Q: baseline doesn't perfectly align in pNiceMatrix/bNiceMatrix

Question

Is it possible to perfectly align the baseline of p/bNiceMatrix? (The misalignment is quite obvious when you have a big matrix...)

MWE

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\begin{equation}
    \rlap{\rule{57mm}{0.11pt}}0 = \begin{NiceMatrix}[baseline=2]
        0 & 1 \
        0 & 1 \
        0 & 1 \
    \end{NiceMatrix}\quad
    0 = \begin{pNiceMatrix}[baseline=2]
        0 & 2 \
        0 & 2 \
        0 & 2 \
    \end{pNiceMatrix}\quad
    0 = \begin{bNiceMatrix}[baseline=2]
        0 & 3 \
        0 & 3 \
        0 & 3 \
    \end{bNiceMatrix}
\end{equation}
\end{document}

Outcome

---

Thank you for all the replies.

Update: The patch suggested by @Ruixi Zhang solves the misalignment.

Update 2 (2021-11-26): There seems to be a side effect of the patch.

Test code

\usepackage{booktabs}
\usepackage{nicematrix}
...
\begin{table}[h]
    \caption{A table}
    \centering
    \begin{NiceTabular}{ll}[cell-space-limits=3pt]
        \toprule
        a & b \\
        \midrule
        \Block{}{new\\line} & c \\
        \bottomrule
    \end{NiceTabular}
\end{table}

Answer 1

This is a “dual” bug. The first part is in the LaTeX kernel itself and the second part is in the nicematrix package.

1. The first part of the bug is in the underlying array environment (and it’s been there since LaTeX2.09). I’ve reported this to the LaTeX team over two years ago (https://github.com/latex3/latex2e/issues/147). Any fixes will break backward compatibility, so the fixes are very unlikely to be implemented in the kernel.

   The problem is that each row of an array has a “strut”. This strut has a height-to-depth ratio of 7:3. But it should instead have a height of <half baseline> + <math axis> and a depth of <half baseline> - <math axis>. The difference is very small in most cases, but still visible enough to cause misalignment.

   For example, a 10pt article with 12pt baseline separation, set in Computer Modern Math, should have strut height 8.5pt and strut depth 3.5pt. But LaTeX sets strut height to about 8.4pt and strut depth to about 3.6pt, causing a tiny 0.1/2=0.05pt misalignment.

2. The second part of the bug is in the environment NiceArrayWithDelims. Even after we fix the kernel strut behavior, there is still a 0.4pt misalignment. The problem seems to be in \vcenter{ \skip_vertical:N -\l_tmpa_dim ... \skip_vertical:N -\l_tmpb_dim }. An offset of \arrayrulewidth seems to be missing.

   There is no easy way to patch LaTeX3 environment, so I will just copy the entire definition of NiceArrayWithDelims below. Only two lines of code are replaced. Thus, this fix is not to be taken seriously. It only serves as a temporary fix.

Complete code:

