[Drawkit] Fix for aliasing of drawing edges

[Graham Cox]

I'm aware of the 0.5 pixel "problem", but I chose not to attempt to  
compensate for it, instead deciding to go for accuracy over crispness.

That said, It definitely does look a lot sharper. One problem with the  
code below is that it causes a big problem when the view is scrolled  
when it's zoomed in to large scale. Not sure why at the moment, but  
it's so bad I can't recommend including this at present. I think the  
update area marking needs to be similarly offset otherwise there is a  
small amount of overlap that doesn't get repainted when scrolling.

Regarding offsetting the grid alignment, I'll look into it, but one  
thing I do want to avoid is the appearance of uneven spacing between  
grid lines by forcing them to integer co-ordinates, and any  
misalignment of objects visually between the grid "real" position and  
the apparent position of the lines. This might not turn out to be a  
problem, just thinking aloud at this stage.


cheers, Graham



On 20 May 2008, at 9:03 am, Brad Larson wrote:

> I noticed that the borders of the drawing, along with the edges of  
> objects, were being aliased (one-pixel-wide black lines were drawn  
> as two-pixel-wide grey lines).  The Quartz drawing model is a little  
> odd in that integer coordinates are considered to be between pixels,  
> so you need to shift your drawing space by a half pixel in X and Y  
> to get things to line up correctly.
>
> By changing
>
> if([NSGraphicsContext currentContextDrawingToScreen])
> {
> 	[[self paperColour] set];
> 	NSRectFill( rect );
> }
>
> to be
>
> if([NSGraphicsContext currentContextDrawingToScreen])
> {
> 	NSAffineTransform* pixelShiftToAdjustForQuartzCoordinates =  
> [NSAffineTransform transform];
> 	[pixelShiftToAdjustForQuartzCoordinates translateXBy:0.5 yBy:0.5];
> 	[pixelShiftToAdjustForQuartzCoordinates concat];
> 		
> 	[[self paperColour] set];
> 	NSRectFill( rect );
> }
>
> in - (void) drawRect:inView: within DKDrawing.m, you can remove this  
> aliasing.  There is still some aliasing with the background grids  
> and objects drawn on them, due to their drawing coordinates not  
> mapping to integer values.  In DrawDemo, you'll need to turn off  
> grid snapping to see how much sharper rectangles and the like are  
> with this setting, because otherwise they'll snap to the non-integer  
> grid locations.
>
> A go-between function, template, or method might be needed to  
> floor() or ceil() the x and y values of grids and drawable objects  
> for display to the screen.  Apple has some tips on this at http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaDrawingGuide/Transforms/chapter_4_section_6.html 
>  .
>
>
> ______________________
> Brad Larson
> SonoPlot, Inc.
> 3030 Laura Lane, Suite 120
> Middleton, WI 53562
>
>
>
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