vq_color test wrong?

[DavidGZ]

Some colour printer drivers do not allow modifying the colour of each register. There is a simple test for checking if the driver permits you to change index colours:
Call vq_color with set_flag value of 0 and save the return
Call vs_color to modify that colour index by a significant value
Call vq_color with set_flag value of 0 and then compare it with what you set
Restore old value
If equivalent values are returned, you can modify each colour index

If I understand this correctly the test described above for vq_color is wrong, the second call to vq_color should have the set flag set to 1 instead of 0 for this test to make sense.

The Compendium has the same test described in its vq_color page with the same error.
If somebody confirms that I'm not missing anything I'll correct this.

[th-otto]

I think it does not matter much whether you use 0 or 1 in the 2nd call. The example states you should change it by a "significant value". The difference between 0 and 1 is only whether you get the exact value or not, but the test is only to check whether you can change the value at all.

[DavidGZ]

If I use set_flag 0 in the second vq_color call I'm going to get always what I requested with vs_color, I really don't understand how this is useful. Note that the vq_color description says that with a set_flag 0 I'll get the last requested value:

"set_flag
0 = Return colour values last requested by user at vs_color
1 = Return realized colour values"

[th-otto]

Hm, maybe it depends how it is implemented in the driver. Then of course feel free to update it.