[Qgis-user] Transparency for vector symbol layers

[Andreas Neumann]

To better illustrate my point, please have a look at the attached 
screenshot of waste-water pipes.  Note that in the middle there is a 
thick transparent purple line with a data-defined line-thickness 
depending on the pipe diameter, defined in map units and at the sides 
there are darker, opaque lines, defined in mm with a data-defined 
offsets. So you can see, you can not only mix transparent/non 
transparent symbol levels, but you can also mix units (map units and 
mm). This is quite powerful!

However, this example also illustrates one shortcoming. Have a look at 
the angle at the manhole symbol, labeled with "52160 ...." - here you 
can see that the two adjacent thick pipes overlap and it gets darker 
(still transparent) because of that. If the transparency wouldn't be 
attached to the individual feature, but on the whole symbol level, this 
wouldn't be the case.

There is always room for improvements ;-)

Andreas

On 20.06.2015 16:02, Andreas Neumann wrote:
> Hi Benoit,
>
> It depends. Yes - you can mix transparent and opaque features. For
> each color you can also set the opacity/transparency value.
>
> However, if two features overlap and you use transparency on the
> feature level, you can see the features shine through - whereas when
> the transparency would be on the symbol level (which isn't implemented
> yet in QGIS) you would first composite all elements in a symbol level
> and then apply the opacity on the whole group. See
> http://www.w3.org/TR/2011/REC-SVG11-20110816/masking.html#ObjectAndGroupOpacityProperties
> for the difference. In
> http://www.w3.org/TR/2011/REC-SVG11-20110816/images/masking/opacity01.png
> - if you look at the bottom row of circles - the second pair of
> circles has the opacity on the group of two circles, whereas the third
> pair has opacity on every single element. I hope you can see the
> difference.
>
> It would be very cool if QGIS could have a more fine-grained opacity -
> new opacity on symbol level and new opacity on a whole group of
> layers. It would make the cartographic options even better ;-)
>
> Andreas
>
> On 19.06.2015 16:40, Benoît Laurent wrote:
>> Hello,
>>
>> I noticed this feature request : https://hub.qgis.org/issues/3438.
>>
>> Is it possible to have semi-transparent and opaque features within
>> the same layer ?
>>
>> Best regards,
>>
>> Benoît
>>
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