[GRASS5] Re: [GRASSLIST:3252] r.colors color= proposal

[Glynn Clements]

Michael Barton wrote:

> > [*] I'm pretty much finished with TrueColor support for core modules
> > now; let me know if there are any others you want done.
> 
> Well, since you mention it, the Cell driver is lacking TrueColor 
> support. If it was there, I could do TrueColor tiff output in 
> d.out.tiff.

Most of the necessary code is already present in the PNG driver. 

However, there's one significant issue: generating a 24-bpp colour
table is *really* slow (this is why r.composite defaults to 15 bpp). 
Realistically, a true-colour CELL driver would need to either:

a) generate three separate R/G/B maps, or

b) reduce the colour depth to a level for which a colour table could
reasonably be generated.

Short version:

Large colour tables aren't practical with the current implementation.

Longer version:

The underlying problem is that the code to add a rule to a colour
table checks for overlap with the existing rules. The time taken to do
this is proportional to the total number of rules, so the total time
taken to construct a colour table is proportional to the square of the
number of rules.

For 15-bpp (5 bits/32 levels per component), you need 32 * 32 = 1024
rules (each covering 32 categories). For 24-bpp (8 bits/256 levels per
component), you need 256 * 256 = 65536 rules, which is 64 times as
many rules and takes ~4000 times as long.

Try comparing "r.composite ... levels=256" with the default
(levels=32); the extra time taken is entirely down to building the
colour table.

BTW, if that was the only issue, I suppose that we could create
additional functions for use in such situations (i.e. constructing a
colour table from scratch where it is known that rules never overlap). 
But there is still the issue that the time taken to perform lookups
would increase in proportion to the number of rules.

> > I commented out the code for non-referenced color-only rules, as a) 
> > they
> > don't work, and b) I can't figure out what they are supposed to be 
> > doing
> > in the first place.
> 
> I'm not clear what you are commenting out. If you mean referring to 
> colors by name (blue, green, etc.) they work for me and are helpful 
> many times. But perhaps you mean something else. Sorry to be dense.

No; he is referring to rules which only specify a colour, but not a
value.

-- 
Glynn Clements <glynn.clements at virgin.net>