How to make a perfect colour filled threeparttable table?

Question

A three line table is used for my report. I want to fill lightgray background color to the whole table, but there is white space between rows, how to eliminate this issue?

1. Without \addlinespace: there is still white space around rules

2. With \addlinespace: this is ideal spacing between rows, but how to fill colour as well for these white spaces?

---

  \usepackage{colortbl}
  \definecolor{Lightgray}{RGB}{235,235,235}

Code:

  \begin{table} [htpb]
  \centering
  \begin{minipage}[b]{0.9 \textwidth}
  \centering
  \begin{threeparttable}[b]

  \caption{Size and shape distribution for a sampled rock riprap} 
  \label{tab:size_shape}
  \begin{tabular} { p{0.15 \textwidth}  p{0.10 \textwidth} p{0.10 \textwidth} p{0.10 \textwidth} p{0.15 \textwidth}  } 
  \toprule
  \rowcolor{Lightgray}& Small    &  Medium &  Large & Shape fraction \\        \cmidrule{1-5}  \addlinespace
  \rowcolor{Lightgray}Compact    & 10\% &  44\% & 7\% & 61\% \\ \addlinespace
  \rowcolor{Lightgray}Flat       & 4\%  &  10\% & 4\% & 18\% \\ \addlinespace       
  \rowcolor{Lightgray}Elongated  & 5\%  &  12\% & 4\% & 21\% \\ \addlinespace
  \midrule
  \rowcolor{Lightgray}Size fraction  & 19\% & 66\% & 15\% & 100\% \\
  \bottomrule
  \end{tabular}
  \end{threeparttable}
  \end{minipage}
  \end{table} 

Edit 1: Using \renewcommand{\arraystretch}{1.3} gets proper spacing between rows without \addlinespace. The remaining issue is: how to get ride of the whihte space for rules?

Edit 2: The solution of Professional-looking tables with alternating row colors still could not eliminate white space after midline or cmidrule. Hence white colour is used for the row after midline to hide the white space. If wrong please correct me.

See the table created by the new commands of that solution:

   \begin{table} [htpb]
   \centering
   \begin{minipage}[b]{0.9 \textwidth}
   \centering
   \begin{threeparttable}[b]

   \caption{Size and shape distribution for a sampled rock riprap} 
   \label{tab:size_shape}
   \begin{tabular} { p{0.15 \textwidth}  p{0.10 \textwidth} p{0.10 \textwidth} p{0.10 \textwidth} p{0.15 \textwidth}  } 
   \topline
   \headcol & Small  &  Medium &  Large & Shape fraction \\% \cmidrule{1-5}  
   \midline
   \rowcol Compact    & 10\% &  44\% & 7\% & 61\% \\ 
   \rowcol Flat       & 4\%  &  10\% & 4\% & 18\% \\ % \cellcolor{lightgray}  
   \rowcol Elongated  & 5\%  &  12\% & 4\% & 21\% \\ 
   \midline
   \rowcol Size fraction  & 19\% & 66\% & 15\% & 100\% \\
   \bottomlinec  %\bottomrule
   \end{tabular}
   \end{threeparttable}
   \end{minipage}
   \end{table}

Besides, if you enlarge the table and look carefully, you will see few thin white lines in the heading row! Is that normal?

See below:

Answer 1

After looking into the new commands provided by Werner, it seems that the mideline is redefined by 3 rules with top colour, middle colour and bottom colour.

