Now that Wade has XSquawkBox 1.0.3 out, I’ve been thinking about CSLs – that is, the collections of simplified airplanes that XSquawkBox uses to draw the other users. The CSL system was invented back in the days of X-Plane 6, and it’s getting a bit long in the tooth. You can’t use OBJ8 files, and it doesn’t understand a lot of the modern rendering tricks that authors use with the standard tool chain.
Plane-Maker has advanced quite a bit since then too – to make the original CSL, I had to create a special one-off hacked build of Plane-Maker to export aircraft as OBJs. This capability is now built into Plane-Maker, works a lot better, and even supports animation.
X-Plane now exports a native OBJ drawing interface to plugins. Besides giving plugins access to the fully optimized native OBJ draw code, this also means that plugins can draw objects with per-pixel lighting.
One more piece of the puzzle: in France Austin announced that we were working on a new ATC engine. One goal of this new engine is to provide ATC to the AI planes, as well as your plane, so that the other aircraft interact seamlessly in one simulated environment. (In X-Plane 9, ATC only directs you, and the AI are rogue planes that try not to buzz you when you’re on the runway.)
This makes me wonder: should there be a next-gen CSL format that is shared between X-Plane and third parties, based on X-Plane doing the rendering work?
Wade just cut the final build for XSquawkBox 1.0.3, and of all of the betas I have been involved in, this was the best one ever. I say this because Wade ran the entire beta and fixed pretty much all of the bugs without me! That is to say, Wade made 1.0.3 happen. So huge thanks to Wade for making a new and much needed XSquawkBox version possible.
Here are some of the features:
- Automatic download and update of the server list.
- Dual VoIP radios.
- Proxy client support for users with multiple copies of X-Plane for multiple views.
- A ton of work on the UI – a lot of little things add up to make a much more mature, usable client.
Now if only I had time to fly on VATSIM…
This isn’t supposed to be a coding blog, but users do ask about DirectX vs. OpenGL, or sometimes start fights in the forums about which is better (and yes, my dad can beat up your dad!). In past posts I have tried to explain the relationship between OpenGL and DirectX and the effect of OpenGL versions on X-Plane.
At the Game Developers Conference 2010 OpenGL 4.0 was announced, and it looks to me like the released the OpenGL 3.3 specs at almost exactly the same time. So…is there anything interesting here?
A Quick Response
In understanding OpenGL 4.0, let’s keep in mind how OpenGL works: OpenGL gains new capabilities by extensions. This is like a new item appearing on a menu at your favorite restaurant. Today we have two new specials: pickles in cream sauce, and fried potatoes. Fortunately, you don’t have to order everything on the menu.
So what is OpenGL 4.0? It’s a collection of extensions: if an implementation has all of them it can call itself 4.0. An application might not care. If we only want 2 of the 4 extensions, we’re just going to look for those 2 extensions, not sweat what “version number” we have.
Now go back to OpenGL 3.0, and DirectX 10. When DX10 and the GeForce 8800 came out, nVidia published a series of OpenGL extensions that allowed OpenGL applications to use “cool DirectX 10 tricks”. The problem was: the extensions were all NVidia specific tricks. After a fairly long time, OpenGL’s architectural review board (ARB) picked up the specs, and eventually most of them made it into OpenGL 3.0 and 3.1. The process was very slow and very drawn out, with some of these “cool DirectX 10 tricks” only making it into “official” OpenGL now.
If there were OpenGL extensions for DirectX 10, who cares that the ARB was so slow to adopt these standards proposed by NVidia? Well, I do. If NVidia proposes an extension and then ATI proposes a different extension and the ARB doesn’t come up with a unified official extension, then application like X-Plane have to have different code for different video cards. Our work-load doubles, and we can only put in half as many new cool features. Applications like X-Plane depend on unity among the vendors, via the ARB making “official” extensions.
So the most interesting thing about OpenGL 4.0 is how quickly they* made official ARB extensions for OpenGL that match DirectX 11’s capabilities. (NVidia hasn’t even managed to ship a DirectX 11 card yet, ATI’s HD5000 series has only been out for a few months, and OpenGL already has a spec.) OpenGL 4.0 exposes pretty much everything that is interesting in DirectX 11. By having official ARB extensions, developers like Laminar Research now know how we will take advantage of these new cards as we plan new features.
