Friday, August 8, 2008

Terminology Rant

Siggraph is coming up, and it’s a great intro to this post! Siggraph is pronounced “sig-raph”, not “see-graph”. (The “si” is pronounced the same as in “sigma”).

Yes, that’s right, this is a terminology rant!

Some mis-used terms just get to me… but posting on just BiNormal and Zoom wasn’t enough, so I invited some friends to add to the pile. ;) I got more responses than I anticipated. Several overlapped from multiple people, so here’s the list:

BiTangent vs BiNormal
A BiNormal is defined in calculus as the cross product between a curve's Normal and Tangent. BiNormal is frequently misused in graphics when people need a basis to use on a 2D manifold surface. In that case, there is only one normal, but an infinite set of tangents. Normal mapping typically uses the tangents oriented by the u or v parameterization on the surface. NBTs are really “Normal, BiTangent, Tangent” sets.

Zoom vs Dolly
Misused when people actually mean dolly in-out. They’re confused that “making something bigger on screen” means “zooming”. Zoom a change in the field of view of a camera (by changing the focal length). This is definitely different than dolly… and we’d have no Alfred Hitchcock’s Vertigo without them both!

Hardware vs. Software (Eric Haines)
I hear this all the time, and it does bug me: "we should run that on the hardware, using a pixel shader". So the GPU is "hardware"? What's the CPU, then? Corrupting the term is kind of pointless, so let's call it the GPU or the graphics accelerator or the graphics card or whatever, but save "hardware" to mean CPU or GPU (or all those other random electronic bits inside the box). The flip side is calling the CPU "software", as in, "well, we can't run it on a pixel shader, so we'll need to run it in software". The CPU and GPU are both controlled by software.

Clipping (Kevin Cristensen)
The most annoying one for me. Reviewers like to use it and so does production or upper management. What they really mean is geometry penetrations between characters and other characters or characters and world geometry/objects. It doesn't affect gameplay at all but Indy got docked major points for it by IGN and other reviewers.

Maybe they are referring to the graphic engine clipping the character by the geometry? Not sure, they never really explain it. They usually say the character clips into the world or something lame.

Orthonormal Matrix (Eric Haines)
In the "don't taunt the mathematicians" category, "orthonormal matrix" is not a term most mathematicians use. A matrix composed from mutually perpendicular vectors, with all vectors normalized, in mathematics is normally called an "orthogonal matrix" - there is no term "orthonormal matrix". Well, there are a few rebellious mathematicians and their engineer lackeys who will daringly use "orthonormal matrix", especially after having a little too much sugar in their tea, but this is not a generally accepted term. It's illogical to me that such a matrix is "orthogonal" and not "orthonormal", since "orthonormal axes" and "orthonormal basis" is perfectly fine usage, but that's how it is in mathematics.

Frustum vs Frustrum (Eric Haines & Kevin Cristensen)
It’s Frustum… not Frustrum

Bezier segments and B-spline
(Bill Baxter)
[Some] seem to refer to a sequence of Bezier segments as a "B-spline". You can convert one to the other, but that doesn't mean they're the same thing!

Phong shading (Eric Haines)
Phong shading: this term means two very different things. One usage is synonymous with "Phong interpolation", or per-pixel lighting. This used to be the main meaning of "Phong shading", vs. "Gouraud shading" (vertex interpolation). The other usage really means "Phong lighting" or "Phong illumination", and this is generally what is meant by "Phong shading" nowadays, as in "Blinn-Phong shading model". "Shading model" has come to mean "lighting model", vs. its ancient meaning of "type of interpolation". We still cope by using context: "Phong shading" usually means the specular-highlight cosine-lobe lighting model, but if we see the word "Gouraud" nearby we know it means interpolation instead. Must confuse newcomers, however, so it's probably better to say "Phong interpolation" if you have to say it at all, and best is probably "per-pixel lighting" and let Phong's association with interpolation die out.

Dot3 Bump Mapping
(Dan Amerson)
It's not bump mapping, it's normal mapping.

Texture vs Texture Map
(Eric Haines)
[People will say,] "Let's apply a texture map of a brick wall here", when what really should be said is simply "texture" instead of "texture map". The "map" part of "texture map" refers to the function used to transform a surface location in space to a location on the texture.

Overload vs. override (Dan Amerson)
People generally mean override when referring to virtual functions in a subclass, but I routinely hear people use overload for that situation.

Thanks to contributors:
Dan Amerson
Bill Baxter
Kevin Christensen
Eric Haines (perhaps because he's an author of a book, he had a lot to offer. ;) more than I could use)

If you have your own favorites… comment away. ;) I’d love to hear them.


  1. If we're going to throw in "frustrums" then there's another good one: "vertice", as in singular of "vertices". Usually heard pronounced "verti-see". It's a vertex, ok?!

    So will we be see-ing you at See-graph this year, Vince?

  2. Glossy reflections.

    Outside of CG-land, "high gloss" means "high polish", with sharp reflections.

    But in CG-land "glossy reflections" is used to mean the exact opposite. ("reflections that get more blurry as the distance increases between surfaces").


  3. "Unified Shader", where everyone confuses 1) different types of shaders that have roughly the same instruction set with 2) hardware design where same computational units can run different types of shaders.

  4. Linux! you were always ranting about Matt C pronouncing it wrong.

    Lin-nux versus Lie-nux.

  5. Maybe the confusion surrounding the term "clipping" stems from the noclip cheat for Doom.

  6. Two frequently-misspelled words in graphics: tessellation and hierarchy (not tesselation nor heirarchy).

    Aras' comment on "unified shader" is an excellent one. I cleared this one up with NVIDIA when writing our new edition, just to be sure. It was wrong on Wikipedia at the time, looks like it's fixed now: - there's the Unified Shader Model, which is the instruction set being common among shaders (software), and the Unified Shading Architecture, which is that a shader on a GPU can be used to execute a vertex, geometry, or pixel shader, with the GPU deciding on the fly where to allocate each one (hardware). You can have the software without the hardware support, or vice versa (e.g. unified shading architectures can run DX8 games).

  7. I think the texture map confusion comes from when the texture itself maps something else. For example, a texture may be applied where each texel corresponds to a terrain value such as water, grass, brush, or mountain. The texture map picked up a texel which mapped to another function.

  8. It's the blinn-phong reflection model, not lighting model. It's about materials, not light sources!

  9. People who spell 'metaballs' 'meatballs' ! Probably just due to aggressive spell-checkers but still: Errg!