That was so fun - let's do it again!

One of the less fun aspects of X-Plane 8.50 was dealing with driver bugs. The basic problem is that PC hardware vendors have a relatively slow cycle for updating the software they bundle with their machines; while the companies that write drivers (ATI and nVidia) have a pretty high latency from when they first ship a card to when the driver support is to the level that X-Plane needs.

The clearest case of this I saw during 8.50 was in the case of PC users with "1.5"*-type ATI drivers. These drivers will cause the terrain textures to swim when hardware-accelerated runway lights are used. Upgrading to "2.0"-type ATI drivers fix the bug. A lot of users were just fine with the 1.5 drivers that came with their machines until X-Plane 8.50 came out, and have only now updated.

8.50 started using pixel shaders; hence the need for newer drivers with bug-free pixel-shader support. But in the future we're going to move from "DSP"-type shaders (that is, shaders written in a very low level language) to shaders writen in a GLSL, a high level abstract way to write shaders.

My fear is that this will induce a second round of "update your driver" for PC and Linux users. (Apple ships new drivers with system update, so users of the current Mac OS can usually just run the updater if it isn't set to update automatically.) But I think it's probably for the best that people use the latest drivers, as ATI and nVidia put software changes in the drivers that increase the throughput of the graphics card, through smarter use of the hardware.

The one group that is really stuck is Macintosh 10.3 users; Apple doesn't provide driver updates to older operating systems. So for example users with 10.3.9 and an ATI card with pixel shaders can't get hardware-accelerated texture lights without buying OS X 10.4. I would go as far as saying that in the future if you have a pixel-shader card it might be worth buying an OS upgrade to unlock the driver support needed to use these features.

Why move to GLSL (and require more driver updates)? In the long term it will provide superior framerate. The current shaders we have now are written in a very low level language, that roughly matches the technology of the first generation pixel-shader cards (Radeon 9700 and GeForce "FX" series). As cards gain new capabilities, Austin and I can't take advantage of the new capabilities using this old language.

GLSL however is a higher level language; when we write a shader in GLSL, the graphics driver translates our shader into code specific to the video card at hand when X-Plane starts. This means that if the video card has some new magic trick it can do, the driver can take advantage of that trick as it translates our shader, resulting in higher framerate. And everyone loves higher framerate. :-)

EDIT: who will be able to use GLSL? Basically any card that has hardware-based pixel shaders** from ATI or nVidia will support GLSL-based shaders if you have a modern enough driver. So if you are seeing hardware-accelerated runway lights in 850 then the GLSL changeover shouldn't cause any more fuss than a driver upgrade if you haven't done one rcently.

Also I should say that if I sound like I am dumping on ATI and nVidia for driver bugs, I shouldn't be; video cards are amazingly powerful, the drivers are very complex, and the technology jumps by huge amounts in a very short time period. I believe the level of driver bugginess we see is a function of fast technology rollout and not the competence of the driver teams, who I am sure are working as hard as they can.

So we, the developers who write programs that use these cards, and you, the users who enjoy these cards, have to ask whether we'd rather have faster framerate and more eye candy or less chaos from our drivers.

*ATI drivers are numbered by ATI to match ATI's internal versioning scheme. But X-Plane can't really see what the ATI driver version number is (e.g. the catalyst version number). So when I say 1.5 or 2.0 I am really talking about the OpenGL specification number that the driver implements. ATI implements newer specs in newer drivers, so we can tell that an ATI driver that supports the 2.0 spec is newer than an ATI driver that supports teh 1.5 spec.

So while I have no idea of either which versions of the ATI drivers had this bug or which version it was fixed in, but we can tell from an X-Plane Log.txt file whether the drivers are going to have this bug.

**Hardware-accelerated shaders? Well, previously I've described a graphics card as either having shaders or not, but this isn't totally accurate. Technically a shader can include a non-programmable shader. Furthermore the driver can present software emulation of a shader. For example, the Intel GMA950 has programmable pixel shaders in hardware. On a Mac the driver reports that it has vertex shaders, but the vertex shadings is actually done by the CPU. (This isn't unreasonable -- all vertex processing is done by the CPU on the GMA950.)

