[GRASS-dev] Help with gsd_color_func() for NVIZ

[Glynn Clements]

Hamish wrote:

> I am trying to understand the OpenGL color setting code in NVIZ for changing
> the North Arrow text color. I am not very good with the bitwise/GL stuff,
> so can anyone give me a hand at understanding how to feed gsd_color_func()
> correctly?
> 
> 
> this is in  lib/ogsf/gsd_objs.c  and  lib/ogsf/gsd_prim.c
> 
> I was expecting 0x00000 to mean 0xRRGGBB, but it's backwards for me.
> The least significant pair comes first.
> (I'm running Pentium4 / 32bit / little endian)

That's how gsd_prim.c defines it:

> #define RED_MASK 0x000000FF
> #define GRN_MASK 0x0000FF00
> #define BLU_MASK 0x00FF0000
> #define ALP_MASK 0xFF000000

> some experiments:
> 
> gsd_color_func(0xFF0000);    r=0  g=0  b=-1 a=0

[snip]

> (am I doing %d with something that should be e.g. %c, %x?)

No; the variables:

>     GLbyte r, g, b, a;

should be GLubyte; GLbyte is signed, i.e. 0xFF == -1.

This won't matter to the glColor4ub() call, which will cast them to
GLubyte, but it will matter to the fprintf() call, which will cast
them to int.

-- 
Glynn Clements <glynn at gclements.plus.com>