Wednesday, February 06, 2008

The Limits of Orthophotos and Meshes in X-Plane

I get asked a lot about the limits of meshes and orthophotos in X-Plane. I'll try to answer this, but the answer isn't as simple as most people expect.

Texture Limits and Orthophotos

The maximum single texture size in X-Plane 8 is 1024x1024, and in X-Plane 9 it is 2048x2048.

I believe the maximum number of unique custom orthophotos that can be attached to a single DSF is at least 32768.

In practice, that number is pretty useless because X-Plane loads all textures for a DSF at the highest user-allowed res when the DSF is loaded. That means you tend to load a lot of textures. Every system is different and drivers have a lot to do with RAM efficiency, but generally you'll run out of virtual address space and crash the sim before you can attach 32768x2048x2048 of pixels.

X-Plane has no limits on how the texturing is applied - that is, you can use your 2028x2048 texture to cover an entire tile or a single meter. So again, the limiting factor on the resolution of your orthophotos is how much total area you want to cover and how much RAM you can spend (remember RAM is also used for mesh complexity, 3-d models, etc.).

You do not need to have enough VRAM to hold all loaded orthophotos; the video driver will paeg the textures into VRAM. Virtual address space is the limiting factor. How far you push it depends on a lot of subjective things:
  • If you expect your users to also run with a lot of trees, 3-d objects, cars on roads, and some plugins, you can't use a lot of RAM.
  • If you expect your users to have /3GB in their boot.ini and use nothing but your add-on, you can use a lot more RAM.
Generally the size of the DDS texture on disk is a good proxy for the virtual memory that is required to hold your textures.

It should be noted that these limits on texturing (due to X-Plane blindly loading a lot of stuff at once) affect all scenery types: objects, draped polygons, very complex airplanes, plugins, and not just terrain mesh orthophotos.

Getting Past the Texture Limit

It will take a future extension to the rendering engine to get past the current limits. Basically X-Plane will have to load textures at lower resolutions when they're farther away. I don't know when that is coming, but when it happens, it will increase the total amount of image data a DSF mesh can contain, because the limiting factor will be how much data is in the small area the user is looking at (since the rest can be stored at much lower res for far-away views). At that point the limiting bottleneck will be resolution (smaller means more data at once), not total image data.

Mesh Limits

Unfortunately, limits to the mesh are even more vague than limits to texture usage. X-Plane uses an adaptive mesh - basically you can put your vertices wherever you want. So the highest resolution you can achieve might be much smaller than 1 meter resolution, but you can only do this for a small area before the total mesh size gets too big. But this is okay - the intention of DSF is to let you put a lot of detail where you need it.

I believe that once again memory provides the first limitation to the mesh. That is - you'll run out of memory loading your insanely huge mesh long before you hit a limit to the DSF container structure. And once again, even the RAM limit isn't a hard limit because that virtual address space is shared with texures. Your mesh density limits actually go down when your textures go up because it's a zero-sum game.

Estimating Memory

Here are some ideas on how to estimate your memory footprint:
  • Run X-Plane over ocean to get an idea of the baseline memory use that the sim needs without extra scenery.
  • Load your mesh without textures (move the textures away) to find the cost of the mesh itself. (I am going on the assumption here that you can rescale your mesh using whatever mesh generation tool you're using).
  • The size of DDS textures is a good proxy for the memory used.


Dan31 said...

When you talk about RAM, is it Video Ram or Main RAM (on mother board) ?

Gayathri said...

Oh..this post is helpful to understand the limits of Orthophoto . Thanks for sharing

digital orthophoto generation
image Ortho rectification