: It's because it's not supposed to be motion blur, if it was you'd see more
: afterimages.
: It's temporal anti-aliasing.
: In traditional anti-aliasing where you average together the lines where two
: objects meet on screen by figuring out what relation they have in screen
: space and looking at their brightness and color. In temporal anti-aliasing
: you take the current frame and the previous frame and average them
: together with a focus on averaging edges (and I think also touching up
: textures through some complicated math).

I know how temporal antialiasing works, and Reach's implementation is not responsible for the large-scale double-image effect in Reach's gameplay. Reach's temporal antialiasing doesn't support nonzero reprojection; when the game detects substantial movement at a pixel, it simply turns the TAA blending off.

Here's an example of Reach with the camera static, exhibiting a 2x quincunx-blend supersample:

Here's the same spot with the camera rotating extremely slowly, and the AA is now totally missing:

Gameplay artifacts from the TAA are mostly pretty subtle, it only happens when you view certain animated objects with a static camera. For instance, you can see double-images on some of the blades of grass in this image:

And here's Halo Reach:

In motion on a TV, when I took this screenshot, the Reach motion blur trails looked like solid ghosts. But in a screenshot, it's clearly the MB. The algorithm is nearly identical to that in Halo 3, featuring the same number of blur samples and the same attenuation of the effect toward screen center... but the blur trails in Reach look MUCH less smooth, and I can't think of any reason this is happening except that Halo 3's blending is gamma-correct and Reach's isn't (which we know to be the case from the Digital Foundry's tech interview for Halo Reach).

: it's when the engine is pushed near its limits (like say a
: big team match on a large map) that you start to see two separate images.

As far as I know, engine load has no effect on the TAA algorithm.