Back in olden times (read: X-Plane 6) X-Plane would look for file names with special extensions to enable special features. For example, in many cases appending the suffix _LIT to your texture name specified that you wanted a second emissive texture attached to your scenery.*
Then I showed up and started replacing the “find by extension” scheme with specific commands in configuration files to induce special behavior. On pretty much all of the modern text file formats, you need to use TEXTURE_LIT to specify your emissive texture, and once you use this command, you can use any suffix you want.
Why the change? I had three reasons:
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Features by file naming convention is ugly. That’s just an aesthetic decision by me, and most users find file extensions simpler, so if this was the only reason, I’d be a bit of a jerk.
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File naming conventions don’t grow over time. With file naming conventions, additional features would mean truly crazy conventions. Consider terrain files, where any terrain can be loaded with wrapped or clamped edges. (Wrapped edges are appropriate for repeating tiles of “land class” and clamped edges are appropriate for orthophotos.) We would end up with my_texture, my_texture_LIT, my_texture_wrapped, my_texture_wrapped_LIT, etc. What about the scale of the texture? What if we implement seasons? my_texture_2000_2000_wrapped_summer_LIT.png it is.*
By putting features in text files, we leave room for future expansion, and we have a lot more room to attach data to an image or control shading.
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Probably most important: going to disk and loading lots of images is slow, so X-Plane loads images in the background while you fly. But checking the disk to see if my_texture_LIT exists is a disk operation too, and it is potentially one that we have to do immediately to understand our art assets. By having information about art assets in the text files and not file naming conventions, we can read one text file, rather than searching for all possible combinations of file extensions.
With X-Plane 9 we’re starting to see enough complexity in shading configurations and options that file names would be a non-starter. Consider some of the new features in version 9: normal maps and paged orthophotos. X-Plane 10 will introduce even more rendering and shading capabilities. These configuration options are better kept in a text file than a file name.
* Actually, Sergio and Austin were heading in that direction. Note that the names of the old ENV land classes were in the form CropCool0_000.dds. What are all the numbers? I don’t know, but Sergio had a plan to use them to select lots of different textures. This scheme was never implemented.
Austin attended the French X-Plane Congress last weekend; in response to some questions I have received, I want to clarify what is planned for DSF and X-Plane 10.
X-Plane 10 will extend DSF scenery capabilities by providing a number of new art asset types, as well as extensions to existing art asset file formats. We will not be changing the DSF file format or breaking compatibility with existing DSF scenery. If your DSF scenery works with version 9, I expect that it will work with version 10 as is.
(In fact, it looks like we do not even need to add new DSF extensions; DSF was designed to be a generic container for geometry data and properties, so we can usually extend X-Plane’s scenery system by simply defining new art asset classes and properties.)
An example will illustrate what I mean by extending the art assets, not DSF.
In X-Plane 9, you create a forest (whether in a base mesh or overlay DSF) by creating a DSF polygon referencing a .for (forest) art asset. X-Plane will render this by filling the area inside the polygon with lots of trees.
In X-Plane 10, you will be able to optionally tell X-Plane to put the trees only on the edge of the polygon, rather than filling the entire inside. (This feature will be controlled by using values on the polygon parameter that are larger than 255, at least I think.) This will mean that you can use .for files and DSF polygon not only to create forest areas, but also to create rows of trees along the edges of roads or fields. A row of trees made by a .for file and DSF polygon will render much faster than a large number of individual OBJ-based trees.
This kind of art asset extension is similar to what we have already seen; X-Plane 850 introduced three new art asset types (.str object strings, .lin line paint, and .pol draped polygons) that all produced new rendering tricks using DSF polygons. These art assets were added to provide high performance rendering of airport surface areas. The new art assets didn’t require any change to DSF because the representation of the position of these art assets is something DSF has always done: simple polygons.
DSF won’t last forever, but at least for X-Plane 10 it looks like we can do a third full cycle of the sim using DSF as our base container format for scenery position data.
In the past I have suggested that there is a limit to the value of additional VRAM when buying a graphics card. When I posted this, Nicholas from the org emailed me to point out that with third party add-ons VRAM was a lot more important.