Things I Like
So are any of the new OpenGL 3.3 and 4.0 capabilities interesting? Well, there are three I like:
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Dual-source blending. It is way beyond this blog to explain what this is or why anyone would care, and it won’t show up as a new OBJ ATTRibute or anything. But this extension does make it possible to optimize some bottlenecks in the internal rendering engine.
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Instancing. Instancing is the ability to draw a mesh more than one time (with slight variants in each copy) with only one instruction to the graphics card. Since many games (like X-Plane) are limited in their ability to use the CPU to talk to the graphics card (we are “CPU bound” when rendering) the ability to ask for more work with fewer requests is a huge win.
There are a number of different ways to program “instancing” with OpenGL, but this particular extension is the one we prefer. It is not available on NVidia cards right now. So it’s nice to see it make it into the core spec – this is a signal that this particular draw path is considered important and will get attention.
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The biggest feature in OpenGL 4.0 (and DirectX 11) is tessellation. Tessellation is the ability for the graphics card to turn a crude mesh with a few triangles into a detailed mesh with lots of triangles. You can see ATI demoing this capability here.
There are a lot of other extensions that make up OpenGL 3.3 and 4.0 but those are the big three for us.
* who is “they ” for OpenGL? Well, it’s the architectural review board (ARB) and the Khronos group, but in practice these groups are made up of employees from NVidia, ATI, Apple, Intel, and other companies, so it’s really a collective of people involved in OpenGL use. There’s a lot of input from hardware vendors, but if you read the OpenGL extensions, you’ll sometimes see game development studios get involved; Transgaming and Blizzard show up every now and then.
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.
I’ve been working on a conformance test for X-Plane. The idea is simple, and not at all mine: X-Plane 945 can output a series of test images that are the same on each run. The images cover a variety of rendering conditions. If a video driver is broken, the images will be corrupted.
You can learn more about how this works here: I am working on the 945 timedemo tarball now.
The main driver for this is to help NVidia, ATI, and Apple to integrate X-Plane into their dedicated testing. With X-Plane as part of their test systems, they can catch driver bugs the easy way – the day after the code is changed, rather than months later after a series of angry web posts. X-plane 945 includes a number of new features as part of its framerate test to help with this process.
My hope is that this will benefit users (who will see less bugs) and the driver writers (who can get feedback on code changes in a uniform and reproducible manner). Here are the eight images in the sample conformance test I wrote, based on the LOWI custom airport scenery.
Version 2.0 of the plugin API (available in all versions of X-Plane 9) introduces three new routines that, for the first time, allow plugins to work with OBJs.
XPLMLoadObject and
XPLMUnloadObject load an OBJ file (a model mesh) into memory, and then purge it when done. That part most users understand. The routine that causes confusion is
XPLMDrawObjects. When you draw an OBJ with
XPLMDrawObjects, you are not creating anything long-lasting or persistent in the world. Your object will be visible for one frame on screen, and then will disappear unless your draw it again. Your object is not part of any of the physics calculations.
To put this in perspective: when you make a new window using the Widgets API, the window is persistent – it exists until (1) you delete it or (2) your plugin is unloaded for some reason. You don’t have to do anything per frame to “maintain” the window – you make it and it exists.
Objects are not like that. You cannot make an object “exist” in the X-Plane world – you can only draw it once per frame using the drawing callbacks. Essentially the draw-objects API is a lower level API.
Building a Layered System
Plugins operate in a layered environment, with lower level code on the bottom and your plugin at the top. The layer stack might only be 2 layers deep (XPLM on the bottom, plugin on top), or there might be several layers. Consider XSquawkBox:
- The UI is drawn using the XPWidgets API. The XPWidgets API gets its drawing from the XPUIGraphics API, and the XPUIGraphics API changes OpenGL state using the XPLMGraphics API. So we’ve built up a layered system: basic OpenGL supports drawing, drawing supports user interface, user interface supports the plugin.
