[Qgis-user] Transparent Shapeburst Fill

[Thomas Colley]

Hi Nyall

Thanks very much, it seems obvious when you explain it! 

What threw me was I chose  the 'Transparent' option under the colour picker rather than choosing a colour and then making it transparent! 'Transparent' uses RGB 0,0,0  alpha 0, explaining the grey shading.

Cheers

Tom
 
-----Original Message-----
From: Nyall Dawson [mailto:nyall.dawson at gmail.com] 
Sent: 06 April 2015 10:17
To: Thomas Colley
Cc: qgis-user
Subject: Re: [Qgis-user] Transparent Shapeburst Fill

On 2 April 2015 at 19:43, Thomas Colley
<t.colley at neath-porttalbot.gov.uk> wrote:
> Hi
>
>
>
> Appearance of shapeburst fill seems strange when going to/from transparent.
>
>
>
> The linked image shows one shapeburst with a colour ramp White > 
> Colour and another that goes Transparent > Colour. They’re both on a 
> white background so my thinking is they should both look the same?

>
> I don’t know if this is down to a setting, or if there’s something 
> that can be done with the blending modes (I’ve tried different modes 
> but don’t have a good understanding).
>

Ready for your mind to be blown? Ok, here goes: not all transparent colours are equal! Let me explain...

When QGIS interpolates between two colours it individually interpolates each individual colour component (red, green and blue) and the alpha channel separately. So, if the shapeburst colours are RGB 0, 255, 0 and alpha 100% and RGB 255, 255, 255 and alpha 50% then a colour 50% of the way will be 127, 255, 127 alpha 75%. Each RGB and alpha channel have been individual interpolated.

Ok, so now let's set the second colour as transparent, with RGB 0,0,0 and alpha 0. Now, the colour 50% of the way will be 0, 125, 0 alpha 50% (dark green). What if instead the transparent colour had an RGB of 255, 255, 255, with alpha 0? Then the 50% colour will be 125, 255, 125 alpha 50% (light green). So even though both colours were totally transparent, they still result in a different gradient.

So, in summary, try with your transparent colour having an RGB of 255, 255, 255 with alpha 0. This will look much different to a transparent colour with RGB 0,0,0 alpha 0. For these kind of effects I usually duplicate the main colour for the secondary and then drop its alpha to
0 - this keeps a constant colour with only the alpha channel varying.

Hope this helps!
Nyall