Re: Patch to allow Cairo-based software to print to laser cutters on Windows

Hairlines are not always 1 pixel wide. For Postscript setting the line
width to 0.0 gives you a hairline that seems to be about 1/150" which is
quite a few pixels on modern printers, and was > 1 even on the first Apple
Laserwriter.

On Fri, Oct 19, 2018 at 6:21 AM Rick Yorgason <[email protected]> wrote:

> It looks like you're right about the device units. In cases where the
> canvas DPI is lower than the device DPI, the "smallest printable line"
> is 1, as in my original patch (because ExtCreatePen only accepts
> integral pen widths), but when the canvas DPI is higher, the "smallest
> printable line" is
> `_cairo_matrix_transformed_circle_major_axis(stroke_ctm_inverse, 1.0)`.
>
> I haven't looked any further into what's culling zero-width lines yet.
> For now, I'm focusing on the "use PS_COSMETIC for smallest printable
> lines" patch, which I believe should be separate from the "allow
> printing zero-width lines" patch.
>
> Tomorrow I'll post a new patch based on the feedback I've received so far.
>
> -Rick-
>
>
> On 2018-10-19 00:56, Adrian Johnson wrote:
> > There were some patches to fix the culling on narrow lines on vector
> > surfaces:
> >
> >
> https://gitlab.freedesktop.org/cairo/cairo/commit/b1192beac7c5b56a8ff356d20af5ebfb65404109
> >
> >
> https://gitlab.freedesktop.org/cairo/cairo/commit/bec8c7508ebc0f69266f9aebe9903539391c519b
> >
> > Those patches should allow zero width lines. I'm not sure what else
> > could be preventing zero width lines from getting through to the win32
> > print surface.
> >
> > You code that sets the cosmetic flag is not checking the line width in
> > device units. The StrokePath() is called in user coordinates so that the
> > line width will be in user coordinates. The reason emitting the path in
> > device space and stroking in user space is demonstrated here
> > https://cairographics.org/tutorial/#L2linewidth
> >
> > To check if the line width is < 1 device unit you would need to do
> > something line in the second commit above to find the line width in user
> > space.
> >
> >
> > On 19/10/18 07:32, Rick Yorgason wrote:
> >> Sure enough, Cairo seems to be culling zero-width lines somewhere, so it
> >> doesn't matter whether or not I check for zero here.
> >>
> >> For now, I propose that I get rid of the zero-check in my patch, so
> >> anything <= 1 device unit is drawn with a PS_COSMETIC pen. This value
> >> gets rounded to an integer, so this approach is consistent with the
> >> definition of a hairline being the smallest thing a printer can print.
> >>
> >> In the long run, it would be nice to track down where Cairo is culling
> >> zero-width lines and allow them through, as I believe postscript and pdf
> >> both treat zero-width lines as hairlines.
> >>
> >> -Rick-
> >>
> >>
> >> On 2018-10-18 12:09, Bill Spitzak wrote:
> >>> I suspect there is code in Cairo that assumes zero-width strokes are
> >>> invisible, so it may be difficult to fix it for only this device.
> >>>
> >>> The main reason for using zero rather than any other number is that it
> >>> survives scaling, which is pretty important for a magic value.
> >>>
> >>> On Thu, Oct 18, 2018 at 11:12 AM Rick Yorgason <[email protected]
> >>> <mailto:[email protected]>> wrote:
> >>>
> >>>      Including zero-width strokes makes sense to me. After all, true
> >>>      hairlines are supposed to act as though they're zero-width.
> >>>
> >>>      The reason I implemented it this way it's because I was worried
> >>>      about two use cases:
> >>>
> >>>      1) Some applications might rely on zero-width lines being
> >>>      invisible. (Imagine an artist who selects all the lines he wants
> >>>      to hide and changes their line width to zero.)
> >>>
> >>>      2) Maybe some CNC machines rely on zero-width lines? PS_COSMETIC
> >>>      lines are always 1 unit wide, so there would be no way to send
> >>>      zero-width lines to the printer any more. This seems unlikely to
> >>>      be a problem, but it's possible.
> >>>
> >>>      (1) can be fixed by the calling application, and (2) isn't likely
> >>>      a real problem, and if we include zero-width strokes it would make
> >>>      it easier for developers to make true zero-width hairlines (since
> >>>      they don't need to know the target device's minimum unit size), so
> >>>      I'm down for it.
> >>>
> >>>      -Rick-
> >>>
> >>>      On October 18, 2018 9:10:07 AM PDT, Carl Worth <[email protected]
> >>>      <mailto:[email protected]>> wrote:
> >>>
> >>>          On Wed, Oct 17 2018, Rick Yorgason wrote:
> >>>
> >>>              With this patch, I can use Inkscape to set my stroke width
> >>>              to 0.001" and it will cut through the material as
> expected.
> >>>
> >>>
> >>>          If we're going to have a magic value here, I think I'd be
> much happier
> >>>          to have 0 be the magic value.
> >>>
> >>>          (I know that your code isn't actually making 0.001" be magic,
> but a
> >>>          whole range of values. I think I would really prefer to have
> that range
> >>>          include rather then exclude 0.0.)
> >>>
> >>>          Does anyone see any reason why a stroke width of 0 shouldn't
> be treated
> >>>          this way?
> >>>
> >>>          I think that would make a much better way to be able to
> document
> >>>          this. ("Use a value of 0 to get a PS_COSMETIC pen which is
> useful when
> >>>          targeting devices such as laser cutters".)
> >>>
> >>>          And speaking of documentation, it seems this patch should
> also be
> >>>          touching up the documentation, such as in
> cairo_set_line_width?
> >>>
> >>>          I know the code is specific to the Windows backend, but I
> think it's
> >>>          reasonable to put a backend-specific note into the general
> documentation
> >>>          in a case like this.
> >>>
> >>>          -Carl
> >>>
> >>>
> >>>      --
> >>>      Sent from my Android device with K-9 Mail. Please excuse my
> brevity.
> >>>      --
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> >>>      [email protected] <mailto:[email protected]>
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> >>>
> >>
> >>
>
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