Untitled Block transparency (Beamer)

Question

I'm doing a beamer presentation and for this I'm using a background image on each slide. To have a readable text I put it on a untitled block with background transparency (cf image).

My problem is the overlap between the block's header and the block's body : as you can see, together, the element's transparency is lower because this overlap.

I think I've two solution :

1. reduce the height of the block's header
2. extend the space between header and body's block

But I don't know how to do this. Can someone explain me how to do this properly ? Or may be existing an other (and simpler) solution…

Thank you

Here is my code :

\documentclass{beamer}

\usepackage{beamerbasethemes}
\usepackage{calc}
\usepackage{ifthen}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[no-math]{fontspec}
\usepackage{ragged2e}
\usepackage{graphics}
\usepackage[frenchb]{babel}
\usepackage{nicefrac}
\usepackage{xcolor}
\usepackage{calc}
\usepackage{tikz}

% link to the backgroundimage
% image should be in the same directory as the other files
\def\blackboard@backgroundimagefile{bg.jpg}
\DeclareOptionBeamer{backgroundimagefile}{\def\blackboard@backgroundimagefile{#1}}
\ProcessOptionsBeamer

\mode<presentation>

\pgfdeclareimage[interpolate=true, height=\the\paperheight, width=\the\paperwidth]{backgroundImage}{\blackboard@backgroundimagefile}

% stretch background image over the entire page
\setbeamertemplate{background}{
    \begin{tikzpicture}
        \useasboundingbox (0,0) rectangle (\the\paperwidth, \the\paperheight); 
        \pgftext[at=\pgfpoint{0cm}{0cm}, left, base]{\pgfuseimage{backgroundImage}}; 
    \end{tikzpicture}
}

\setbeamertemplate{blocks}[rounded][shadow=false]
\addtobeamertemplate{block begin}{\pgfsetfillopacity{0.75}}{\pgfsetfillopacity{1}}

\begin{document}

    \begin{frame}{Projet}{Axe 1 : Petite enfance et découverte}
        \begin{block}{}
            \begin{itemize}
                \item A deux ans, 90\% de psychomotricité globale et 10\% de jeux et d'activités sur surface élastique
                \item A 3 ans, 80\% de psychomotricité globale et 20\% de jeux et d'activités sur surface élastique
                \item A 4 ans, 65\%  de psychomotricité globale et 35\% d'activités sur surface rebondissante
                \item A 5 ans, 50\% de psychomotricité globale et 50\% d'éducation aérienne.
            \end{itemize}
            \vfill
            En introduisant ainsi les surfaces élastiques, cela nous permettrait également une détection précoce des talents et aptitudes des enfants, pouvant alors à les orienter vers des disciplines plus appropriées une fois l’âge venu.
        \end{block}
    \end{frame}
\end{document}

Answer 1

The "problem" is that the title area and the body overlap a bit. I added the line

                \advance\bmb@dima by0.95pt%NEW

to the definition of the rounded blocks. You might have to experiment a bit to finetune this value.

\documentclass{beamer}

\usepackage{beamerbasethemes}
\usepackage{calc}
\usepackage{ifthen}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
%\usepackage[no-math]{fontspec}
\usepackage{ragged2e}
\usepackage{graphics}
\usepackage[frenchb]{babel}
\usepackage{nicefrac}
\usepackage{xcolor}
\usepackage{calc}
\usepackage{tikz}

% link to the backgroundimage
% image should be in the same directory as the other files
\def\blackboard@backgroundimagefile{bg.jpg}
\DeclareOptionBeamer{backgroundimagefile}{\def\blackboard@backgroundimagefile{#1}}
\ProcessOptionsBeamer

\mode<presentation>

\pgfdeclareimage[interpolate=true, height=\the\paperheight, width=\the\paperwidth]{backgroundImage}{\blackboard@backgroundimagefile}

% stretch background image over the entire page
\setbeamertemplate{background}{
    \begin{tikzpicture}
    \useasboundingbox (0,0) rectangle (\the\paperwidth, \the\paperheight); 
    \pgftext[at=\pgfpoint{0cm}{0cm}, left, base]{\pgfuseimage{backgroundImage}}; 
    \end{tikzpicture}
}

