On 12.5.2009, at 06:23, Graham Cox wrote:
On 12/05/2009, at 12:44 AM, Sahana A wrote:
We have a feature in our application wherein we have to draw a 1
pixel line. This drawing also varies relative to the zoom factor
of our view. For example
if we are drawing at a zoom factor of 200% we draw 0.5 pixel line.
( basically: 1/zoomfactor width of line; 1 if the zoomfactor <
100 ).
For most of the times we succeed in drawing the 1 pixel line.
However after we change the zoom factor, our drawing fails. Our 1
pixel
line is now drawn in 2 pixels.
For example: On changing the zoom values from say 100% to 300% and
back to 100% again, the single pixel line sometimes draw in 2 pixels.
Even if the stroke width is set to 0.0 or 1.0, the stroked line
will draw 2 pixels.
It's not just a case of setting the line width, but also of placing
the line where you want it. Strokes are placed so that they are
centred on the co-ordinate, so half the line is above and half below
the integral co-ordinate position. This is true even if you suppress
anti-aliasing, which make matters worse since the lightening of the
line is disabled and it'll draw as a solid 2 pixel line.
You need to truncate the line's co-ordinates to a whole number then
add or subtract 0.5 so that the line is drawn on the pixel, not on
the co-ordinate.
This goes into the right direction, but things are a little more
involved. Basically you need to convert to device coordinates first,
to the correct rounding, convert the result back and then draw.
The 10.5-only NSView methods convertPointToBase: and friends are quite
helpful for this.
For more details see Apples "Resolution Independence Guidelines".
Kai
--Graham
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