[reportlab-users] RGBA Colors

[Kevin D Smith]

On Nov 24, 2009, at 10:53 AM, Robin Becker wrote:

> Kevin D Smith wrote:

>> A little while ago I asked if there was a way to get RGBA colors  

>> (like in CSS3) where the A component is an alpha value to set  

>> transparency.  While doing some searching last night, I realized  

>> that the svn version of ReportLab already has alpha support;  

>> however, it didn't go quite as far as I had hoped.  Methods were  

>> added to the canvas to set the fill and stroke opacity, which is  

>> nice, but if you are using Platypus, you don't have access to them.

>> I messed around with the code a bit in textobject.py and colors.py  

>> to add alpha arguments to all of the color methods as well as an  

>> alpha component to rgb and cmyk colors.  With the attached patch,  

>> you can specify colors with an alpha value that can be used in  

>> Platypus or anywhere else colors are used.  I would like this patch  

>> to be considered for official use so that I don't have to remember  

>> to add this support for my projects.  Does anyone know what that  

>> would take?

> .........

>

> This looks like a nice patch, but there might be hidden problems.  

> The opacity patch was contributed and one of the approaches  

> considered was that opacity should go into the colour objects.  

> However, since opacity is actually stored on the canvas there are  

> problems related to using a mixture of colours containing alpha and  

> non-alpha colours.

>

> In the colour+a model setting a colour with a in [0,1] means an  

> action is carried out on the canvas.

>

> So

>

> setFill(red+alpha=0.5) ==> canvas alpha = 0.5

>

> follow that by

>

> setFill(black+alpha=None)   canvas alpha remains 0.5

>

> which I would argue is unexpected. Forgive me if your patch doesn't  

> exactly follow this model, but that's what my quick scan seems to  

> imply.

>

> We seem to need to get the incoming colour+alpha=None to restore the  

> appropriate alpha value. The problem then is what that value should  

> be; who owns the alpha value?

>

> I'm not against this idea, I did already consider such a mechanism.  

> Perhaps others would like to comment.

>

> One other point is that it is very easy to make a flowable that  

> allows platypus to mess with canvas properties.

> -- 

> Robin Becker



I see your point about alpha not being reset.  I was using the None  
value in the methods and the colors themselves so that they wouldn't  
force a PDF document to be versioned as 1.4 unless alpha was  
explicitly used.  If I didn't care about that fact, I would have set  
alpha to 1 in all of those places.  But it is a bit problematic making  
it both work properly and be backwards compatible.

As far as writing flowables to use the canvas properties, at the  
moment, I use very basic components of platypus like paragraphs and  
tables.  I would prefer not to have to write customized ones just to  
get alpha values in the colors.

Kevin D Smith
Kevin.Smith at sixquickrun.com