enable transparent background of beamercolorbox (#884)

implementation based on https://tex.stackexchange.com/questions/75839/transparency-of-beamercolorbox-and-opaqueness-of-its-text

additional things that needed fixing:
- implement transparent background for `rounded=false`
- by default, don't change anything by not defining the `bgopacity`
  macros
- more exact clipping of shadows when `bgopacity` is set

Co-authored-by: samcarter <[email protected]>

Author: chrjabs

AUTHORS.md

@@ -29,6 +29,7 @@
 - Andreas Scherer <[email protected]> (documentation fixes)
 - Juergen Spitzmueller <[email protected]> (diabel auto-loading amssymb)
 - Gustav Tiger <[email protected]> (typos in manual)
+- Christoph Jabs <[email protected]> (transparency in beamercolorbox and beamerboxesrounded)
 
 # Theme contributors
 

CHANGELOG.md

@@ -11,6 +11,7 @@ a major and minor version only.
 ### Added
 
 - doc: added sentence about limitations of xelatex
+- added `bgopacity` option to `beamercolorbox` and `beamerboxesrounded` for transparent background drawing
 
 ### Changed
 

base/beamerbaseboxes.sty

@@ -20,6 +20,7 @@
 \define@key{beamerboxes}{lower}{\def\bmb@lower{#1}}
 \define@key{beamerboxes}{width}{\edef\bmb@width{#1}}
 \define@key{beamerboxes}{shadow}[true]{\csname bmb@shadow#1\endcsname}
+\define@key{beamerboxes}{bgopacity}{\def\bmb@bgopacity{#1}}
 
 \newif\ifbmb@shadow
 \newbox\bmb@box
@@ -80,6 +81,7 @@
       \else%
         \color{upper.bg}%
       \fi%
+      \ifdefined\bmb@bgopacity\pgfsetfillopacity{\bmb@bgopacity}\fi
       \pgfpathqmoveto{-4bp}{-1bp}
       \pgfpathqcurveto{-4bp}{1.2bp}{-2.2bp}{3bp}{0bp}{3bp}
       \pgfpathlineto{\pgfpoint{\bmb@temp}{3bp}}
@@ -88,7 +90,17 @@
       {\pgfpoint{\bmb@dimb}{1.2bp}}%
       {\pgfpoint{\bmb@dimb}{-1bp}}%
       \bmb@dima=-\ht\bmb@box%
-      \advance\bmb@dima by-2pt%
+      \ifdefined\bmb@bgopacity%
+        % Only with explicity specified opacity, make sure that there is no
+        % overlap between top and bottom part of the box
+        \ifdim\wd\bmb@box=0pt%
+          \advance\bmb@dima by .5pt%
+        \else%
+          \advance\bmb@dima by -1.07pt%
+        \fi%
+      \else%
+        \advance\bmb@dima by -2pt%
+      \fi%
       \pgfpathlineto{\pgfpoint{\bmb@dimb}{\bmb@dima}}
       \pgfpathlineto{\pgfpoint{-4bp}{\bmb@dima}}
       \pgfusepath{fill}
@@ -105,13 +117,16 @@
       \advance\bmb@dima by8bp%
       \pgfpathrectangle{\pgfpoint{-4bp}{-1bp}}{\pgfpoint{\bmb@dima}{8bp}}
       \pgfusepath{clip}
+      \ifdefined\bmb@bgopacity\pgfsetfillopacity{\bmb@bgopacity}\fi%
       {\pgftransformshift{\pgfpoint{-4bp}{0bp}}\pgftext[left,base]{\pgfuseshading{bmb@transition}}}%
     \end{pgfpicture}%
   }%
   \nointerlineskip%
   \vskip-0.5pt%
   \fi%
-  \setbox\bmb@colorbox=\hbox{{\pgfpicturetrue\pgfsetcolor{lower.bg}}}%
+  \setbox\bmb@colorbox=\hbox{{\pgfpicturetrue%
+    \ifdefined\bmb@bgopacity\pgfsetfillopacity{\bmb@bgopacity}\fi%
+    \pgfsetcolor{lower.bg}}}%
   \setbox\bmb@box=\hbox\bgroup\begin{minipage}[b]{\bmb@width}%
     \vskip2pt%
     \usebeamercolor[fg]{\bmb@lower}%
@@ -173,6 +188,16 @@
             \pgfsetcolor{beamer@safeblack}
             \pgfpathrectangle{\pgfpoint{4bp}{8.1bp}}{\pgfpoint{\bmb@boxwidth}{\bmb@boxheight-2bp}}
             \pgfusepath{fill}
+            \ifdefined\bmb@bgopacity%
+              % More precise clipping if bgopacity is specified
+              \pgfpathqmoveto{-4bp}{8.2bp}
+              \pgfpathqlineto{-4bp}{8bp}
+              \pgfpathqcurveto{-4bp}{5.8bp}{-2.2bp}{4bp}{0bp}{4bp}
+              \pgfpathlineto{\pgfpoint{\bmb@boxwidth}{4bp}}
+              \pgfpathqcurveto{\dimexpr\bmb@boxwidth+2.2bp}{4bp}{\dimexpr\bmb@boxwidth+4bp}{5.8bp}{\dimexpr\bmb@boxwidth+4bp}{8bp}
+              \pgfpathqlineto{\dimexpr\bmb@boxwidth+4bp}{8.2bp}
+              \pgfusepath{fill}
+            \fi%
           \end{pgfscope}
         \end{pgfpicture}%
         \hskip4bp%      
@@ -204,6 +229,10 @@
       {\pgfpoint{\the\bmb@dimb}{1bp}}%
       {
         \bmb@dima=\ht\bmb@box%
+        \ifdefined\bmb@bgopacity%
+          % Remove overlap with bgtransparency specified
+          \advance\bmb@dima by-.5pt%
+        \fi%
         \pgfpathlineto{\pgfpoint{\bmb@dimb}{\bmb@dima}}
         \pgfpathlineto{\pgfpoint{-4bp}{\bmb@dima}}
         \pgfusepath{fill}