- Similarly, multiplayer is done using a library that isn’t part of the basic plugin system (but is open source): libXplaneMP. So here the XPLM supports drawing airplanes, libXplaneMP uses that to create a full multiplayer API, then XSquawkBox uses it.
The alternative to a layered system would be a “monolithic” one. Under a monolithic system, the only API for airplanes would be libXplaneMP, and the only way to create user interface would be widgets. Sandy and I usually prefer the layered approach because it provides a lot more flexibility. If you like widgets, great, use them. If not, no problem – roll your own on top of XPLMDisplay.
The Plugin System Is a Foundation
When Sandy and I cannot provide all of the layers, we have a strong bias toward providing the bottom layer, for an obvious reason: if the bottom layer isn’t in the plugin system, it may be impossible for anyone else to create it. So typically if we have a choice between a high level vs. low level API, we’ll put the low level API in first.
This is precisely what is happening with object drawing – we have the low level API (“draw an object”) but not the high level one (“create an object in the scenery system”). Since we have provided the lowest level, it is possible to code persistent objects in your plugin by layering on top of our API. By comparison, had we only provided “create an object” it would be pretty close to impossible to draw an object for one frame – if you didn’t want a scenery object, the API would be inflexible and useless.
I’ve been on the road a lot for work, so my apologies to everyone whose email I am sitting on. Most of my time these days is being spent on new next-gen tech. But there are a few things I’m hoping to get done in the short term:
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Cut a new time-demo test. This might seem like a low priority item, but it’s not. Apple, ATI and NVidia all run continuous automatic tests of their video drivers, with many applications and games. They have rooms full of computers that continuously run through 3 minute sections of Quake and Call of Duty, etc. If they introduce a driver bug while doing new development, these machines catch the problem immediately.
The new time demo (based on 945) will have a number of features to make X-Plane a more useful test case. If we can make X-Plane into a test case, then they can catch bugs early, and that means you don’t have to see them.
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Bring WED 1.1 to beta. The only thing holding it back is the DSF exporter, and I did have about two hours to poke at it last week. I’m hoping if I can find just a few more hours, I can finish off the exporter.
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Examine 950 bugs. I have half a dozen bug reports against 950 beta 1. 950 will be a small beta but also a slow one, because Austin and I have a lot of other things on our plates. If you haven’t heard back from me on a bug report, probably it’s still on my to-do list.
We’ll see how much of that I can get to in the next week.
There’s basically one reason why WorldEditor developer preview 2 is a “developer preview” and not a real beta: the DSF overlay export code isn’t complete.
The problem is that, unlike an airport, an overlay has to be “cut” to the DSF tile boundaries. This is made slightly tricky by the fact that the overlay can have (1) bezier curved segments and (2) a UV map on those segments. My existing toolkit of polygon editing routines doesn’t handle this case yet.
I have no idea when I will have time to complete this code. It is the number one piece of code that, if I had a quiet single afternoon of unexpected time to code, I’d pound it out. If I were stuck in an airport with my laptop, I’d pound it out. It should give you some idea of how busy things are that it still isn’t done.
In the meantime, there is the scenery tools bugbase. By filing a bug, your issue won’t get lost even if it’s a while before I get to it.
A few quick rants about the bug base:
- Most likely the first thing I’ll do when I do get to your bug is just ask you for more info. Consider the bug to be as much a business card so I can make contact as well as a bug report.
- Some bugs may get kicked out.
- Do not file X-Plane bugs in the scenery tools bug base! The scenery tools bug base is not where we store sim bugs.
Do not bother to ask for direct bug base access for X-Plane itself. You cannot have it. The ratio of submitted “bugs” for X-Plane to actual bugs is at least 10:1. That is, 90% of you think that you should file a bug when you have a tech support question. Now you might be in that 10% (particularly if you’ve made it all the way down to this blog post), but we can’t set up open infrastructure with those numbers. My hope is that the scenery tools are self-selecting to the point where people who are using developer-preview tools know what a bug report is.
