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Comment by somat | Hacker News Reader

I love the quake .map format. It is like this glimpse into an weird alternate history. A sort steampunk ascetic to graphics.

The article touches on this, but for them unfamiliar, the quake .map format is the editable text based format. but it does not use the common method of vertices into edges into faces as the base building block. it uses planes, the graphics primitive is is the three points that define a plane, where these planes intersect are the edges.

I never dove into the subject enough to figure out if this plane representation was also internal to the compiled bsp format, but I suspect it is. My guess is that it is the natural conclusion when you need a map interchange format for your highly optimized space partitioned engine.

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> I never dove into the subject enough to figure out if this plane representation was also internal to the compiled bsp format, but I suspect it is.

Not exactly. The BSP format stores planes as a normal (3 floats) + distance to the origin + an int that flags axis aligned planes. Which is the representation the engine uses.

I believe “3 points on the plane” was chosen for the map file so the coordinates could be exact integers in the text file, to avoid precision loss from storing the normal+distance representation. The latter representation uses less space and is easier to work with.

Now I'm curious. How does BSP encode the orientation of a plane wrt its normal?

The normal is the orientation

It is the orientation of the plane, but not the orientation inside the plane. How does BSP decides in which direction it must display the textures inside a plane?

The BSP file stores faces, which define a number of edges (or vertices). Faces are used in rendering, and contain the texture being drawn.

Faces also refer to a plane and a “side”. If the side is 1, the normal of the plane is flipped which has the effect of using the back side of the plane.

Three points on a plane also give you a basis for texture mapping that’s reasonably intuitive to the level designer. I believe this is the main reason for the representation.

While texture mapping is done via box projection based on the plane itself, the actual texture mapping parameters are separate values than the plane (specifically they are scaling, shifting and rotation).

Some .map extensions do store vectors instead of scale/shift/rotate that allow much more flexible texture mapping (though still need to be on a plane, but can represent things like skewing), but these are still stored separately from the plane.

The planes are important for collision and occlusion queries.

And yes. Intersection of planes is a higher level language than vertices and UVs. It pushes more work onto the computer instead of the human.

PowerVR supported in hardware something called "infinite planes", wonder if its connected. Afaik no game, program or API ever used it.

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