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:
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.
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.)
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.
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
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.
A number of people are working on an update to Jonathan’s Blender X-Plane export scripts; this post is aimed at shedding some light on some of the recent changes to the OBJ format. X-Plane 9 introduced a number of new OBJ features (manipulators, invisible geometry and camera collisions, dataref-driven control of emissive texturing, normal maps, and a number of new light billboard options). If you simply read the new OBJ commands in the order they were added to the format, it’s just a soup of funny names. But there is some logic to how the OBJ format is extended.
The World’s Simplest OBJ
Here is a very simple OBJ file, broken up by my annotations. First we have the header and global section:
POINT_COUNTS 24 0 0 36
the global section describes properties universal to the entire OBJ. For example, what textures will be used to draw the object?
We picked up a few new global properties in the version 9 run:
- Normal maps are declared globally for the entire OBJ.
- The metrics of any panel regions to be used are declared globally.
We may pick up new global attributes in the future; if we do, they will be properties that apply to the entire obj.
Next comes the data section:
VT 0.449997 0.300003 0.860001 1.000000 0.000000 0.000000 0.000000 0.000000
VT 0.449997 0.300003 0.000000 1.000000 0.000000 0.000000 0.000000 1.000000
VT 0.449997 -0.509995 0.860001 1.000000 0.000000 0.000000 1.000000 0.000000
IDX10 0 1 2 3 2 1 4 5 6 7
I have removed a lot of the data section, because there’s not much to be said about it. The data section contains the raw data for the meshes in your OBJ, and it hasn’t changed since the OBJ 8 format was introduced.
The third and final section is the most interesting one: the commands section.
ATTR_LOD 0 3000
TRIS 0 36
The commands section describes how the data is used in the form of serial instructions to X-Plane. Most changes to the OBJ format have come in the form of new commands. We can categorize our commands into a few buckets:
- Drawing commands create “stuff”. There aren’t very many drawing commands, and new ones don’t appear very frequently. TRIS and LINES are the main commands, but the smoke commands also fall into this category, as do the light billboard commands. The new light billboard command LIGHT_PARAM is the only new drawing command for version 9, and it probably warrants its own blog post.
- Attribute commands change how stuff is drawn – they effectively define properties for drawing on all triangles that can be modified. We picked up a number of new attributes: manipulators (controlling how the mouse works), light level control, solid camera, draw disabling, deck style hard surfaces, and panel regions. (While you must declare the panel region locations globally, a panel region is enabled for a specific batch.)
- ATTR_LOD is sort of an exception, because it defines the structure of the model (e.g. a model with LOD really contains several separate command lists, of which only one is used).
Most new extensions to the OBJ format come in the form of new attributes. Attributes generally apply to a specific mesh within your model, not to the entire model.
Note that attributes can be thought of as “per mesh” or “per batch” properties, because they affect only the batches of mesh (TRIS commands) between the attribute being turned on and turned back off again.
Where Will New Features Appear?
I try to post some of my crazier ideas regarding OBJ on the scenery system RFCs page. Looking at the extensions, we can see how these extensions will all be either global, drawing primitive, attribute, or OBJ structure extensions. (I am not promising that any of these RFCs will be implemented, just showing how the OBJ format grows.)
- Additive LOD. This is a change to the structure of an OBJ, but it doesn’t actually change the format, just the legal LOD values.
- Explicit OBJ Height. This is a global property on the OBJ.
- Global Texture Variants would be a global property on the OBJ’s textures.
- Global Object Attributes are new global properties – they move some per-batch attributes to be object-wide.
- Draped Object Geometry would be per batch.
In summary, the vast majority of proposed extensions are new per batch or per object properties.
Next: what are the implications on performance of the various sections of an OBJ?
Meshes in X-Plane, whether modeled in an OBJ, or generated as the results of other “3-d clutter” (road .net files, .for forest files, etc.) can be either one or two sided. So first: two-sided geometry is bad in most cases.
In order to understand why two sided geometry is bad, we must consider the alternative. The alternative to two-sided geometry is to simply create each triangle twice, with one facing in each direction. We can do this in an OBJ without making new vertices – because vertices are referenced by indices, we only need more indices, and indices are cheap.
Thus we have an alternative to two sided geometry, namely “doubled” one-sided geometry.
The first problem with two sided geometry is performance: for a small number of triangles, it is must faster to simply emit additional indices than to change the drawing mode to two-sided drawing on the CPU.
Thus in an object it is virtually never a good idea to use two-sided geometry. That ATTRibute will always be worse.
What about for the other clutter? Forests are currently always two-sided, but that’s okay; X-Plane enables two sided drawing just once, then draws a huge number of trees. Same with roads. For facades, there is a cost to using two sided geometry, so only use it for facades that must be two sided, like fences; do not use it for buildings.
