9.2ProblemsinS3DUniquetoGames 139
to merge or fuse two images correctly requires that the focal plane be aligned in a
coplanar fashion between the two images. Instead, the correct projection matrices
are asymmetrical, off-axis projection matrices. These contain shearing so that the
corners of the focal plane exactly match between the left and right stereo images
but have a different point of origin. We don’t reproduce the math here because it
can be found elsewhere with better descriptions of the derivation [Bourke 1999,
Schertenleib 2010, Jones et al. 2001].
2DImage‐BasedEffects
A staple of modern monoscopic games are the image-based effects, commonly
called “post effects.” These cheap adjustments to rendered images make up for
the lack of supersampling hardware, low-resolution rendering buffers, inability
for game hardware to render high-quality motion blurs, poor-quality lighting and
shadowing with image-space occlusion mapping, blooms and glows, tone map-
ping, color gamut alterations, etc. It is a major component of a good-quality ren-
dering engine but, unfortunately, is no longer as valuable in S3D. Many of these
tricks do not really work well when the left/right images deviate significantly
because the alterations are from each projection’s fixed perspective. Blurring
around the edges of objects, for example, in an antialiasing pass causes those
edges to register more poorly with the viewer. Depending on the effect, it can be
quite jarring or strange to see pixels that appear to be floating or smeared in
space because they do not seem to fit. The result can be somewhat akin to win-
dow violations in the middle of the screen and, in large enough numbers, can be
very distracting. When considering that post effects take twice the processing
power to modify both images, a suggestion is to dial back these effects during
S3D rendering or remove them entirely since the hardware is already being
pushed twice as hard just to render the scene for both eyes separately.
HUDandSubtitles
The most difficult problem to solve for S3D and games is the heads-up display
(HUD). User interfaces (UIs) generally have some issues with current-generation
LCD monitors that do not change colors rapidly enough to prevent ghosting be-
tween eyes (but this technological problem will probably improve in the next few
years to become less of an issue), which makes interesting 3D UIs difficult to
enjoy with high-contrast colors.
A systemic problem, though, is where to draw subtitles and UI elements in
the game. If they are drawn at the focal plane, then there is no ghosting on LCD
monitors since both left and right images display the UI pixels in the same place.