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

That shouldn't be. See page 675 of the Postscript Language Reference: 
https://www.adobe.com/content/dam/acom/en/devnet/actionscript/articles/PLRM.pdf

> A line width of 0 is acceptable, and is interpreted as the thinnest 
> line that can be rendered at device resolution—1 device pixel wide. 
> However, some devices cannot reproduce 1-pixel lines, and on 
> high-resolution devices, they are nearly invisible.

-Rick-


On 2018-10-19 09:27, Bill Spitzak wrote:
> 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] 
> <mailto:[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]>
>     >>> <mailto:[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]>
>     >>>      <mailto:[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.
>     >>>      --
>     >>>      cairo mailing list
>     >>> [email protected] <mailto:[email protected]>
>     <mailto:[email protected] <mailto:[email protected]>>
>     >>> https://lists.cairographics.org/mailman/listinfo/cairo
>     >>>
>     >>
>     >>
>
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>

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