\documentclass{article}
\usepackage{nicematrix}
% Fix the LaTeX kernel part of the bug
\usepackage{etoolbox}
\makeatletter
\AtBeginDocument{%
  % nicematrix loads array.sty
  \patchcmd@array
    {\arraystretch @tempdima}
    {\dimexpr\arraystretch\baselineskip/2
      +\arraystretch\extrarowheight
      +\fontdimen22\textfont\tw@\relax}
    {\typeout{Fixed array strut height for array.sty}}
    {\typeout{Couldn't patch \string@array}}%
  \patchcmd@array
    {\arraystretch \dp \strutbox}
    {\dimexpr\arraystretch\baselineskip
      -\arraystretch\baselineskip/2
      -\fontdimen22\textfont\tw@\relax}
    {\typeout{Fixed array strut depth for array.sty}}
    {\typeout{Couldn't patch \string@array}}%
  \let@@array@array
}
\makeatother
% `Fix' the package bug.
% Only two lines are replaced!!!
% Not to be taken as a serious fix!!!
\makeatletter
\ExplSyntaxOn
\RenewDocumentEnvironment { NiceArrayWithDelims }
  { m m O { } m ! O { } t \CodeBefore }
  {
    \bool_if:NT \c__nicematrix_revtex_bool __nicematrix_patch_for_revtex:
    __nicematrix_provide_pgfsyspdfmark:
    \bool_if:NT \c__nicematrix_footnote_bool \savenotes
    \bgroup
    \tl_gset:Nn \g__nicematrix_left_delim_tl { #1 }
    \tl_gset:Nn \g__nicematrix_right_delim_tl { #2 }
    \tl_gset:Nn \g__nicematrix_preamble_tl { #4 }
    \int_gzero:N \g__nicematrix_block_box_int
    \dim_zero:N \g__nicematrix_width_last_col_dim
    \dim_zero:N \g__nicematrix_width_first_col_dim
    \bool_gset_false:N \g__nicematrix_row_of_col_done_bool
    \str_if_empty:NT \g__nicematrix_name_env_str
      { \str_gset:Nn \g__nicematrix_name_env_str { NiceArrayWithDelims } }
    __nicematrix_adapt_S_column:
    \bool_if:NTF \l__nicematrix_NiceTabular_bool
      \mode_leave_vertical:
      __nicematrix_test_if_math_mode:
    \bool_if:NT \l__nicematrix_in_env_bool { __nicematrix_fatal:n { Yet~in~env } }
    \bool_set_true:N \l__nicematrix_in_env_bool
      \cs_gset_eq:NN __nicematrix_old_CT@arc@ \CT@arc@
    \cs_if_exist:NT \tikz@library@external@loaded
      {
        \tikzexternaldisable
        \cs_if_exist:NT \ifstandalone
          { \tikzset { external / optimize = false } }
      }
    \int_gincr:N \g__nicematrix_env_int
    \bool_if:NF \l__nicematrix_block_auto_columns_width_bool
      { \dim_gzero_new:N \g__nicematrix_max_cell_width_dim }
    \seq_gclear:N \g__nicematrix_blocks_seq
    \seq_gclear:N \g__nicematrix_pos_of_blocks_seq
    \seq_gclear:N \g__nicematrix_pos_of_stroken_blocks_seq
    \seq_gclear:N \g__nicematrix_pos_of_xdots_seq
    \tl_gclear_new:N \g__nicematrix_code_before_tl
    \tl_gclear:N \g__nicematrix_row_style_tl
    \bool_gset_false:N \g__nicematrix_aux_found_bool
    \tl_if_exist:cT { c__nicematrix _ \int_use:N \g__nicematrix_env_int _ tl }
      {
        \bool_gset_true:N \g__nicematrix_aux_found_bool
        \use:c { c__nicematrix _ \int_use:N \g__nicematrix_env_int _ tl }
      }
     \tl_gclear:N \g__nicematrix_aux_tl
    \tl_if_empty:NF \g__nicematrix_code_before_tl
      {
        \bool_set_true:N \l__nicematrix_code_before_bool
        \tl_put_right:NV \l__nicematrix_code_before_tl \g__nicematrix_code_before_tl
      }
    \bool_if:NTF \l__nicematrix_NiceArray_bool
      { \keys_set:nn { NiceMatrix / NiceArray } }
      { \keys_set:nn { NiceMatrix / pNiceArray } }
    { #3 , #5 }
    \tl_if_empty:NF \l__nicematrix_rules_color_tl
      { \exp_after:wN __nicematrix_set_CT@arc@: \l__nicematrix_rules_color_tl \q_stop }
    \IfBooleanTF { #6 } __nicematrix_pre_array_i:w __nicematrix_pre_array:
  }
  {
    \bool_if:NTF \l__nicematrix_light_syntax_bool
      { \use:c { end __nicematrix-light-syntax } }
      { \use:c { end __nicematrix-normal-syntax } }
    \c_math_toggle_token
    \skip_horizontal:N \l__nicematrix_right_margin_dim
    \skip_horizontal:N \l__nicematrix_extra_right_margin_dim
    \hbox_set_end:
    \bool_if:NT \l__nicematrix_width_used_bool
      {
        \int_compare:nNnT \g__nicematrix_total_X_weight_int = 0
          { __nicematrix_error:n { width~without~X~columns } }
      }
    \int_compare:nNnT \g__nicematrix_total_X_weight_int > 0
      {
        \tl_gput_right:Nx \g__nicematrix_aux_tl
          {
            \bool_set_true:N \l__nicematrix_X_columns_aux_bool
            \dim_set:Nn \l__nicematrix_X_columns_dim
              {
                \dim_compare:nNnTF
                  {
                    \dim_abs:n
                      { \l__nicematrix_width_dim - \box_wd:N \l__nicematrix_the_array_box }
                  }
                  <
                  { 0.001 pt }
                  { \dim_use:N \l__nicematrix_X_columns_dim }
                  {
                    \dim_eval:n
                      {
                        ( \l__nicematrix_width_dim - \box_wd:N \l__nicematrix_the_array_box )
                        / \int_use:N \g__nicematrix_total_X_weight_int
                        + \l__nicematrix_X_columns_dim
