# Draw a transparency gradient arc over a rectangle with tikz

Consider the following code:

% DOCUMENT TYPE
\documentclass[12pt,a4paper]{article}

% PACKAGES
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
\usepackage[margin=10pt,font=footnotesize,labelfont=bf]{caption}
\usepackage{lipsum}
\usepackage{fancyhdr}
\usepackage{fancybox}
\usepackage{amsmath}
\usepackage{array}
\usepackage{makeidx}
\usepackage{graphicx}
\usepackage{floatrow}
\usepackage{subfigure}
\usepackage{enumitem}
\usepackage{appendix}
\usepackage{listings}
\usepackage{multicol}
\usepackage{sistyle}
\usepackage{color}
\usepackage{tikz}
\usepackage{lmodern}
\usepackage{multicol}
\usepackage{seqsplit}
\usepackage[listings,skins,theorems]{tcolorbox}
\usepackage{libertine}
\usepackage[normalem]{ulem}
\usepackage[top=1.25in, bottom=1.25in, left=1.25in, right=1.25in]{geometry}
\usepackage{calc}
\usetikzlibrary{calc}
\usepackage{hyperref}

% PRESENTATION
\pagestyle{fancy}
\fancyhf{}
\renewcommand{\footrulewidth}{0pt}

% DOCUMENT BEGINNING
\begin{document}
\lipsum~{}
\begin{center}
\makebox[\textwidth]{
\begin{tikzpicture}
\draw[fill=blue,line] ($(current page.west)+(1cm, 0cm)$) rectangle ($(current page.east)+(-1cm, 2cm)$);
\end{tikzpicture}
}
\end{center}
\lipsum
\end{document}
% DOCUMENT END


which produces the following output:

I have very few knowledge with tikz. So what I would like to do is :

• draw the rectangle without a line around it (just the blue filling)
• create a blank shape (see below) aligned at the north west up to the south west of the rectangle
• with the east side being an arc
• with a transparency gradient perpendicular to the arc of 1cm

Here is the final result I want to obtain:

How to do that with tikz?

• The arc is not fully described. What is its radius? – Qrrbrbirlbel May 17 '15 at 16:28
• You can clip a very simple shape with the rectangle such as \usetikzlibrary{fadings} \begin{tikzpicture} \clip[preaction={blue,fill}] (0,0) rectangle (6,1); \fill[white,path fading=circle with fuzzy edge 20 percent] circle (3cm); \end{tikzpicture} – percusse May 17 '15 at 17:26
• @Qrrbrbirlbel I don't know the radius yet. I will test several options. – Vincent May 17 '15 at 18:21

In general, one can define a radial shading and try drawing a rectangle on it.

\documentclass[border=9,tikz]{standalone}
\begin{document}
color(0bp)=(white);
color(12bp)=(white);
color(13bp)=(blue);
color(50bp)=(blue)}

\tikz{

Once you think the rectangle is good, change the \draw to \clip.
\tikz{