[Qgis-developer] Transparency/Opacity (was: Outline/border -> stroke? )

[Nyall Dawson]

On 19 February 2017 at 14:02, John Hawkinson <jhawk at mit.edu> wrote:

> As for transparency vs. opacity, as I wrote in that email:
>
> * Why does QGIS use Transparency sliders instead of Opacity sliders?
>   Isn't Opacity much more common in graphics software?
>
> To expand a little more, both Photoshop and Illustrator use Opacity
> sliders -- far left is 0% opacity / full transparency. (Although
> curiously, Illustrator's is inside the Transparency window.)
>
> Are there prominent examples of software packages (in the GIS world?)
> that use Transparency instead of Opacity in sliders?
>
> For the tols I use, Opacity wins hands-down. I gather that's not true
> for everyone, but I wonder what the tools are that use Transparency
> sliders (far left is 100% opacity / no transparency).
>
> I would say, also, that Opacity is a little more intuitive, because
> it basically translates to Brightness. If you ask a user which way
> they turn the knob to make a line darker, they'll usually turn it in
> the positive director (err, s/knob/slider/).

My preference would be for opacity too.

An outstanding question is what range we should allow? Behind the
scenes it's generally going to be converted to a 0-255 value, but for
users a 0-100% may be more explanatory. Opinions?

> I would say, also, that the internal API is very different from the
> UI. What's the justification for changing the API on this? Confusion
> to API developers? Is that really worth it?

The API is ALL OVER THE PLACE here, and should also be fixed. We have
a mix of transparency/opacity/alpha and ranges of 0-1, 0-100, 0-255.
It's confusing for developers too!

Nyall