Now the second problem with two sided geometry is lighting: X-Plane does not calculate lighting values separately for the two sides of the two-sided geometry. So if you have directionally lit models with two sided geometry, the lighting will look wrong. This is the second reason to use doubled geometry instead.
Things Are Starting To Look Up
There is a work-around to this problem of incorrect lighting on two-sided geometry: “up normals”. With up normals, the normal vector for the triangle (which is used to determine how light “bounces” off the triangle) is set to face straight up. The result is a triangle with brightest lighting at high noon, regardless of which way the triangle actually faces.
The good: the triangle looks the same on both sides and has sort of a “flat” lighting – it doesn’t look wrong when the sun is setting. The bad: the triangle has “flat” lighting – it looks non-3d.
We use up normals for forests because the forests are made of two-quad trees…the trees look less fake if directional lighting hints don’t make the two quads as obvious. You can simulate this in ac3d using the “make up normal” command for vegetation quads you put in your own models.
For roads, the geometry is two-sided, so we use up normals to avoid having the back of a road element look funny. Some day we may do something more sophisticated.
Fixing Facade Lighting
Facade lighting behavior will be changed in the next 950 release candidate. Before 950, facades would receive up normals, always. Starting in 950, facades will get correct normals if they are one sided and up normals if they are two-sided. This avoids artifacts with two-sided facades, but will make one-sided closed buildings look much better.
Normal maps in X-Plane 940 have a funny property: if you flip the normal map horizontally or vertically, the bumps change direction. Things that “stuck out” now “stick in” and vice versa. (If you flip the normal map horizontally and vertically, the two cancel out and the normal map is not reversed.)
You can understand this by thinking of your normal map as a physical piece of metal with bumps punched in it. Flipping it horizontally really means rotating it horizontally to see the other side – now you see the back side of the bumps. Same with the vertical flip. Flip both and you have flipped it twice and it’s front-side forward again.
In a move that is either just in the nick of time or dangerously reckless, I have tweaked the normal map shader for 950 RC1 (coming out “real soon”) to detect and “fix” a flipped normal. In 950 rc1 the bumps in your normal map will always point in the same direction as the normal of your mesh, even if your UV map is flipped horizontally or vertically.
What does this mean to you, the modeler? It means that you can now mirror your normal map from the left to the right side of the airplane and the normal map will still have the bumps “sticking out” like you want.
I crammed this into 950 RC 1 because it looks like it’s a useful change that will restore flexibility to authors making highly detailed airplanes. Mirroring a symmetric airplane (which results in a horizontally mirrored normal map) is a pretty common thing to do, and if the text is applied as decals, this can be a big win in texture space savings.
I figured best to get the tweak in here now so people could take advantage of it. Plus, what’s an RC1 without an RC2?
Tom has a new video on youtube of his just finished Falco. The video shows what screen-shots cannot: that the mouse interactions on the plane are really well crafted.
If you’re just discovering X-Plane (or just discovering that X-Plane’s 3-d cockpits can be very interactive), here’s X-Plane’s “raw” capabilities for manipulation:
- The simplest manipulations are based on mapping the mouse from the 3-d cockpit back to the 2-d panel. This can only be done when the 3-d cockpit is textured using a piece of 2-d panel. This is the oldest way to make a clickable cockpit in X-Plane, dating back to the original X-Plane 3-d cockpits. The advantage of this method is that it’s very easy to set up; the disadvantage is that the mouse click gestures tend to be “flat” in their operation.
- As Tom’s plane demonstrates, you can manipulate just about any dataref or command via a drag along a specific axis. Axes are subject to animation, so there’s a lot of potential for “grabbing” things with this interface.
- X-Plane also supports direct “click” manipulation – this can be handy for buttons where you don’t want to require the user to move the mouse around. There are several types of click manipulation.
Click and drag manipulations can be tied into the plugin system – your plugin sees a manipulation as a change to a plugin-created dataref. This makes it possible to create almost any imaginable mouse effect. If you don’t want to write a plugin, you can still write up the manipulators to any of X-Plane’s datarefs (there are thousands) or commands (we’re getting up toward the 1000 mark on these too).
To create manipulators on your cockpit, you can use the latest plugin for AC3D. A manipulator is a property on a mesh within your object – each mesh can have its own manipulation with its own properties.
X-Plane does not have an IK solver. Rather, movement of “stuff” in your cockpit is indirect.
- Your manipulator changes a dataref as the user drags along an axis.
- The dataref change shows as an animation on your mesh.
Fortunately, ac3d has a “Guess” button for the axis manipulators. If you set a mesh to be manipulated by dragging along an axis, the guess button will examine your animations and suggest an axis that will create the most “natural” looking animation for the manipulation. For example, if you have a throttle handle that rotates, the guess button will provide a drag axis perpendicular to the throttle (to push the levers); if you have a throttle lever that pushes, the guess button will make a drag axis that runs along the lever.