So really when we talk about "cards with shaders" we mean: any nVidia card with a number of 5000 or higher (plus some Quadros, sorry I don't know the numbres on those), and any ATI card with a number of 9500 or higher, and any ATI card with an "X" in it, e.g. X850. At this point we've had shaders in cards for 3 generations, so if you buy a new computer and it has an ATI or nVidia graphics card, it should have fully programmable hardware shaders.

Of course now not all shaders are equal; the newest cards have even more powerful shaders. But this isn't likely to matter a whole lot to X-Plane for a while. But generally I'd recommend buying the mid-range latest generation cards over high-end obsolete cards at this point.

Using GLSL does not mean changing the system requirements!

X-Plane 860 uses GLSL pixel shaders. But this doesn't mean you need any new hardware. It does mean you might need to update your drivers. Let's break this down a little bit.

X-Plane never talks directly to your video card. It always talks to the video driver, a piece of software that acts as a translator between X-Plane and your video card. (That's a gross simplification - video drivers do a lot of amazing things these days.)

Your video card has certain capabilities - different cards can do different tricks. The video driver tells X-Plane what tricks the video card can do.

So there are really four possibilities for any given trick:

1. Your video card cannot do that trick. The video driver will tell X-Plane "I can't do it."
2. Your video card can do that trick. The video driver will tell X-plane "I can do it".
   
3. Your video card can do the trick, but the driver is old and will tell X-Plane "I can't do it" because the driver doesn't realize what the card can do. (This can happen because the video driver you use might come with your operating system, before you bought the new video card.)
4. Your video card cando the trick, but the driver has a bug. The driver will tell X-Plane "I can do it", but when X-Plane says "go and do it", the video driver will crash your whole computer. This happens when drivers are buggy and can usually be fixed by getting newer drivers.

Something to note about this last case - we only crash if X-Plane actually says "go and do it". So when we release a new version of X-Plane that utilizes new video driver tricks...

• Some users have cards that can't do it and will see no change.
• Some users have cards that can do it, so they see the new feature.
• Some users have cards that can do it but old drivers and see no change.
• Some users have buggy drivers and see a crash for the first time.

GLSL shaders is a certain way to use the pixel shaders built into some cards. (GLSL is an interface and language to talk to those shaders.)

GLSL shaders are definitely not required for X-Plane 860. If you don't have them, X-Plane will run the way it always did.

But if your drivers are buggy and crash when GLSL shaders are used, you will need a software update to get good drivers. (There is a way out of this...if you really can't get a driver fix, there are command-line options that will tell X-Plane "don't use this feature even if the card has it".)

Doing the Numbers

I think the framerate test may be the best piece of code I've ever written for X-Plane. It speeds up my own work checking X-Plane's performance (and removes the risk of careless error), allows us to easily collect user-based stats, and has changed the dialog on performance in the beta from he-said/she-said (the result of having no precise quantitative measurements) to a fact-based dialog box where we can discover problems more quickly.

Today in a noble sacrifice (using this card is nothing less than torture) I put a GF-FX5200
back in my G5 to look at bugs and performance. (The terrain night lighting and landing lights should be back in all nVidia hardware with shaders in the next beta.) Some things I've learned:

Turning off GLSL shaders boosts fps from 46 to 55 fps with my Radeon 9600XT but goes from 22 to 45 fps with the FX5200. From that we can learn:

• The penalty for GLSL shaders is really really high on the FX5200.
• The shader implementation for the FX5200 just doesn't run at the same speeds we see in R300-based cards...it's not in the ballpark.

Looking at a profile of the sim, with the FX5200 running shaders, about 50% of the time is spent waiting for the graphics card to chug.

Other things I've found:

• GLSL shaders are slower on terrain, but still worth it for runway lights - having those be hardware accelerated is definitely a positive.
• Per-pixel fog doesn't affect fps on the 9600XT but is a measurable penalty on the FX5200.
• The landing light also hits the FX5200 about 20% - again it's negligible on the 9600XT.

