Re: NSAffineTransform & bezier path line widths

FROM : Shamyl Zakariya
DATE : Thu Mar 20 22:24:03 2008

So, I managed to solve it on my own.

The trouble is I was resizing the image and then using 
compositeAtPoint:, when I should have been using drawInRect: fromRect: 
( which is better, anyway )

For some reason using the former approach left some odd transform in 
the drawing context during a window resize, but not during a draw 
operation caused by a call to setNeedsDisplay.



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On Mar 12, 2008, at 12:22 PM, Shamyl Zakariya wrote:
> I'm using an NSAffineTransform to scale and offset drawing a bezier 
> path into a view. The bezier path *vertices* are correctly 
> transformed. The oddity is that the line width sometimes is 
> transformed, and sometimes isn't.
>
> Specifically, if the view is redrawn as a result of [self 
> setNeedsDisplay: YES] the line width is scaled by the affine 
> transform. I.e., if the scaling is 2 and the line width is 2, the 
> line on screen appears to be 4 pixels thick, which I read as correct 
> behavior. However, if the view is redrawn as a result of resizing 
> the window, the line width on screen is 2! And more bafflingly, the 
> lines are still otherwise correct with regards to position. They're 
> just not the right width.
>
> I presume that I'm not grokking something, and perhaps somebody here 
> can point me to some documentation that clarifies this. I *have* 
> read documentation, but there's so much it's possible I overlooked 
> something.
>
> Here's my testing code. In this code I draw an image ( image 
> processing of a slice of voxels, the color marks visitation 
> information, etc ) scaled to fit the view and centered. And over the 
> image I draw a test bezier path. I'm not rendering the actual path 
> data because I'm still developing the algorithm which analyses the 
> voxel slice.
>
> - (void)drawRect:(NSRect)rect
> {
> if ( voxelSlice )
> {
>    [[NSGraphicsContext currentContext] saveGraphicsState];
>    [[NSGraphicsContext currentContext] setImageInterpolation: 
> NSImageInterpolationNone];
>
>    NSRect bounds = [self bounds];
>    NSSize imageSize = [voxelSlice size];
>
>    CGFloat scale = bounds.size.width / imageSize.width;
>    if ( imageSize.height * scale > bounds.size.height )
>    {
>        scale *= bounds.size.height / (imageSize.height * scale);
>    }
>
>    NSSize scaledImageSize = NSMakeSize( imageSize.width * scale, 
> imageSize.height * scale );
>    NSPoint origin = NSMakePoint( bounds.size.width/2 - 
> scaledImageSize.width/2,
>                                  bounds.size.height/2 - 
> scaledImageSize.height/2 );
>
>    [voxelSlice setSize: scaledImageSize];
>    [voxelSlice compositeToPoint: origin 
> operation:NSCompositeSourceOver];
>
>    /*
>        Set up an affine transform for line rendering.
>        After this, we're in the coordinate system of the image.
>    */
>    NSAffineTransform *transform = [NSAffineTransform transform];
>    [transform translateXBy: origin.x yBy: origin.y];
>    [transform scaleBy: scale];
>    [transform concat];
>
>    // dummy code for testing -- just draw an X across the image
>    [[NSColor whiteColor] set];
>    NSBezierPath *test = [NSBezierPath bezierPath];
>    [test setLineWidth: 2];
>    [test moveToPoint: NSZeroPoint];
>    [test lineToPoint: NSMakePoint( imageSize.width, 
> imageSize.height )];
>    [test moveToPoint: NSMakePoint( imageSize.width, 0 )];
>    [test lineToPoint: NSMakePoint( 0, imageSize.height)];
>    [test stroke];
>
>
>    [[NSGraphicsContext currentContext] restoreGraphicsState];
> }
> }
>
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