At the time I wasn’t really convinced, but I’ve finally come around; it takes a while for the trend to get back to me. (I just don’t have time to look at everyone’s add-ons the way I used to 4 or 5 years ago.) It seems clear that airplane authors (and to some extent scenery authors) are using VRAM pretty aggressively. If you want to use third party add-ons and you care about texture res and texture sharpness, spring for some VRAM. It doesn’t cost as much as it used to, and authors are starting to use it.
What Lives in VRAM
Bear in mind that in any discussion of how your video card operates, anything I post is informed speculation. The driver provides an abstraction (OpenGL) of what the hardware does, and a lot of the bookkeeping isn’t visible to X-Plane at all. So what I am describing is typical of past video drivers that we have had insight to in the past, but it’s not universal, and it’s not at all guaranteed. (X-plane can’t demand any of this behavior of the video driver.)
In order of how “stuck” in VRAM things are we have:
- Video memory used for on-screen rendering. Depending on rendering settings you can lose anywhere from 12 to 24 MB of VRAM per million pixels on screen. So if you’re running at 1920 x 1200, you might be using 50 MB of VRAM just for the screen. If you use FSAA, you’re going to chew up VRAM even more aggressively. (Costs vary depending on the scheme; you might lose 4-16 MB x the FSAA level per million pixels on screen, depending on your GPU and driver.)
- Off-screen rendering for things like the water reflections, the panels, the cloud shadows, the airplane shadow, etc. These don’t have to be in VRAM all of the time, but they have to be in VRAM almost all of the time. Because they are created by the GPU, the driver tries hard not to move these out of VRAM. You might lose 6 to 16 MB of VRAM for these, depending on the airplane you use and settings. (Given 4 1024×1024 panel regions, the panel will chew up 32 MB!)
- Textures end up in VRAM, but only when they are used. The key here is “working set“. Only texures that are drawn need to be in VRAM, so over time stuff that isn’t on screen will be removed from VRAM. This is why when you see “600 MB of texture memory” in the rendering settings, there is no need to panic. The working set is usually much smaller.
- OBJ geometry actually lives in VRAM too, sometimes. Again this is a working set issue; objects that aren’t drawn don’t get cached there.
- Textures from airplanes and scenery packages that are not loaded don’t ever end up VRAM or even system memory; we only load what we need. Paged orthophotos have their resolution reduced while you fly, which makes their VRAM footprint quite small, even when drawn.
There are a few things authors and users are doing now that use up a lot of VRAM.
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Monitors have gotten bigger; the VRAM used for the screen itself can never be swapped out, so the advent of the 1920×1200 LCD has taken its toll.
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Since the panel texture is drawn off-screen, the panel texture is in an expensive category of VRAM use. Authors can limit the cost of this by using a single 1024×1024 panel region texture, if possible.
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There is a hidden cost here: we pack together instrument textures into “atlases” to help with performance. The problem is that we pack for fit. Some of your instruments may be hidden but loaded into VRAM anyway because they sit in the same atlas texture as other instruments that are drawn. Thus you may be paying for the VRAM used by your entire panel even if a lot of it is hidden.
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Because a lot of this VRAM is going to airplanes, reducing texture resolution doesn’t have as much impact as it used to; X-Plane tries to keep the user’s plane’s resolution as high as possible since it is viewed up close. The panel cannot have its resolution reduced at all.
But It Works Sometimes
I think what drives users crazy about VRAM exhaustion is that X-Plane will sometimes run smoothly, and then fail later. And sometimes really strange things, like moving X-Plane to the background, then the foreground, or changing liveries or rendering settings in a trivial way, will change performance.
I have discussed this a bit in past posts. But the key here is “working set”:
- In any one frame, we can access everything that is permanently in VRAM, plus as much data as we can put through the PCIe bus from the CPU to the GPU.
- We only need to access data in the working set (what is on screen).
- Some of VRAM is permanently used (e.g. memory for the screen itself).
- We lose PCIe bandwidth to both drawing from main memory (terrain is in your system memory and must go over the PCIe bus per frame) and from bandwidth spent juggling textures.
So the actual maximum throughput will have a lot to do with whether the video driver makes good decision about what lives in VRAM and what does not.