The moral of the story may be: we need a way to not use shaders for terrain on the FX5200. Between the little things that shaders give us (that we can't use) that cost too much and the overall lack of performance, GLSL terrain and the FX5200 may not be a good idea.

If you have an FX5200 and OS X 10.3, there may not be any benefit to updating to 10.4 - you'll have the option of using GLSL shaders but you may not want to.

If you have a G5 and an FX5200 and the option to put a new card in, you may want to consider it - you'll almost certainly get a fps boost (especially if you use something like an X850).

LSD.glsl

Too trippy not to post:

Pixel Shaders and Scenery

X-Plane 850 was the first version of X-Plane to start employing pixel shaders -- they make the new hardware-accelerated lights possible. Our next step will be to start converting existing OpenGL code over to pixel shaders.

DISCLAIMER: these are my predictions for the future of X-Plane, not promises or guarantees. Do not base your purchasing decisions on what I say, and do not come back quoting this blog saying "you promised X". This is a statement of our intentions, but we cannot predict what limitations we could hit in the future, such as graphics problems we did not anticipate!

Right now X-Plane works based on individual selection of features - that is, each separate part of X-Plane examines our summary of a driver's capabilities and does what it can based on those capabilities. The result is good for users in that X-Plane's visual features turn off one-by-one as a card becomes less capable, leaving as much eye candy as possible.

What's bad about this technique is that there are huge combinations of possible configurations, more than we can test individually, so we sometimes have bugs that show up only with a peculiar combination. Since most of the code that uses this driver spec has been relatively stable we haven't seen a lot of bugs due to this in 830-850, but I definitely fixed a lot of configuration-related OpenGL bugs in the early v8 run when the rendering engine was new.

Pixel shaders (we use GLSL for the OpenGL nerds out there) will change that; the OpenGL shader spec is powerful enough that if we have it, we can do everything we want. So the new world of shaders will make X-Plane's visuals be either "you have it" or "you don't". This definitely helps us for testing - we can test the shader version, the legacy version, and that's it.

Remember the dark old days of X-Plane 6 when GeForce 4 MX's had two pixel shaders, but GeForce 4 Ti's had 4, and ATI cards had 2,3,4 or 6? Fortunately we're past that. The last three generations of video cards have pixel shaders and every card with shaders has 16 texture units, which is more tha enough for us.

Here are the cards that will support shaders, at leaset I think from the specs posted on the web (all suffixes have equal pixel shader capabilities - the suffix usually is about the speed of the card):

• ATI Radeon 9500-9800.
• ATI Radeon X300-x850.
• ATI Radeon X1000-X1900.
• nVidia GeForce 5200-7900.
• Any card that hasn't been released yet will surely support shaders.

Here are the cards that X-Plane will run on but don't support shaders:

• ATI Radeon 7000-8500.
• GeForce 2,3, and 4.
• Intel GMA950 (and probably a few other Intel chipsets I don't know abot).
  

I don't know about the old Rage 128 or original GeForce 256 - technically they probably meet minimum requirement, but they're so underpowered you wouldn't want to run X-Plane 8 on them. Also I don't think they ever came in 16 MB variants. I don't know about Matrox's compatibility.

To use pixel shaders you will also need reasonably new and modern drivers. Both ATI and nVidia ship modern drivers that support shaders fully, but very often what you get on your machine or with Windows XP does not. So if you have a card that supports shaders but don't see them, it might be time to check the vendor website.

For Mac users Apple shipped full pixel shader support in 10.4. So: if you have a card with pixel shaders and you use 10.3.9 you should upgrade to either 10.4 now or 10.5 when it comes out! Otherwise x-Plane will not have the right driver to utilize your hardware! An X850 or 7300 on OS 9 is definitely a waste of potential. (X-Plane 8.50 already has features that are only available to pixel-shader users on 10.4!)

I am excited about pixel shaders because we'll be able to finally fix some of the bugs we just couldn't fix before. For example, when you turn on the landing light, X-Plane's ambient lighting levels drop to pitch black. With pixel shaders we will finally have the capability to fix this!