So, what exactly is the great bump map disaster? When the PC version of Poser 3 or 4
converts a graphic file (.BMP, .TIF, .JPG) into a bump map (.BUM), it does it
wrong. In a lot of cases, this doesn't matter; or it just looks
"a bit wrong". Sometimes, like when you really need it, it is
obviously wrong; see below for an example.
The Macintosh version of Poser 4 does not display this problem, and later versions
of Poser from Pro Pack through to 5 and 6 don't use the .BUM format. However, if
you download bump maps which have already been converted to .BUM (rather than in .JPG
format which is the norm), you should try to find out what system did the
conversion since those produced by a PC will be wrong. If you can't find out,
follow the instructions below and try both versions to see which is best.
This problem was first pointed out by gillan on the Renderosity forum. You can
probably still read the thread here:
Bug in poser bump mapping and solution
Since then, MartinC (a Mac user, ironically) has unofficially taken up the cause.
All credit should go to these people; I'm just passing on what they've found.
• The Problem
Here's a very simple bump map:
The white area should produce the effect of a "sticking out" disc.
But surprise surprise, if we let Poser convert this, it doesn't look right.
On the left, a simple square with our bump map applied to it; on the right,
an actual cylinder poking through a square to show where the light is
really coming from. As you can see, the bump mapped version is
about 90 degrees out. It should look like this:
Now read on to find out how to do it.
• What to do about it
You'll need an image editing program which can handle the red, green and blue
colour channels separately; I originally used Paint Shop Pro 5, but the general method is
applicable to later versions of PSP, and others such as Photoshop.
Open up the .BUM file that Poser made in your image editor. You might have
to tell it that it's a bitmap, and it might complain, but press on anyway.
In our white blob example, we get this:
Now you need to invert (make negative) the green channel only. Photoshop
can do this directly, in Paint Shop we need to split it up first: select
Split to RGB.
Select the green image that has been created, and choose
Negative Image from the Colors menu.
Now select Combine from RGB to put the channels back together again.
Make sure you select the right components to go in each channel.
Save the image as a bitmap (.BMP) file - you won't be able to save as .BUM.
Now rename the file to have a .BUM extension. Windows will complain;
ignore it as you usually do.
Make nice pictures! Just select your corrected bump map in the surface materials
dialogue as usual.