But how does the video card know what should be in VRAM? The answer is that it has to guess. It looks at frames going by and tries to use heuristics (that is computer-science geek speak for “carefully formulated wild guesses”) to decide what goes in VRAM and what does not. When the heuristics happen to make good decision, your video card kicks ass. And when it does not, your framerate tanks.
The only way to guarantee good framerate is to use so little VRAM that everything that needs to be in VRAM can be in VRAM, without depending on the video driver to make lucky guesses with its juggling.
And this helps us understand why strange things like livery reloads and backgrounding the sim can affect framerate (for better or worse). These operations seriously reshuffle VRAM – either by deleting textures and loading new ones, or by forcing everything out of VRAM so the video driver must try to repack video RAM all over again.
Unfortunately as a user this means that there’s not much you can do about this as a user. The main things would be: reduce screen size or FSAA or texture resolution, use fewer add-ons, or get more VRAM. Those peak bursts of framerate you see, they’re not going to be sustainable .
REX is here for X-Plane. That’s really exciting to me because REX is a very successful add-on to FS X, and it’s been understandably difficult for us to convince companies that have a functioning and profitable business model on FS X to jump over to X-Plane, which requires building new capabilities in-house, starting at the bottom of the learning curve, and developing new processes. I hope these guys have a ton of success. Heck, I’d buy the product just because REXPlane is such an awesome name!
What surprises me is that we haven’t seen more ports of airport scenery. These packages can be partly converted mechanically using Jonathan Harris’ FS2XPlane tools, and in the case of an airport, the potential for recycling is huge – that is, the largest part of the development process is modeling, creating textures, and connecting the two. Since X-Plane can use these art assets with minimal mods, it seems like a very viable type of port.
A slightly surprising note about X-Plane’s rendering settings: increased detail in the water reflections mostly increases load on the CPU! Here’s what is going on.
X-Plane’s water reflections are a two-step process:
- A (simplified) version of the world is drawn into a texture .
- That texture is used (and perturbed) to draw the surface of the water.
The water reflection detail setting affects a number of this: the size of the texture used to draw the world, the level of simplification of the world drawn into the texture, and I think some details about the shader used to draw the water surface itself.
Of these factors, the biggest impact on frame-rate is almost always simplification of the world as drawn into the water texture. At its most detailed (“complete” reflections) that rendering is almost identical to the main sim rendering, and thus reflective water should cut your framerate in half!
In particular, the big hit of drawing the world a second time is that it means drawing objects a second time. Thus the higher the reflection setting, the more objects are being drawn. In a scenario where objects are already the main drag on frame-rate, using complete water reflections will make this even worse.
There are two things to conclude from this:
- A bigger, faster CPU won’t let you turn up the water reflection detail. Most decent GPUs handle the shader-side just fine. It’s the objects that are the problem.
- You shouldn’t turn up the water reflection all the way unless you’re taking screenshots. It just doesn’t add much visual improvement for the fps hit.*
* This is one of those cases where I ask myself: why did I let the setting go up this high?
I’m trying to post weird in-development screenshots as I get them because, well, sometimes they are entertaining, in a nerdy sort of way.
This is experimental test code for road draping.
The idea will be to let X-Plane “drape” the road over the base mesh, so that an overlay can contain customized roads that work with any base mesh.
From one of the podcasts I listen to I just discovered Edward Burtynsky. He takes these amazing photographs of industrial landscapes – really scary post-apocalyptic images of oil refineries, chopped up cargo ships, etc. Take a few minutes to look through some of the images.
One of the side effects of working on X-Plane scenery for the last few years is that it has made me look a lot more closely at the world. Once you try to recreate the world on a computer (and watch your digital creation fall way, way short) you realize how much intricacy and detail every-day phenomena have.
So when I saw Burtynsky’s photos I immediately thought “he sees the complexity and beauty* and sadness of industrial landscapes the same way we do!”
* Beauty? I am not suggesting that the SOCAR oil fields are beautiful, a particularly good idea, or something I want more of. But I think that there can be a poetry in the image – perhaps a poetry of despair.
With the latest updates to X-Plane for iPad, you can now purchase the Apollo lunar lander game inside X-Plane.
A quick note: if you lose your preferences (and thus X-Plane thinks you have not already bought Apollo) you will not be charged a second time if you click “buy it” again. When you go to repurchase Apollo, X-Plane will notice that you already have bought the upgrade and will simply re-enable the update.
