[cairo] Patch to allow Cairo-based software to print to laser cutters on Windows

Bill Spitzak spitzak at gmail.com
Mon Oct 29 17:12:20 UTC 2018 

Seems like it would be better to consistently use one device unit rather
than the maximum of one device unit and one unit in the CTM.

The zero stuff is a real pain but I suspect trying to fix this in Cairo
(and also support hairlines on other output devices) will be a lot of work.

On Fri, Oct 26, 2018 at 7:10 PM Rick Yorgason <rick at firefang.com> wrote:

> Apologies for the delay. Here's the promised updated patch.
>
> The difference is now it considers the minimum size of both the canvas and
> the printer instead of just the canvas, zero-width strokes count as
> hairlines (although Cairo still seems to be culling strokes from getting to
> this function — that's a separate bug) and the documentation was updated
> for clarity.
>
> Cheers,
>
> -Rick-
>
> On 2018-10-19 12:12, Rick Yorgason wrote:
>
> 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 <rick at firefang.com> 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 <rick at firefang.com
>> >>> <mailto:rick at firefang.com>> 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 <
>> cworth at cworth.org
>> >>>      <mailto:cworth at cworth.org>> 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
>> >>>      cairo at cairographics.org <mailto:cairo at cairographics.org>
>> >>>      https://lists.cairographics.org/mailman/listinfo/cairo
>> >>>
>> >>
>> >>
>>
>> --
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>> cairo at cairographics.org
>> https://lists.cairographics.org/mailman/listinfo/cairo
>
>
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