[cairo] line width = 0.0 ?
spitzak at d2.com
Mon Jan 9 12:53:47 PST 2006
Keith Packard wrote:
> On Sat, 2006-01-07 at 19:01 +0100, Christian Krause wrote:
>>1. What is the meaning of a line width = 0.0? Should the line be visible?
> It doesn't draw anything. Because cairo can usefully draw sub
> pixel-width lines, and because it has an affine transform that affects
> the visible line width, it really can't do anything different; you'd
> have lines 'underflow' to 0.0 line width and suddenly become visible
As defined by Cairo, 0.0 is invisible.
However underflow is *not* a problem or excuse for this. It would be
trivial to remember whether the number passed in was non-zero, no matter
what happens to the transform. In fact I expect an internal "line width
is zero" switch would be the actual implementation. The main reason for
using 0.0 linewidth to trigger it is that it avoids adding another call
to Cairo, and for compatability with every other graphics API I have
>>2. How can the following achieved in cairo (as e.g. required in the
>>"Draw the thinnest line which can be rendered in the current context"?
> So, if you're interested in drawing Postscript '0 width' lines, you
> might consider just using 1/300th of an inch as the nominal width. These
> will still be faintly visible on the X screen at 1-1 resolution, and
> when zoomed will appear with a reasonable relative width.
No this is not what is wanted. The resulting lines should be the same no
matter what the current transformation is. If you scaled by .5, these
72/300 lines would draw 1/2 as bright. We need lines that draw exactly
the same (and are thus visible) no matter what the transform is.
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