DDS Has No Gamma (Which is Very Sad)

A few years ago I blogged about gamma correction for png files. Here's the very, very short version:

• PC and Mac monitors are calibrated differently. Dark tones on a PC appear darker than on a Mac. The curve of how colors are mapped to the monitor is the gamma correction curve, typically expressed as a number like 1.8 for Mac and 2.2 for PC. The higher the number, the more Gothic your dark tones.
• A png file can have a gamma value written into the file, which tells X-Plane (and anyone else) what kind of monitor the png was drawn on. This lets X-Plane brighten a png from a Mac when you are on a PC, and darken a png from a PC when you are on a Mac.
• If you leave off the gamma value on your png, we assume 1.8 (Mac) which can be bad if you're a PC author.

While this is confusing, it was an improvement over the BMP situation (where everything was set up for a Mac and PC users had to simply crank their monitor brightness).

In version 9 we added a gamma correction setting to X-Plane. The setting you enter in the rendering settings is how "dark" your monitor is (bigger number = darker). We then attempt to compensate by lightening the textures more; thus a bigger number results in a lighter looking X-Plane (because you told us your monitor was dark and we tried to "fix it").

There are two other developments since the original png situation which have unfortunately been a step backward in terms of X-Plane color correction.

DDS and Gamma

The handling of DDS and gamma is, to put it mildly, quite problematic. The problem is two-fold:

• DDS doesn't actually have gamma information, so we can't tag DDSes as having originated on Macs and PCs. So we assume a DDS is authored at a gamma of 1.8 (Mac). I think DDSTool correctly does a gamma correction when grinding files at other gammas.
  
• (If you are a real graphics programmer, please do not read this next sentence.) X-Plane attempts to adjust the color of the DDS in its compressed form. This is a big hack designed to keep framerate high, but it's really not a very good idea. The result can be color distortion when a DDS is viewed at 2.2 gamma.

So that's not good, but what happened next made things a lot worse.

Apple Goes Gothic

Apple adopted the sRGB color profile for OS X 10.6, which has a gamma curve of about 2.2. So now the situation with DDS is particularly ugly:

• All DDS are authored at a gamma of 1.8.
• All users are moving toward a display gamma of 2.2.
• X-Plane thus has to always color correct, but its color correction is low quality for performance reasons.

This is...very sad.

There are two things we can do about this:

• In the short term, we can provide post-decompression color correction. This will cost a (hopefully) small amount of framerate and improve color fidelity for users with 2.2 gamma. This is the kind of thing that any user with a modern card would want, but that we might make optional for users with very old hardware.
• In the long term, we can provide a gamma calibration in the text files that wrap DDS files so that authors can mark their DDS as already being 2.2. This will mean that for most users X-Plane won't have to do any color correction at all.

Lighting Rheostats in X-Plane 9

There were a few threads regarding lighting rheostats in X-Plane 9. Here's a short version of the issue and why we're not changing X-Plane 9's behavior.

X-Plane 9's policy toward lighting rheostats is a little bit arbitrary. The sim will pre-position every lighting rheostat in the cockpit to 75% intensity on sim startup, and from that point on, we never touch them. We do not reset them when you load a new plane or reset your flight.

The result of this is that when you load a new airplane, it "inherits" the rheostat positions of the last airplane loaded. This can cause a problem if the newly loaded plane doesn't have controls to adjust the lights (e.g. it has no instruments on the 2-d panel or manipulators on the 3-d panel, and there is no keyboard shortcut defined). A plane can be "stuck dark".

It would be nice if X-Plane would pre-initialize the lighting rheostats on startup, but X-Plane deos not have enough information to do this. For example, on plane load, the instrument brightness should be set fairly high (so a glass cockpit can be read during the day) but the flood lights should be fairly low (to prevent loss of night vision). But X-Plane has no idea which rheostats control instruments and which control floods. So if we wanted to correctly initialize the cockpit, we wouldn't have enough information.

To make it more complicated, some airplane authors have taken it upon themselves to initialize the cockpit via plugin code. If X-Plane were to start changing the rheostats at startup, it might undo some of what these plugins have done.

Given the difficulty of maintaining compatibility and the lack of a "correct" set of values, we decided not to change the behavior in 9.50 or 9.55.

If there's any take-away point for airplane authors, I think it is this: provide controls for the lighting rheostats that you use in your airplane. Otherwise the user can't turn the lights on if they are off for any reason. You can control the lighting rheostats with a generic instrument, manipulator, or the built-in instruments.

Ugly Glow

There is a separate issue that sometimes comes up: X-Plane panels can look bad when the flood lights are turned all the way up during the day. A panel can look very red and washed out, for example.

This problem comes from a mismatch of real-world lighting levels. In the real world, the sun is approximately four gajillion times more powerful than the little dome lights in an airplane. So when the sun is out, the dome light isn't visible even if it's turned all the way up. The dome light only looks bright when your eyes have adjusted to a no-sun condition.

What X-Plane should do (and may do in the future) is scale all cockpit lights relative to the overall daytime brightness, which would effectively dim the effect of flood lights during the day. Simply turning down flood lights when a flight is started during the day is not a full solution, as the user can simply turn them right back up again and end up with an unrealistic scene.

Suffice it to say, I think we will address these things in a v10 time frame, not a v9 time frame; in the short term it's better to have airplanes continue to function as the author intended.

Moving the Scenery Site

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:

1. 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.
2. 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.

What is the Difference between ANIM_hide and ATTR_draw_disable?

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.