Posted by johnfound on Dec 20, 2024; 5:05pm
URL: https://forum.librecad.org/Lines-pattern-scaling-tp5725741p5725845.html

dxli wrote

1. in rs_linetypepattern.cpp, it is stated the patterns are defined in pixels. I few we may have to redefine this one. Better to make LibreCAD patterns defined in physical distances, like mm/inches. By defining line patterns in display/rendering space allows zooming in/out, with roughly the same line pattern appearance;

Actually, the patterns are not defined in pixels and I think they never was. In the official versions, they are converted to pixels by multiplying with some constant and displayed this way.

In my version, they are interpreted as a millimeters and displayed this way.

dxli wrote

2. Qt line patterns are defined in line widths, and line width is taken as 1 pixels, if defined to be 0;
3. If we define LibreCAD line patterns in mm,
    p_Qt = p_LC * dpmm/(lineWidth)

with p_Qt and p_LC as the pattern values for QPen and LibreCAD (rs_linetypepattern), dpmm as the standard dots per mm, and lineWidth the line width in pixels.

This formula is not working in the current implementation, because there dpmm is simply a constant, somehow related to the screen/paper size. For correct work, we need dpmm = (pixes_size_on_the_screen/object_mm_size). But if so, then for some line dpmm = lineWidth_px/lineWidth_mm and then the formula becomes:

p_Qt = p_LC_mm/lineWidth_mm

My changes, provide exactly this - correct conversion from mm (in rs_linetypepattern.cpp) to relative units, needed by Qt.
Here is a link to rs_painter.cpp/ RS_Painter::SetPen() in my version:  

The conversion

How the k constant is computed

And you can check it simply by pulling the recent commit from my working repo: jf_fixes branch and test it.

If you don't want to mess with fossil, here is a download link for the source: LibreCad_jf_fixes.tar.gz