The file loading code in 950 beta 1 for Windows is slower than 945. Sometimes. This will be “fixed” in beta 2. Here’s what happened:
The scenery system uses a number of small files. .ter files, multiple images, .objs, etc. This didn’t seem like a problem at first, and having everything in separate text files makes it easier to take apart a scenery pack and see what’s going on.
The problem is that as computers get bigger and faster, rather than a scenery pack growing bigger files, they are growing more files. The maximum texture size has doubled from 1024×1024 to 2048×2048. But with paged orthophotos, multicore, and a lot of VRAM, you could easily build a scenery pack with 10,000 images per DSF.
That’s exactly what people are doing, and the problem is that loading all of those tiny files is slow. Your hard drive is the ultimate example of “cheaper by the dozen” – it can load a single huge file at a high sustained data rate. But the combination of opening and closing files and jumping between them is horribly inefficient. 10,000 tiny .ter files is a hard drive’s worse nightmare.
In 950 beta 1 I tried to rewrite part of the low level file code to be quicker on Windows. It appeared to run 20% faster on my test of the LOWI demo area, so I left it in beta 1, only to find out later that it was about 100% slower on huge orthophoto scenery packs. I will be removing these “optimizations” in beta 2 to get back to the same speed we had before. (None of this affects Mac/Linux – the change was only for Windows.)
The long term solution (which we may have some day) is to have some kind of “packing” format to bundle up a number of small files so that X-Plane can read them more efficiently. An uncompressed zip file (that is, a zip where the actual contents aren’t compressed, just strung together) is one possible candidate – it would be easy for authors to work with and get the job done.
In the short term, for 950 beta 2, I am experimenting with code that loads only a fraction of the paged orthophoto textures ahead of time – this means that some (hopefully far away part) of the scenery will be “gray” until loaded, but the load time could be cut in half.
There is one thing you can do if you are making an orthophoto scenery pack: use the biggest textures you can. Not only is it good from a rendering perspective (fewer, larger textures means less CPU work telling the video card “it’s time to change textures”) but it’s good for loading too – fewer larger textures means fewer, larger total files, which is good for your hard disk.
(Thanks to Cam and Eric for doing heavy performance testing on some of the 950 beta builds!)
This blog post is for amateur plugin developers. By amateur I mean: some plugin developers are professional programmers by day, and are already familiar with all aspects of the software development progress. For those developers, the SDK is unsurprising and performance is simply a matter of applying standard practice: locate the worst performance problem, fix it, wash-rinse-repeat.
But we also have a dedicated set of amateur plugin developers – whether they had programming experience before as hobbyists, or learned C to take their add-ons to the next level, this group is very dedicated, but doesn’t have the years of professional experience to draw on.
If you’re in that second group, this post is for you. Explaining how to performance tune code is well beyond the scope of a blog post, but I do want to address some fundamental ideas.
I receive a number of questions about plugin performance (to which the answer is always “that’s not going to cause a performance problem”). It is understandable that programmers would be concerned about performance; X-Plane is a high performance environment, and a plugin that wrecks that will be rejected by users. But how do you go from worrying about performance to fixing it?
Measure, Measure, Measure, Measure.
If I had to go crazy and recite a sweaty and embarrassing mantra about performance tuning so that I could be humiliated on YouTube it would go: measure, measure, measure, measure.
If you want your plugin to be fast, the single most important thing to know is: you have to find performance problems by measurement, not by speculation, guessing or logic.
If you are unfamiliar with a problem domain (which means you are writing new code or a new algorithm – that is, doing something interesting), there is no way you are going to make a good guess as to where a performance problem is.
If you have a ton of experience in a domain, you still shouldn’t be guessing! After 5 years of working on X-Plane, I can make some good guesses as to where performance problems should be. But I only use those guesses to search in the likely places first! Even with good guesses, I rely on measurement and observation to make sure my guess wasn’t stupid. And even after 5 years of working on the rendering engine, my guesses are wrong more often than they are right. That’s just how performance tuning is: it’s really hard for us to guess where a performance problem might be.*
Fortunately, the most important thing to do, measuring real performance problems, is also the easiest, and requires no special tools. The number one way to check performance: remove the code in question! Simply remove your plugin and compare frame-rate. If removing the plugin does not improve fps, your plugin is not hurting fps.
