[OpenJDK 2D-Dev] X11 uniform scaled wide lines and dashed lines; STROKE_CONTROL in Pisces

[Denis Lila]

Hello Jim.

Thanks for that. I'll get to work on implementing it.

One thing though, about normalizing the control points of bezier
curves: pisces never gets any bezier curves as input. It only gets
lines that are the product of flattening bezier curves.

Pisces receives its input from flattening path iterators which get it
from other path iterators. Of course we can't require these to send out
normalized points. In order to normalize the control points we need to
be able to look at the bezier curves in Pisces, so we can't just take
all the input from the flattener. However, pisces can't handle curves
(yet, hopefully), so after the normalization, they must be flattened, and
this is the problem. I think it's a pretty good idea to do this by 
storing the input form the iterator into pisces (after normalization), 
creating a nameless path iterator that just iterates through all that,
and using this iterator to create a flattening iterator, which then
is used as before.

Does anyone have any other ideas?

Thank you,
Denis.


----- "Jim Graham" <james.graham at oracle.com> wrote:

> For AA this is exactly what we do (round to nearest pixel centers for
> 
> strokes).  Note that this is done prior to any line widening code is 
> executed.
> 
> For non-AA we normalize coordinates to, I believe the (0.25, 0.25) 
> sub-pixel location.  This is so that the transitions between widening
> of 
> lines occurs evenly (particularly for horizontal and vertical wide 
> lines).  If you round to pixel edges then you have the following 
> progression (note that the line width grows by half on either side of
> 
> the original geometry so you have to consider the "line widths" where
> 
> you encounter the pixel centers to your left and right (or above and 
> below) which govern when that column (or row) of pixels first turns
> on):
> 
> width 0.00 => 0.99      nothing drawn (except we kludge this)
> width 1.00 => 1.00      1 pixel wide (col to left turns on)
> width 1.01 => 2.99      2 pixels wide (col to right turns on)
> width 3.00 => 3.00      3 pixels wide (etc.)
> width 3.01 => 4.99      4 pixels wide
> 
> Note that it is nearly impossible to get an odd-width line.  You 
> basically have to have exactly an integer width to get an odd-width 
> line.  This is because at the odd widths you reach the "half pixel" 
> locations on both sides of the line at the same time.  Due to the 
> "half-open" insideness rules only one of the pixels will be chosen to
> be 
> inside this path.  Just below these sizes and you fail to hit either 
> pixel center.  Just at the integer size you reach both pixel centers
> at 
> the same time.  Just slightly larger than that width and now you've 
> fully enclosed both pixel centers and the line width has to increase
> by 
> nearly 2.0 until you reach the next pixel centers.
> 
> (The kludge I talk about above is that we set a minimum pen width so 
> that we never fail to draw a line even if the line width is set to
> 0.0, 
> but the above table was a theoretical description of the absolute
> rules.)
> 
> If we rounded them to pixel centers, then the transitions look like
> this:
> 
> width 0.00 => 0.00      nothing drawn (modulo kludge)
> width 0.01 => 1.99      1 pixel wide (column you are in turns on)
> width 2.00 => 2.00      2 pixels wide (column to left turns on)
> width 2.01 => 3.99      3 pixels wide (column to right turns on)
> width 4.00 => 4.00      4 pixels wide (etc.)
> width 4.01 => 5.99      5 pixels wide
> 
> We have a similar effect as above, but biased towards making even line
> 
> widths harder.
> 
> So, by locating lines at (0.25, 0.25) subpixel location we end up with
> a 
>   very even progression:
> 
> width 0.00 => 0.50      nothing drawn (modulo kludge)
> width 0.51 => 1.50      1 pixel wide (column you are in turns on)
> width 1.51 => 2.50      2 pixel wide (column to left gets added)
> width 2.51 => 3.50      3 pixel wide (column to right gets added)
> width 3.51 => 4.50      4 pixel wide (etc.)
> 
> This gives us nice even and gradual widening of the lines as we
> increase 
> the line width by sub-pixel amounts and the line widths are fairly 
> stable around integer widths.
> 
> Also, note that we don't say "when stroking" as you might want to 
> normalize both strokes and fills so that they continue to match.  I 
> believe that we normalize both strokes and fills for non-AA and we
> only 
> normalize strokes for AA (and leave AA fills as "pure").  AA is less 
> problematic with respect to creating gaps if your stroke and fill 
> normalization are not consistent.
> 
> The rounding equations are along the lines of:
> 
> 	v = Math.floor(v + rval) + aval;
> 
> For center of pixel you use (rval=0.0, aval=0.5)
> For 0.25,0.25 rounding use  (rval=0.25, aval=0.25)
> For edge of pixel you use   (rval=0.5, aval=0.0)
> 
> Also, we came up with an interesting way of adjusting the control
> points 
> of quads and cubics if we adjusted their end points, but I don't know
> if 
> what we did was really the best idea.  For quads we adjust the control
> 
> point by the average of the adjustments that we applied to its 2 end 
> points.  For cubics, we move the first control point by the same
> amount 
> as we moved the starting endpoint and the second control point by the
> 
> amount we moved the final endpoint.  The jury is out on whether that
> is 
> the most aesthetic technique...
> 
> 			...jim
> 
> Denis Lila wrote:
> > Regarding VALUE_STROKE_NORMALIZE the API says:
> >                 Stroke normalization control hint value -- geometry
> should
> >                 be normalized to improve uniformity or spacing of
> lines and
> >                 overall aesthetics. Note that different
> normalization 
> >                 algorithms may be more successful than others for
> given 
> >                 input paths. 
> > 
> > I can only think of one example where VALUE_STROKE_NORMALIZE makes a
> visible
> > difference between the closed source implementation and OpenJDK:
> > when drawing anti-aliased horizontal or vertical lines of width 1,
> Pisces 
> > draws a 2 pixel wide line with half intensity (because integer
> coordinates
> > are between pixels). Sun's jdk with VALUE_SROKE_NORMALIZE turned on
> draws
> > a 1 pixel line with full intensity. This could to achieved by just
> > checking for normalization and rounding coordinates to the nearest
> half 
> > pixel, but this solution seems too simple, and I'm not sure whether
> I'm missing
> > anything. It would also probably cause problems when drawing
> anti-aliased 
> > short lines (which is done when drawing any sort of curve)
> > Unless, of course, this rounding was restricted to just horizontal
> and 
> > vertical lines.
> > 
> > Regards,
> > Denis.