X-Plane 9 has a number of recent features to let you customize the exterior lighting of your aircraft; see the wiki for notes and a sample plane.
X-Plane 940 introduced the concept of parameterized lights to support these features. Here’s the basic idea:
Named lights (available for quite a while now) let you add a light billboard to your model that we define. The idea is that since the lights are specified against a real world model (this light billboard should look roughly like a landing light) it lets us upgrade art assets and back the light with the fastest path on the graphics card.
The problem with landing lights is that they are one-size-fits all, and this is particularly problematic for airplanes, where the lights can look quite different in size and angle based on the size of the airplane. Parameterized lights fix this by letting you specify a limited number of parameters in your OBJ. By limiting the parameters that you can set, it means that we can still optimize the light when possible.
I took a few minutes today to round out the list of parameterized lights, and I think there will be 9.46 patch in which we can release them*. When we put 9.46 in beta I’ll update the example plane; the new set of lights will give you parameterized control over the navigation and taxi lights, as well as the generics, beacons, strobes and landing lights.
* We have a few small bug fixes we’ll roll out in 9.46.
I am trying to be disciplined and put documentation on the X-Plane wiki, and limit the blog to announcements, rants, and explanations of what’s going on inside X-Plane.
You can read about custom lights here. The short of it is that a custom light is a billboard on an object where you (the author) texture the billboard (with part of the object texture), pick the texture coordinates and color, and optionally run all of these parameters through a dataref* that can modify them.
For named lights, the light texture comes from a texture atlas that Sergio made a few years ago – it’s a nice grid 8×8 pretty lights.
So…why can’t you use it with custom lights? Why do custom lights use the object texture?
The answer is: future compatibility. Sergio and I are actually already working on a new texture atlas for the sim’s built-in lights. (This has been a back-burner project for a while … I have no idea when we’ll actually productize this currently experimental work.) What happens when we create a new texture atlas with all of the lights moved around and scrambled? If your object referenced that texture, the texture coordinates would be incorrect.
Thus, for the lights where you specify texture coordinates (custom lights) you use your own texture. For named lights (where the texture coordinate is generated by X-Plane) it’s safe to use ours.
A Dangerous Bug
I found a bug in 940 that’s been in the sim for a while now: given the right strange combination of named and custom lights in a row, the sim would accidentally use Sergio’s texture atlas rather than the object’s texture for custom lights.
This is a mistake, a bug, and it will be fixed in the next 941 release candidate. I certainly hope there aren’t any objects out there relying on this erroneous behavior, which violates the OBJ spec and is pretty dangerous from a future compatibility standpoint.
* Dataresfs are normally thought of as data we read, so the idea of using them to “process” data is a bit of a bastardization of the original abstraction. You can read about the dataref scheme in detail here.
X-Plane 940 has these generic light things…what the heck are they? Here’s the story:
X-Plane has been growing a larger number of independently simulated landing lights with each patch. We started with one, then four, now we’re up to sixteen. Basically each landing light is a set of datarefs that the systems code monitors.
- You use a generic instrument to hook a panel switch up to a landing light dataref.
- The sim takes care of matching the landing light brightness with the switch depending on the electrical system status.
- Named lights can be used to visualize the landing lights.
See here for more info.
But what else lights up on an airplane? Sergio sent me the exterior lighting diagram for an MD-82, and it would make a Christmas tree blush. There are lights for the staircases, for the inlets, on the wings, pointing at the wings, the logo lights, the list goes on.
We have sixteen landing lights, so we could probably “borrow” a few to make inlet lights, logo lights, etc. But if we do that, the landing light will light up the runway when we turn on any of those other random lights.
Thus, generic lights were born. A generic light is a light on the plane that can be used for any purpose you want. They aren’t destined for a specific function like the strobes and nav lights. There are 64 of them, so divide them up and use them how you want. Just like landing lights, you use a generic light by:
- Using a generic instrument to control its “switch” from the panel.
- Using a named light to visualize it somewhere on the OBJs attached to your airplane.
Generic lights don’t cast any light on any other part of the plane – sorry. You can use ATTR_lit_level to light up part of your mesh dynamically when the generic light comes on though – the effect can be convincing if carefully authored.
Normally I try to make the X-Plane scenery and modeling system as opaque as possible — I want to make sure that nobody ever actually uses the rendering features that I spend weeks and weeks developing.
But the other night I had a little bit too much to drink, got distracted, and posted these:
In all seriousness, I have been trying to find time to put more documentation up on the Wiki. For these features, you will find an explanation of how the planes work, as well as a link to the planes (with plugins) to download, and a link to the plugin source code (on the SDK site, with sample makefiles for 3 operating systems).
Plugins? Do not panic! While plugins are necessary for some of the features demonstrated here, others can be created without additional programming.
BTW, if the existing documentation uses a concept that is not explained anywhere, please email me. I sometimes leave holes in the documentation by accident.