                      }
                  }
              }
          }
      }
    \int_compare:nNnT \l__nicematrix_last_row_int > { -2 }
      {
        \bool_if:NF \l__nicematrix_last_row_without_value_bool
          {
            \int_compare:nNnF \l__nicematrix_last_row_int = \c@iRow
              {
                __nicematrix_error:n { Wrong~last~row }
                \int_gset_eq:NN \l__nicematrix_last_row_int \c@iRow
              }
          }
      }
    \int_gset_eq:NN \c@jCol \g__nicematrix_col_total_int
    \bool_if:nTF \g__nicematrix_last_col_found_bool
      { \int_gdecr:N \c@jCol }
      {
        \int_compare:nNnT \l__nicematrix_last_col_int > { -1 }
          { __nicematrix_error:n { last~col~not~used } }
      }
    \int_gset_eq:NN \g__nicematrix_row_total_int \c@iRow
    \int_compare:nNnT \l__nicematrix_last_row_int > { -1 } { \int_gdecr:N \c@iRow }
    \int_compare:nNnT \l__nicematrix_first_col_int = 0
      {
        \skip_horizontal:N \col@sep
        \skip_horizontal:N \g__nicematrix_width_first_col_dim
      }
    \bool_if:NTF \l__nicematrix_NiceArray_bool
      {
        \str_case:VnF \l__nicematrix_baseline_tl
          {
            b __nicematrix_use_arraybox_with_notes_b:
            c __nicematrix_use_arraybox_with_notes_c:
          }
          __nicematrix_use_arraybox_with_notes:
      }
      {
        \int_compare:nNnTF \l__nicematrix_first_row_int = 0
          {
            \dim_set_eq:NN \l_tmpa_dim \g__nicematrix_dp_row_zero_dim
            \dim_add:Nn \l_tmpa_dim \g__nicematrix_ht_row_zero_dim
          }
          { \dim_zero:N \l_tmpa_dim }
        \int_compare:nNnTF \l__nicematrix_last_row_int > { -2 }
          {
            \dim_set_eq:NN \l_tmpb_dim \g__nicematrix_ht_last_row_dim
            \dim_add:Nn \l_tmpb_dim \g__nicematrix_dp_last_row_dim
          }
          { \dim_zero:N \l_tmpb_dim }
        \hbox_set:Nn \l_tmpa_box
          {
            \c_math_toggle_token
            \tl_if_empty:NF \l__nicematrix_delimiters_color_tl
              { \color { \l__nicematrix_delimiters_color_tl } }
            \exp_after:wN \left \g__nicematrix_left_delim_tl
            \vcenter
              {
%                \skip_vertical:N -\l_tmpa_dim
                \skip_vertical:n { -\l_tmpa_dim - \arrayrulewidth }
                \hbox
                  {
                    \bool_if:NTF \l__nicematrix_NiceTabular_bool
                      { \skip_horizontal:N -\tabcolsep }
                      { \skip_horizontal:N -\arraycolsep }
                    __nicematrix_use_arraybox_with_notes_c:
                    \bool_if:NTF \l__nicematrix_NiceTabular_bool
                      { \skip_horizontal:N -\tabcolsep }
                      { \skip_horizontal:N -\arraycolsep }
                  }
%                \skip_vertical:N -\l_tmpb_dim
                \skip_vertical:n { -\l_tmpb_dim + \arrayrulewidth }
              }
            \tl_if_empty:NF \l__nicematrix_delimiters_color_tl
              { \color { \l__nicematrix_delimiters_color_tl } }
            \exp_after:wN \right \g__nicematrix_right_delim_tl
            \c_math_toggle_token
          }
        \bool_if:NTF \l__nicematrix_delimiters_max_width_bool
          {
            __nicematrix_put_box_in_flow_bis:nn
              \g__nicematrix_left_delim_tl \g__nicematrix_right_delim_tl
          }
          __nicematrix_put_box_in_flow:
      }
    \bool_if:NT \g__nicematrix_last_col_found_bool
      {
        \skip_horizontal:N \g__nicematrix_width_last_col_dim
        \skip_horizontal:N \col@sep
      }
    \bool_if:NF \l__nicematrix_Matrix_bool
      {
        \int_compare:nNnT \c@jCol < \g__nicematrix_static_num_of_col_int
          { __nicematrix_error:n { columns~not~used } }
      }
    \group_begin:
    \globaldefs = 1
    __nicematrix_msg_redirect_name:nn { columns~not~used } { error }
    \group_end:
    __nicematrix_after_array:
    \egroup
    \iow_now:Nn @mainaux { \ExplSyntaxOn }
    \iow_now:Nn @mainaux { \char_set_catcode_space:n { 32 }  }
    \iow_now:Nx @mainaux
      {
        \tl_gset:cn { c__nicematrix_ \int_use:N \g__nicematrix_env_int _ tl }
          { \exp_not:V \g__nicematrix_aux_tl }
      }
    \iow_now:Nn @mainaux { \ExplSyntaxOff }
    \bool_if:NT \c__nicematrix_footnote_bool \endsavenotes
  }
\ExplSyntaxOff
\makeatother
\begin{document}
\begin{equation}
    \rlap{\rule{57mm}{0.11pt}}0 = \begin{NiceMatrix}[baseline=2]
        0 & 1 \
        \rlap{\color{red}\rule{5.7mm}{0.11pt}}0 & 1 \
        0 & 1 \
    \end{NiceMatrix}\quad
    0 = \begin{pNiceMatrix}[baseline=2]
        0 & 2 \
        \rlap{\color{red}\rule{5.7mm}{0.11pt}}0 & 2 \
        0 & 2 \
    \end{pNiceMatrix}\quad
    0 = \begin{bNiceMatrix}[baseline=2]
        0 & 3 \
        \rlap{\color{red}\rule{5.7mm}{0.11pt}}0 & 3 \
        0 & 3 \
    \end{bNiceMatrix}
\end{equation}
\end{document}

