PCX is a bitmap image format originally used by PC Paintbrush from ZSoft Corporation. Originally, it was a relatively simple 128-byte header + uncompressed bitmap format, but latest versions introduced more complicated palette support and RLE compression.
There's an option to encode 32-bit or 16-bit RGBA pixels, and thus it can potentially include transparency. Theoretically, it's possible to encode resolution or pixel density in the some of the header fields too, but in reality there's no uniform standard for these, so different implementations treat these differently.
PCX format was never made a formal standard. "ZSoft Corporation Technical Reference Manual" for "Image File (.PCX) Format", last updated in 1991, is likely the closest authoritative source.
This page hosts a formal specification of .pcx file format using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
digraph {
rankdir=LR;
node [shape=plaintext];
subgraph cluster__pcx {
label="Pcx";
graph[style=dotted];
pcx__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="hdr_pos">0</TD><TD PORT="hdr_size">128</TD><TD>Header</TD><TD PORT="hdr_type">hdr</TD></TR>
</TABLE>>];
pcx__inst__palette_256 [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="palette_256_pos">(_io.size - 769)</TD><TD PORT="palette_256_size">769</TD><TD>TPalette256</TD><TD PORT="palette_256_type">palette_256</TD></TR>
</TABLE>>];
subgraph cluster__header {
label="Pcx::Header";
graph[style=dotted];
header__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="magic_pos">0</TD><TD PORT="magic_size">1</TD><TD></TD><TD PORT="magic_type">magic</TD></TR>
<TR><TD PORT="version_pos">1</TD><TD PORT="version_size">1</TD><TD>u1→Versions</TD><TD PORT="version_type">version</TD></TR>
<TR><TD PORT="encoding_pos">2</TD><TD PORT="encoding_size">1</TD><TD>u1→Encodings</TD><TD PORT="encoding_type">encoding</TD></TR>
<TR><TD PORT="bits_per_pixel_pos">3</TD><TD PORT="bits_per_pixel_size">1</TD><TD>u1</TD><TD PORT="bits_per_pixel_type">bits_per_pixel</TD></TR>
<TR><TD PORT="img_x_min_pos">4</TD><TD PORT="img_x_min_size">2</TD><TD>u2le</TD><TD PORT="img_x_min_type">img_x_min</TD></TR>
<TR><TD PORT="img_y_min_pos">6</TD><TD PORT="img_y_min_size">2</TD><TD>u2le</TD><TD PORT="img_y_min_type">img_y_min</TD></TR>
<TR><TD PORT="img_x_max_pos">8</TD><TD PORT="img_x_max_size">2</TD><TD>u2le</TD><TD PORT="img_x_max_type">img_x_max</TD></TR>
<TR><TD PORT="img_y_max_pos">10</TD><TD PORT="img_y_max_size">2</TD><TD>u2le</TD><TD PORT="img_y_max_type">img_y_max</TD></TR>
<TR><TD PORT="hdpi_pos">12</TD><TD PORT="hdpi_size">2</TD><TD>u2le</TD><TD PORT="hdpi_type">hdpi</TD></TR>
<TR><TD PORT="vdpi_pos">14</TD><TD PORT="vdpi_size">2</TD><TD>u2le</TD><TD PORT="vdpi_type">vdpi</TD></TR>
<TR><TD PORT="palette_16_pos">16</TD><TD PORT="palette_16_size">48</TD><TD></TD><TD PORT="palette_16_type">palette_16</TD></TR>
<TR><TD PORT="reserved_pos">64</TD><TD PORT="reserved_size">1</TD><TD></TD><TD PORT="reserved_type">reserved</TD></TR>
<TR><TD PORT="num_planes_pos">65</TD><TD PORT="num_planes_size">1</TD><TD>u1</TD><TD PORT="num_planes_type">num_planes</TD></TR>
<TR><TD PORT="bytes_per_line_pos">66</TD><TD PORT="bytes_per_line_size">2</TD><TD>u2le</TD><TD PORT="bytes_per_line_type">bytes_per_line</TD></TR>
<TR><TD PORT="palette_info_pos">68</TD><TD PORT="palette_info_size">2</TD><TD>u2le</TD><TD PORT="palette_info_type">palette_info</TD></TR>
<TR><TD PORT="h_screen_size_pos">70</TD><TD PORT="h_screen_size_size">2</TD><TD>u2le</TD><TD PORT="h_screen_size_type">h_screen_size</TD></TR>
<TR><TD PORT="v_screen_size_pos">72</TD><TD PORT="v_screen_size_size">2</TD><TD>u2le</TD><TD PORT="v_screen_size_type">v_screen_size</TD></TR>
</TABLE>>];
}
subgraph cluster__t_palette_256 {
label="Pcx::TPalette256";
graph[style=dotted];
t_palette_256__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="magic_pos">0</TD><TD PORT="magic_size">1</TD><TD></TD><TD PORT="magic_type">magic</TD></TR>
<TR><TD PORT="colors_pos">1</TD><TD PORT="colors_size">3</TD><TD>Rgb</TD><TD PORT="colors_type">colors</TD></TR>
<TR><TD COLSPAN="4" PORT="colors__repeat">repeat 256 times</TD></TR>
</TABLE>>];
}
subgraph cluster__rgb {
label="Pcx::Rgb";
graph[style=dotted];
rgb__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="r_pos">0</TD><TD PORT="r_size">1</TD><TD>u1</TD><TD PORT="r_type">r</TD></TR>
<TR><TD PORT="g_pos">1</TD><TD PORT="g_size">1</TD><TD>u1</TD><TD PORT="g_type">g</TD></TR>
<TR><TD PORT="b_pos">2</TD><TD PORT="b_size">1</TD><TD>u1</TD><TD PORT="b_type">b</TD></TR>
</TABLE>>];
}
}
pcx__seq:hdr_type -> header__seq [style=bold];
pcx__inst__palette_256:palette_256_type -> t_palette_256__seq [style=bold];
t_palette_256__seq:colors_type -> rgb__seq [style=bold];
}