It is very, very important to make frame-rate comparison measurements under equal conditions. If you measure with your plugin in the ocean and without your plugin at LOWI, the results are meaningless. Here’s a trick I use in X-Plane all the time: I set my new code to run only if the mouse is on the right half of the screen. That way I can be sitting at a fixed location, with the camera not moving, and by mousing around, I can very rapidly compare “with code”, “without code”. The camera doesn’t move, the flight model is doing the same thing – I have isolated just the routine in question. You can do the same thing in your plugin.
Understand Setup Vs. Execution
This is just a rule of thumb, and you cannot use this rule instead of measuring. But generally: libraries are organized so that “execution” code (doing stuff) is fast, while setup and cleanup code may not be. The SDK is definitely in this category. To give a few examples:
- Drawing with a texture in OpenGL is very fast. Loading up a texture is not fast.
- Reading a dataref is fast. Finding a dataref is not as fast.
- Opening a file is usually slower than reading a file.
- You can run a flight loop per frame without performance problems. But you should only register it once.
If you want to pick a general design pattern, separate setup from execution, and performance-tune them separately. You want things that happen all the time to be very fast, and you can be quite intolerant of performance problems in execution code. But if you have setup code in your execution code (e.g. you load your textures from disk during a draw callback) you are fighting the grain; the library you are using probably hasn’t tuned those setup calls to be as fast as the execution code.
Math And Logic Is Fast
Modern computers are astoundingly fast. If you are worried that doing a slightly more complex calculation will hurt frame-rate, don’t be. One of the most common questions about performance I get is: will my systems code slow down X-Plane. It probably won’t – the things you calculate in systems logic are trivial in computer-terms. (But – always measure, don’t just read my blog post!)
In order to have slow code you basically need one of two things:
- A loop. Once you start doing some math multiple times, it can add up. Adding numbers is fast. Adding numbers 4,000,000,000 times is not fast. It only takes one for-loop to make fast code slow.
- A sub-routine. The subroutine could be doing anything, including a loop. Once you start calling other people’s code, your code might get slow.
This is where the professionals have a certain edge: they know how much a set of standard computer operations “cost” in terms of performance. What really happens when you allocate a block of memory? Open a file? If you understand everything going on to make those things happen, you can have a good idea of how expensive they are.
Fortunately, you don’t need to know. You need to measure!
SDK Callbacks Are Fast (Enough)
The SDK’s XPLM library serves as a mediator between plugins and X-Plane. Fortunately, the mediation infrastructure is reasonably fast. Mediation includes things like requesting a dataref from another plugin, or firing off a draw callback. This “callback” overhead contains no loops internally, and thus it is fast enough that you won’t have performance problems doing it correctly. One draw callback that runs every frame? Not a performance problem. Read a dataref? Not a performance problem. (Read a dataref 4,000,000 times inside a for-loop…well, that can be slow, as can anything!)
However you should be aware that some plugin routines “do work”. For example, XPLMDrawObject doesn’t just do mediation (into X-Plane), it actually draws the object. Calls that do “real work” do have the potential to be slower.
Be ware of one exception: a dataref read looks to you like a request for data. But really it happens in two parts. First the SDK makes a call into the other plugin that provides the data (often but not always X-Plane itself) and then that other plugin comes up with the data. So whenI say “dataref reads are fast” what I really mean is: the part of a dataref read that the SDK takes care of is fast. If the dataref read goes into a badly written plugin, the read could be very, very slow. All of the datarefs inside X-Plane vary from fast to very fast, but if you are reading data from another plugin, all bets are off.
Of course, all bets are off anyway. Did I mention you have to measure?
* Why can’t we guess? The answer is: abstraction. Basically well structured code uses libraries, functions, etc. to hide implementation and make the computer seem easier to work with. But because many challenging problems are hidden from view (which is a good thing) it’s hard to know how much real work is being done inside the black box. Build a black box out of black boxes, then do it again a few time, and the information about how fast a function is has bee
n obscured several times over!