So two new commands were added to make a perfect colour filled table:

1. Middle line connecting heading row and the second row

   \colorlet{blcolor}{gray!80}
   
   \newcommand{\rowmidlineHR}{\arrayrulecolor{tableheadcolor}
     \specialrule{\aboverulesep}{0pt}{0pt}%
     \arrayrulecolor{black}\specialrule{\lightrulewidth}{0pt}{0pt}%
     \arrayrulecolor{tablerowcolor}\specialrule{\belowrulesep}{0pt}{0pt}%
     \arrayrulecolor{black}}
     % Command \rowmidlinewc consists of 3 rules
     % (top colour tableheadcolor, middle colour black, bottom colour tablerowcolor)
   

2. Secondary gray middle line

   \newcommand{\rowmidlineG}{\arrayrulecolor{tablerowcolor}%
     \specialrule{\aboverulesep}{0pt}{0pt}%
     \arrayrulecolor{blcolor}\specialrule{\lightrulewidth}{0pt}{0pt}%
     \arrayrulecolor{tablerowcolor}\specialrule{\belowrulesep}{0pt}{0pt}%
     \arrayrulecolor{black}}
   % Command \rowmidlineG consists of 3 rules
   % (top colour tablerowcolor, middle colour black, bottom colour tablerowcolor)
   

Final table:

and the codes:

\renewcommand{\arraystretch}{1.3}

\begin{table} [htpb]
\centering
\begin{minipage}[b]{0.9 \textwidth}
\centering
\begin{threeparttable}[b]

\caption{Size and shape distribution for a sampled rock riprap} 
\label{tab:size_shape}
\begin{tabular} { p{0.15 \textwidth}  p{0.10 \textwidth} p{0.10 \textwidth} p{0.10 \textwidth} p{0.15 \textwidth}  } 
\topline
\headcol & Small  &  Medium &  Large & Shape fraction \\
\rowmidlineHR  % Head Rule
\rowcol Compact    & 10\% &  44\% & 7\% & 61\% \\ 
\rowcol Flat       & 4\%  &  10\% & 4\% & 18\% \\ 
\rowcol Elongated  & 5\%  &  12\% & 4\% & 21\% \\ 
\rowmidlineG % Gray midline
\rowcol Size fraction  & 19\% & 66\% & 15\% & 100\% \\
\bottomlinec
\end{tabular}
\end{threeparttable}
\end{minipage}
\end{table}

Answer 2

With {NiceTabular} of nicematrix, you have a command to color the whole tabular. You have directly the expected result.

However, in order to use {NiceTabular} in {threeparttable}, you have first to tell threeparttable that it should take into account the environment {NiceTabular} (with the dedicated hook of threeparttable):

\makeatletter
\AddToHook{env/threeparttable/begin}
{\TPT@hookin{NiceTabular}\TPT@hookin{NiceTabular*}}
\makeatother

For the whole MWE:

\documentclass{article}
\usepackage{xcolor}
\usepackage{threeparttable}
\usepackage{nicematrix}
\usepackage{booktabs}
\usepackage[left=2.5cm,right=2.5cm]{geometry}

\begin{document}

\makeatletter
\AddToHook{env/threeparttable/begin}
{\TPT@hookin{NiceTabular}\TPT@hookin{NiceTabular*}}
\makeatother


\begin{table} [htpb]
\centering
\begin{minipage}[b]{0.9 \textwidth}
\centering
\begin{threeparttable}[b]

\caption{Size and shape distribution for a sampled rock riprap} 
\label{tab:size_shape}
\begin{NiceTabular} { p{0.15 \textwidth}  p{0.10 \textwidth} p{0.10 \textwidth} p{0.10 \textwidth} p{0.15 \textwidth}  } 
\CodeBefore
  \arraycolor[gray]{0.9}
\Body
\toprule
& Small    &  Medium &  Large & Shape fraction \\ \cmidrule{1-5}  \addlinespace
Compact    & 10\% &  44\% & 7\% & 61\% \\ \addlinespace
Flat       & 4\%  &  10\% & 4\% & 18\% \\ \addlinespace       
Elongated  & 5\%  &  12\% & 4\% & 21\% \\ \addlinespace
\midrule
Size fraction  & 19\% & 66\% & 15\% & 100\% \\
\bottomrule
\end{NiceTabular}
\end{threeparttable}
\end{minipage}
\end{table} 

\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).