[Qgis-developer] Vienna hackfest: QGIS Legend discussion

[Nyall Dawson]

On 26 March 2014 07:16, Martin Dobias <wonder.sk at gmail.com> wrote:
> Hi Vienna hackfest participants
>
> tomorrow (Wednesday) I would like to have a session on QGIS Legend
> (the thing on the left side of map canvas) and its future - starting
> from 10:00. I am planning some refactoring of the legend widget and it
> would be great to discuss various possible future use cases - the
> refactoring may help to make our dreams come true :-)
>

Martin - I'm not sure if it falls outside of the scope of this but an
idea which has been bouncing around my head is the ability to set
transparency and blending options for a layer group. Here's how I
think it should work:

- Have a setting for layer groups for something like "render child
layers together"
- If this is set, then all child layers from that group are first
composited together onto a blank image using their individual layer
blending/transparency settings
- This entire group image is then composited onto the map using the
group's transparency/blending setting

There's two main use cases I can see which would benefit from this:
1. I've played with exposing the various QPainter masking blend modes
to QGIS (see DestinationOver, SourceIn/DestinationIn and
SourceOut/DestinationOut at
http://doc.qt.digia.com/qq/qq17-compositionmodes.html ). However,
since these modes currently apply to all underlying layers in the
stack (including the canvas background color) the end result is
confusing and not very useful. If the scope of the masking operation
was restricted by the process outlined above then it would be possible
for these masking modes to only apply to underlying child layers from
the same layer group.

2. Sometimes it's cartographically desirable to have several layers
completely obscure each other (100% opacity), but be rendered as a
group with some level of transparency.

I'm happy to code this up myself sometime, but if changes are being
made to how layer groups operate then it may be beneficial to consider
the possibility of layer group settings like this for the future.

Nyall