We have Tyler working for us again for the summer (last summer he did a very nice and much needed rework of the X-Plane manual), and among his projects is cleaning up the mess that is scenery documentation.
We are moving the scenery documentation from their own dedicated site to the X-Plane wiki. As of this writing, as you can tell, it’s a work in progress. There are a few reasons why we decided to consolidate to the wiki:
- The scenery website is the very best of 1995 technology – unmanaged php in a big mess of files. We wanted to get the site under some kind of content management system, and MediaWiki is already working well for us for other docs.
- There is a lot of overlap between modeling techniques for scenery and modeling techniques for airplanes, so having all of the third party authoring docs in one place makes sense.
It’ll be a few more weeks before everything is organized.
Tyler is also reviewing all of the documentation. I have had a lot of trouble trying to document the scenery system, partly because I have been working on it for years, and thus I have no sense of what people don’t know. Since Tyler hadn’t done any scenery work before, he was able to read the documents and go “hey Ben, you keep talking about X but you never defined what it is.” The resulting edits should make the docs a lot clearer.
In order to understand the difference between hiding geometry and disabling drawing, you need to understand that an OBJ triangle can serve many purposes. Broadly, those purposes are:
- Drawing (the most basic use).
- Collision detection, of which we have three flavors: collision of the plane with the object (“hard surfaces”, or “physics”), collision of the mouse with the panel (manipulators, or clickable triangles) and collisions of the camera with the airplane (“solid camera”, which constrains the camera).
Any given triangle can be drawn and/or used to check for any of these collisions*; attributes change what the triangle is used for.
By default, all triangles are drawn; ATTR_draw_disable marks future triangles as not being drawn. This allows you to make a triangle that is used only for collisions. Examples might include a “hot spot” in front of a region on the panel (the hot spot might be easier to click than a small switch) and an invisible simple mesh to constrain the camera.
By comparison, ANIM_hide effectively removes some triangles from your model (temporarily) for all uses – drawing and collision detection of any kind. If a door is hidden, it’s not only not drawn, but it’s not going to stop the camera moving through it either.
Some key points to these distinctions:
- Categorizing what a triangle is used for (drawing, various flavors of collisions) is static – that is, it is always the same for the object and never changes with datarefs or animation. This is intentional for performance reasons!
- Animation to hide triangles affects the triangle in every way consistently – drawing and collisions.
Generally, you will get better performance improvements by removing categories from a triangle than by hiding it. (That is, it is better to not have manipulators on your cockpit, so it isn’t mouse-click collision-checked, than to hide it.) But the purpose of ATTR_draw_disable and ANIM_hide are different enough that which you use will be determined by the effect you are trying to create.
Finally, note that hiding an object completely (that is, the object does no drawing) does not provide the maximum performance benefit of not having an object at all. ANIM_hide was created to allow authors to create clever effects, not as a performance enhancer!
* This is not quite accurate: airplane-object collision checks are only available in scenery objects, and camera/airplane or mouse/panel collision checks are only available in the cockpit object.
I was discussing plugin-controlled prop discs with a third party developer. The developer wanted to know if custom prop disc control would end up inside Plane-Maker. It may end up doing so, but I don’t think this would be nearly as useful as it would seem. What follows is my explanation to him of why this is.
Let me draw an analogy: when it comes to systems modeling, using a plugin is to Plane-Maker as using Blender is to using Plane-Maker.
Users who cannot use Blender are frustrated because they cannot make something as nice as those who are building planes out of OBJs. Sometimes they ask for more features in Plane-Maker, like: more stations! This new editing mode! Make the UI better!
But…you tell me: will Plane-Maker’s UI ever be as flexible and powerful as Blender? And if it ever did get to be that good, would that have turned out to be a good use of LR’s time, when Blender is already available?
The motivation for OBJ-based airplane geometry via third party tools is that what users want to do cannot be easily generalized into a few simple cases. Every plane is different, so a truly flexible platform is needed.
The prop disc (and other systems modeling problems) are the same way. In developing the prop disc graphics, I spoke with a number of third party developers who had already tried to push prop discs as far as they could go, were using plugins, were drawing themselves, as well as 25 other crazy hacks. I also spoke to our internal art team. And what I found was: no one had any consensus on how the prop disc system should work. Everyone wanted to tune a very specific set of behaviors to their peculiar art assets.