base/beamerbasecolor.sty

@@ -141,6 +141,7 @@
 \define@key{beamercolbox}{rounded}{\csname beamer@colbox@rounded#1\endcsname}
 \define@key{beamercolbox}{ignorebg}[true]{\csname beamer@colbox@ignorebg#1\endcsname}
 \define@key{beamercolbox}{vmode}[]{\def\beamer@vmode{}}
+\define@key{beamercolbox}{bgopacity}{\def\beamer@colbox@bgopacity{#1}}
 
 \newif\ifbeamer@colbox@rounded
 \newif\ifbeamer@colbox@ignorebg
@@ -190,13 +191,24 @@
   \ifx\beamer@colbox@dp\@empty\else\dp\beamer@tempbox=\beamer@colbox@dp\fi%
   \ifbeamer@colbox@rounded%
     \if@tempswa%
-      \begin{beamerboxesrounded}[%
-        shadow=\beamer@colbox@shadow,%
-        lower=\beamer@colbox@color,%
-        upper=normal text,%
-        width=\beamer@colbox@wd]{}%
-        \box\beamer@tempbox%
-      \end{beamerboxesrounded}%
+      \ifdefined\beamer@colbox@bgopacity%
+        \begin{beamerboxesrounded}[%
+          shadow=\beamer@colbox@shadow,%
+          lower=\beamer@colbox@color,%
+          upper=normal text,%
+          width=\beamer@colbox@wd,%
+          bgopacity=\beamer@colbox@bgopacity]{}%
+          \box\beamer@tempbox%
+        \end{beamerboxesrounded}%
+      \else%
+        \begin{beamerboxesrounded}[%
+          shadow=\beamer@colbox@shadow,%
+          lower=\beamer@colbox@color,%
+          upper=normal text,%
+          width=\beamer@colbox@wd]{}%
+          \box\beamer@tempbox%
+        \end{beamerboxesrounded}%
+      \fi%
     \else%
       \ifdim\wd\beamer@tempbox>\textwidth%
         \setbox\beamer@tempbox=\hbox to\textwidth{\hss\box\beamer@tempbox\hss}%
@@ -206,8 +218,10 @@
   \else%
     \if@tempswa\setbox\beamer@tempbox=\hbox{%
       \usebeamercolor{\beamer@colbox@color}%
+      \ifdefined\beamer@colbox@bgopacity\pgfsetfillopacity{\beamer@colbox@bgopacity}\fi%
       \hskip-\beamer@colbox@colseps%
       \fboxsep=\z@\colorbox{bg}{%
+        \pgfsetfillopacity{1}%
         \hskip\beamer@colbox@colseps%
         \box\beamer@tempbox%
         \hskip\beamer@colbox@colseps%

doc/beamerug-localstructure.tex

@@ -842,6 +842,8 @@ \subsection{Framed and Boxed Text}
     \declare{|ignorebg|} causes the background color of the \meta{beamer color} to be ignored, that is, to be treated as if it were set to ``transparent'' or ``empty.''
   \item
     \declare{|vmode|} causes \TeX\ to be in vertical mode when the box starts. Normally, \TeX\ will be in horizontal mode at the start of the box (a |\leavevmode| is inserted automatically at the beginning of the box unless this option is given). Only \TeX perts need this option, so, if you use it, you will probably know what you are doing anyway.
+  \item
+    \declare{|bgopacity|=\marg{opacity}} sets the opacity with which the background is drawn. To draw a box with 30\% transparent background, set |bgopacity=0.7|. Note that combining rounded corners and a transparent background can lead to fine gaps appearing in the box because the box is drawn in multiple segments. If |bgopacity| is not specified, |\pgfsetfillopacity| is never set so that the surrounding opacity setting is adopted.
   \end{itemize}
 \end{environment}
 
@@ -856,6 +858,8 @@ \subsection{Framed and Boxed Text}
     \declare{|width=|\meta{dimension}} causes the width of the text inside the box to be the specified \meta{dimension}. By default, the |\textwidth| is used. Note that the box will protrude 4pt to the left and right.
   \item
     \declare{|shadow=|\meta{true or false}}. If set to |true|, a shadow will be drawn.
+  \item
+    \declare{|bgopacity|=\marg{opacity}} sets the opacity with which the background is drawn. To draw a box with 30\% transparent background, set |bgopacity=0.7|. Note that with a transparent background fine gaps can appear in the box because the box is drawn in multiple segments. If |bgopacity| is not specified, |\pgfsetfillopacity| is never set so that the surrounding opacity setting is adopted.
   \end{itemize}
   If no \meta{head} is given, the head part is completely suppressed.
   \example