The code inside \AtBeginDocument{...} is extracted from my own patch file (https://github.com/RuixiZhang42/font-pairing-guide/blob/master/mtpro2-patch.tex), lines 852–866 as of this writing. It covers only array.sty. A more thorough fix is provided as the code from line 850 to line 911.

Answer 2

The version 6.4 of nicematrix (2021/11/23) solves that bug of nicematrix.

Here is a MWE.

Eventually, I don't use the Ruixi Zhang's patch of {array} of LaTeX (see other answer) because it seems that it has consequences when the command \Block is used with \\ somewhere in the matrix.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}


\begin{equation}
    \rlap{\rule{57mm}{0.11pt}}0 = \begin{NiceMatrix}[baseline=2]
        0 & 1 \
        0 & 1 \
        0 & 1 \
    \end{NiceMatrix}\quad
    0 = \begin{pNiceMatrix}[baseline=2]
        0 & 2 \
        0 & 2 \
        0 & 2 \
    \end{pNiceMatrix}\quad
    0 = \begin{bNiceMatrix}[baseline=2]
        0 & 3 \
        0 & 3 \
        0 & 3 \
    \end{bNiceMatrix}
\end{equation}
\end{document}

As usual with nicematrix, you need several compilations.

Answer 3

This is not a real answer, but only a possible \patchcmd way to patch the (end of) LaTeX3 environment NiceArrayWithDelims, in order to shorten the corresponding part of the "complete code" example from @RuixiZhang's answer.

\makeatletter
\ExplSyntaxOn
% patch \cs{environment NiceArrayWidthDelims end aux }
\char_set_catcode_space:n {32} % restore catcode of space char
\expandafter\patchcmd
  \csname environment~ NiceArrayWithDelims~ end~ aux~ \endcsname
  {\skip_vertical:N -\l_tmpa_dim}
  {\skip_vertical:n {-\l_tmpa_dim -\arrayrulewidth }}
  {}{\fail}
\expandafter\patchcmd
  \csname environment~ NiceArrayWithDelims~ end~ aux~ \endcsname
  {\skip_vertical:N -\l_tmpb_dim }
  {\skip_vertical:n {-\l_tmpb_dim +\arrayrulewidth }}
  {}{\fail}
\ExplSyntaxOff
\makeatother