Hopefully in our next patch we will make it clear in the UI that there is no double-purchase. The upgrade to Apollo is a “persistent” in-app purchase – once you buy it, you own it forever, as you would expect. (And because the iTunes store has a copy of your receipt, you don’t lose it even if your iPad or computer’s hard drive gets wiped out.)
Also, the latest update should fix crashes on iPads that were running for a while. X-Plane was very close to the memory limit so iPads that had been running for a while wouldn’t have enough free RAM for the sim. The new patch is a little bit leaner to work around this problem.
Sometimes I get reports of a slow airplane, and I do a quick audit for performance problems. The trick to spotting performance problems is to divide and conquer: turn off various aspects of the airplane to see which aspect is really causing performance problems, then optimize that aspect.
Here are some of the specific tricks I do:
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Change views; the panel will be drawn differently in the 2-d view, 3-d cockpit view, external view (when close or far – zoom out and the panel won’t be updated) vs. 2-d no HUD.
If the 2-d view is slow but forward-no-HUD is not, your panel is expensive. If the 3-d view is slow but 2-d is not, one of your panels may be more expensive than the other (copy them in Plane-Maker from one to the other to see) or it could be that the preparation of the cockpit texture is slow.
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Remove 3-d objects from your plane to test the cost of OBJs. Turn down X-Plane’s texture res or shrink your textures to see if texture memory is at issue. (Some airplane textures are not affected by the texture res settings, so you may have to manually shrink them.)
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Be sure to play with X-Plane’s rendering settings; the GPU-specific options don’t always cost “the same”. For example, per pixel lighting is more expensive when there is more translucency on screen. If your airplane has a lot of overlapping surfaces or translucency this otherwise manageable setting might become too slow.
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If you use panel regions, try switching to regular ATTR_cockpit. Panel regions provide superior lighting effects but can take more CPU time when you have a lot of instruments.
The key is to divide the many possible causes of performance problems to isolate one thing that can be optimized.
In my previous post I tried to break an OBJ down into a few basic sections:
- Global properties of the OBJ.
- Raw Mesh Data
- Commands, which in turn set per-batch state and then draw the batches.
The performance cost of an OBJ feature often has a lot to do with where in the OBJ the command shows up, e.g. is it global or per batch.
Global properties tend to affect OBJ performance on a one-time basis. For example, if you use cockpit regions, you pay a fairly large penalty for having the panel texture be set up even if you only apply that panel texture to a single texture. Sure enough, COCKPIT_REGION is in the global properties section of an OBJ.
Per-batch properties affect the OBJ in two ways:
- Every command you see in the commands section is going to involve some CPU intervention. A very long commands section is more work for an OBJ.
- Every time there are attributes between TRIS commands, it defines a new “batch” – that is, a separate instruction to the graphics card to draw a new and distinct setup. Think of this as shutting down the factory to reconfigure the assembly line.
Generally batch count is more important than total commands. In other words, in evaluating this:
TRIS 0 300
ATTR_light_level 0 1 some_dataref
ATTR_no_blend 0.5
TRIS 300 12
the fact that there are two attributes is less interesting than the fact that there are two batches (the two TRIS commands run with different state). Even if you got rid of the no-blend attribute, you’d still have two batches because of the light-level change.
The most powerful aspect of the OBJ format is bulk data handling – that is, you have to add a huge number of triangles before the number of triangles becomes a performance problem.
For this reason, you should never use an attribute to reduce geometry count. A few examples:
- Don’t use ATTR_no_cull to reduce triangle count – simply issue the indices of the triangle twice.
- Don’t use ATTR_flat_shade to reduce vertex count – simply use more vertices with correct per-vertex normals to simulate flat shading.
- Prefer texturing to materials whenever possible.
Finally a note on weighting: for airplanes, where the total number of objects is low (a few dozen) global object properties often matter most. For example, on an airplane, choosing to use huge panel regions, or huge textures can make a big difference in performance. By comparison, batches aren’t that expensive unless you do something really crazy.
By comparison, for scenery, batches matter more; X-Plane will share the global properties of objects across hundreds or thousands of objects, but each batch hurts framerate. So when making autogen-style scenery, batches are most important.
