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

Rick Yorgason rick at firefang.com
Mon Nov 5 05:41:32 UTC 2018


If it's helpful, I've created a gitlab branch here: 
https://gitlab.freedesktop.org/Skrapion/cairo/commit/9c81cdd255cd2553ff99cf11e086f905d577e4a0

Thanks for the reviews! Let me know if you would prefer for this to be 
based on just the CTM instead of both the CTM and device pixel.

-Rick-

On 2018-10-29 13:29, Rick Yorgason wrote:
>
> I think it makes sense to do both, but if we're only going to have one 
> check, it should be the CTM, like i originally had.
>
> If we only test the device unit, then we run into weird scenarios. 
> Let's say 1 device pixel is 0.3 CTM units. That means anything from 
> [0-0.3] units *will* draw as a hairline, but anything from (0.3-1) 
> will get rounded to 0 and won't draw at all. (Remember, ExtCreatePen 
> takes the pen width as an integer.) So 0.3 and 1.0 draw, but 0.5 doesn't.
>
> -Rick-
>
>
> On 2018-10-29 10:12, Bill Spitzak wrote:
>> 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 
>> <mailto: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 <mailto: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>
>>>>         >>> <mailto: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>
>>>>         >>>      <mailto: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>
>>>>         <mailto:cairo at cairographics.org
>>>>         <mailto:cairo at cairographics.org>>
>>>>         >>> https://lists.cairographics.org/mailman/listinfo/cairo
>>>>         >>>
>>>>         >>
>>>>         >>
>>>>
>>>>         -- 
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>>>>         https://lists.cairographics.org/mailman/listinfo/cairo
>>>>
>>>
>>>
>>>
>>
>

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