\makeatletter
\renewcommand\beamerboxesrounded[2][]{%
    \pgfsetfillopacity{0.75}
    \global\let\beamer@firstlineitemizeunskip=\relax%
    \vbox\bgroup%
    \setkeys{beamerboxes}{upper=block title,lower=block body,width=\textwidth,shadow=false}%
    \setkeys{beamerboxes}{#1}%
    {%
        \usebeamercolor{\bmb@lower}%
        \globalcolorstrue%
        \colorlet{lower.bg}{bg}%
    }%
    {%
        \usebeamercolor{\bmb@upper}%
        \globalcolorstrue%
        \colorlet{upper.bg}{bg}%
    }%
    %
    % Typeset head
    %
    \vskip4bp
    \setbox\bmb@box=\hbox{%
        \begin{minipage}[b]{\bmb@width}%
            \usebeamercolor[fg]{\bmb@upper}%
            #2%
        \end{minipage}}%
        \ifdim\wd\bmb@box=0pt%
        \setbox\bmb@box=\hbox{}%
        \ht\bmb@box=1.5pt%
        \bmb@prevheight=-4.5pt%
        \else%
        \wd\bmb@box=\bmb@width%
        \bmb@temp=\dp\bmb@box%
        \ifdim\bmb@temp<1.5pt%
        \bmb@temp=1.5pt%
        \fi%
        \setbox\bmb@box=\hbox{\raise\bmb@temp\hbox{\box\bmb@box}}%
        \dp\bmb@box=0pt%
        \bmb@prevheight=\ht\bmb@box%
        \fi%
        \bmb@temp=\bmb@width%
        \bmb@dima=\bmb@temp\advance\bmb@dima by2.2bp%
        \bmb@dimb=\bmb@temp\advance\bmb@dimb by4bp%
        \hbox{%
            \begin{pgfpicture}{0bp}{+-\ht\bmb@box}{0bp}{+-\ht\bmb@box}
                \ifdim\wd\bmb@box=0pt%
                \color{lower.bg}%
                \else%
                \color{upper.bg}%
                \fi%
                %               \pgfsetfillopacity{\opacitylevel}%NEW
                \pgfpathqmoveto{-4bp}{-1bp}
                \pgfpathqcurveto{-4bp}{1.2bp}{-2.2bp}{3bp}{0bp}{3bp}
                \pgfpathlineto{\pgfpoint{\bmb@temp}{3bp}}
                \pgfpathcurveto%
                {\pgfpoint{\bmb@dima}{3bp}}%
                {\pgfpoint{\bmb@dimb}{1.2bp}}%
                {\pgfpoint{\bmb@dimb}{-1bp}}%
                \bmb@dima=-\ht\bmb@box%
                \advance\bmb@dima by0.95pt%NEW
                \pgfpathlineto{\pgfpoint{\bmb@dimb}{\bmb@dima}}
                \pgfpathlineto{\pgfpoint{-4bp}{\bmb@dima}}
                \pgfusepath{fill}
            \end{pgfpicture}%
            \copy\bmb@box%
        }%
        \nointerlineskip%
        \vskip-1pt%
        \ifdim\wd\bmb@box=0pt%
        \else%
        \hbox{%
            \begin{pgfpicture}{0pt}{0pt}{\bmb@width}{6pt}
                \bmb@dima=\bmb@width%
                \advance\bmb@dima by8bp%
                \pgfpathrectangle{\pgfpoint{-4bp}{-1bp}}{\pgfpoint{\bmb@dima}{8bp}}
                \pgfusepath{clip}
                {\pgftransformshift{\pgfpoint{-4bp}{0bp}}\pgftext[left,base]{\pgfuseshading{bmb@transition}}}%
            \end{pgfpicture}%
        }%
        \nointerlineskip%
        \vskip-0.5pt%
        \fi%
        \ifbmb@shadow%
        \setbox\bmb@boxshadow=\hbox{\pgfuseshading{bmb@shadow}}%
        \setbox\bmb@boxshadowball=\hbox{\pgfuseshading{bmb@shadowball}}%
        \setbox\bmb@boxshadowballlarge=\hbox{\pgfuseshading{bmb@shadowballlarge}}%
        \fi%
        \setbox\bmb@colorbox=\hbox{{\pgfpicturetrue\pgfsetcolor{lower.bg}}}%
        \setbox\bmb@box=\hbox\bgroup\begin{minipage}[b]{\bmb@width}%
            \vskip2pt%
            \usebeamercolor[fg]{\bmb@lower}%
            \colorlet{beamerstructure}{upper.bg}%
            \colorlet{structure}{upper.bg}%
            %\color{.}%
        }
\makeatother


\setbeamertemplate{blocks}[rounded][shadow=false]
%\addtobeamertemplate{block begin}{\pgfsetfillopacity{0.75}}{\pgfsetfillopacity{1}}


\begin{document}

    \begin{frame}{Projet}{Axe 1 : Petite enfance et découverte}
        \begin{block}{}
            \begin{itemize}
                \item A deux ans, 90\% de psychomotricité globale et 10\% de jeux et d'activités sur surface élastique
                \item A 3 ans, 80\% de psychomotricité globale et 20\% de jeux et d'activités sur surface élastique
                \item A 4 ans, 65\%  de psychomotricité globale et 35\% d'activités sur surface rebondissante
                \item A 5 ans, 50\% de psychomotricité globale et 50\% d'éducation aérienne.
            \end{itemize}
            \vfill
            En introduisant ainsi les surfaces élastiques, cela nous permettrait également une détection précoce des talents et aptitudes des enfants, pouvant alors à les orienter vers des disciplines plus appropriées une fois l’âge venu.
        \end{block}
    \end{frame}
\end{document}