That’s what drove me to put it into a plugin. When we need an equation or a strategy we reach the point where we need more flexibility than Plane-Maker can exhibit. A plugin can encapsulate a strategy or technique in a way that Plane-Maker radio buttons cannot.
Consider what would happen if custom prop disc parameters were built into Plane-Maker. Everyone would have to wait until Austin implemented the prop disc algorithm they wanted. How would this be bad? Let me count the ways?
- How many algorithms do you think Austin has time to code? Not more than he has fingers on his right hand. Only the five lucky third party developers who get their algorithm coded will be happy with this.
- Austin code exactly what you want? Don’t get your hopes up.
- , what you asked for wasn’t what you wanted? We can’t change the behavior now, that would break compatibility!
- oh…your email got eaten by a spam filter? Too bad…no custom prop disc for you!
- Sorry, we don’t have a release vehicle lined up for the next 3 months. You’ll have to wait.
This problem is already happening across pretty much every aspect of systems modeling: airplane have unique, quirky systems which are usually useful for exactly one plane. It is not even remotely sustainable for X-Plane to code these things one at a time with a set of check-boxes. We might as well have a pop-up menu for every airplane ever invented, and simulate every single airplane inside the sim itself. Imagine the development costs…if a single high quality MSFS add-on sells for $30-$50…
Think of the prop disc via plugins situation (and the strobe lights are the same way) as an experiment in generic instruments for systems. By transitioning to a generic abstaction for instruments we’ve let a lot more users do exactly what they want with a small, high performance piece of code. The original instrument strategy (one of everything) reached a point where we simply couldn’t meet user needs in a cost-effective manner.
In X-Plane there are two fundamental ways that a texture can be painted “over” a background image:
- Blending, whereby the alpha channel of the new top layer decides how much we see the top layer vs. the background.
- Additive Lighting, whereby the new texture makes the background lighter.
Blending is more common. For example, if you build an OBJ, the object appears “in front of” the terrain via blending. With blending, you put the color of your new layer in the RGB channels and use the alpha channel to indicate opacity. 1.0 alpha = 100% opaque, 0.0 alpha = 0% opaque, and alpha in between will create a blend. If you omit an alpha channel, X-Plane will treat the entire layer as 100% opaque.
When a layer is applied using additive lighting, the resulting color is the sum of the background plus the new color, clamped to the maximum brightness we can show on screen. Additive lighting is good for simulating effects that really “add light”. Some examples of additive lighting in X-Plane:
- All lighting billboards are drawn additively.
- Instrument overlays are additive if you pick the appropriate mode in Plane-Maker. (The option is labeled “glass” for generics, since most glass instruments work by adding light to a nearly black screen.)
- The emissive (_lit) texture of an object is added to the albedo (daytime) texture using additive lighting.
Now just a little bit of math. In RGB color terms, black = 0,0,0 and white = 1,1,1. So if we add a pure black texture to a background additively we get:
new_r = old_r + overlay = old_r + 0 = old_r
new_g = old_g + overlay = old_g + 0 = old_g
new_b = old_b + overlay = old_b + 0 = old_b
In other words,when using additive light, adding “black” does nothing, preserving the background.
And this brings me to my main point: any time you have additive lighting you don’t need an alpha channel. You can simply make your additive lighting texture black for the parts you want to be “transparent”.
This is why I generally recommend that emissive _LIT textures for objects not have an alpha channel. In fact, for “back-lit” and “additive” instruments (these are instruments that use a second emissive _LIT texture) Plane-Maker will indicate a warning if the texture has an alpha channel. If you have a texture that is applied additively, you don’t need alpha.
At this point you might be wondering: Ben, if additive lighting doesn’t require alpha, and all lighting billboards are drawn additively, why the heck is there an alpha channel for custom lights?
The short answer is: there probably doesn’t need to be one; the original setup for lighting billboards inherited a number of idioms from older versions of X-Plane, going back to versions where lighting billboards were not additive* (and thus alpha was necessary).
The long answer is: the alpha channel is often as a general “intensity” control to turn the light up and down in amplitude, while the RGB channels are often re-interpreted in strange ways. So while RGBA color is not necessary from a graphics standpoint, it is handy that there are four color channels in the custom lights because that gives us one more parameter to play with when designing really compicated lights (like VASIs).
* Note that lighting billboards that aren’t additive don’t look very good…hence Austin switched to additive billboards.
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.
