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

Rick Yorgason rick at firefang.com
Fri Oct 19 13:19:29 UTC 2018

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.


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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