POV-Ray

Due to a recent thread in povray.general (7th September 2013), I dive
into the code of height field creation.

There is 2 TODO in source/backend/support/imageutil.cpp, for
image_height_at() (circa line 512)

The first one is about using the index of palette-image: As far as *257
would indeed perform a better job to cover the full range than *256, it
would break backward compatibility with previous versions of povray (3.6
included) which only promoted the index as the Most significant byte,
keeping the least one at 0.

But on the second one, the new formula is plain wrong: (r*255+g)*255
should be (r*256+g)*255.
In previous versions, r*255 was the Most significant byte, and g*255 was
the least one.
Ergo, the value was r*255*256 + g*255, which can and should be only
factored as (r*256+g)*255;

I know it is damn late in the release schedule, but can that be either
be fixed before final official delivery or a memo added to the release
note that it would be fixed later and backward-bug will not be
maintained for that specific point (using rgb-8 or less bit per
channel-image for height field)

(it was not bugged in 3.6.1 nor before, it’s just that tiny little bit
of 3.7 that would should that bug)

If you look carefully at the attached pictures (povray +I... +H700 +W700 +A0.01) of scene and scene36, there is a significant difference at the top.
With 3.6, it matched exactly (hence the noise on the top pixels row) the view.
With 3.7, it cannot reach the top and leave a white area
(another difference is the slope on the side are also a bit more lower with 3.7, easier to spot when alternating the display of both pictures)

If you want to regenerate hf.png, it was rendered with povray 3.7RC7, +W400 +H400 +A0.01 +Ihf.pov

The white line at the bottom is expected, as the minimal value of the height field is “full green”