what shadow mapping does. By shadow samples I meant the individual depth values making up the shadow map's buffer. Still, the game engine is generating a new depthmap for each frame, so 4096*4096*60 depth values would have to be generated per second with a 4096 shadow map size and a game running at 60 FPS, for the sunlight shadows alone, and I think we both agree that's not happening

[mbangali]

I have my game all tweaked out (uGrid and far off Detail and stuff) and I get 60 FPS "average", some locations where there is a far off view will dip 50-55. Cities of course, going to dip no matter what, 35-45 usually, although 60 when near buildings and not viewing a lot of far off stuff. But I too noticed the Shadows. From default Ultra settings (no tweaks), using High Shadows nets me 60 FPS in a certain spot that I get 50-55 with my above settings, change to Ultra, boom, 45 FPS. I hate the stupid shadow system.

[LotusN]

Not sure if the game still processes them if there is 0 distance to draw them but might as well be safe and turn those settings down to minimum just in case ! I had initially done this because the flickering with the updates of position every few seconds was giving me headaches so did not check to see if there was a performance difference afterwards was just happy to have the flickering go away.