The BITMAP_TYPE nomenclature is used for bump_map, image_map, 
image_pattern and material_map. HF_TYPE is used for height_fields. Over 
time I think the documentation has gotten somewhat scrambled with 
respect to current functionality.

bump_map:
   http://wiki.povray.org/content/Reference:Bump_Map

height_field:
http://wiki.povray.org/content/Reference:Height_Field

image_pattern:
   http://wiki.povray.org/content/Reference:Image_Pattern

image_map:
   http://wiki.povray.org/content/Reference:Image_Map

material_map:
   http://wiki.povray.org/content/Reference:Patterned_Textures#Material_Maps

We sometimes mark BITMAP_TYPE | HF_TYPE as optional when I believe from 
3.7 onward it is everywhere optional.  When specified, this is the file 
type POV-Ray will try and read no matter the filename given or the 
output file type.

Our docs often specify the filename as "bitmap.ext" or "bitmap[.ext]" 
except for height_fields where it is "filename".  I think "filename" is 
best because that string needs to be the filename POV-Ray looks to read 
no matter format.

If the user has not specified an explicit BITMAP_TYPE | HF_TYPE, POV-Ray 
will first look for any known extension in the "filename" string.  If 
found, that extension is the image format POV-Ray will try and read.

Where the filename has no recognizable extension and BITMAP_TYPE | 
HF_TYPE has not been specified, POV-Ray will try and read the "filename" 
as whatever the current output image format is. Only the image_map 
documentation mentions this latter behavior.

There is too in some documentation - material_map for example - this note:

"Note: Earlier versions of POV-Ray allowed some modifiers before the 
BITMAP_TYPE but that syntax is being phased out in favor of the syntax 
described here."

which probably we can drop. At least for the modifiers I tried before 
BITMAP_TYPE, I got parser errors in 3.7 and 3.8.

Bill P.

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