Cocoa color with alpha weirdness

[John C. Randolph]

"Liz S. Reynolds" wrote:

>
> Code snippet below. It goes in a drawRect method in a subclass of NSView.
>
> The first path fills a rectangle in the right half of the view colored
> transparent blue. the second path fills a rectangle in the bottom half of
> the view colored transparent yellow. Either of these by itself looks just
> like you'd expect.
>
> +------------------+
> | no color | blue |
> |------------------|
> | no color | blue |
> +------------------+
>
> +---------------------+
> | no color | no color |
> |---------------------|
> | yellow | yellow |
> +---------------------+
>
> When both are drawn, they intersect in the bottom right
> quadrant of the view. However, the quartered view looks like this:
>
> +------------------+
> | no color | green |
> |------------------|
> | yellow | blue |
> +------------------+
>
> while I expected this:
>
> +------------------+
> | no color | blue |
> |------------------|
> | yellow | green |
> +------------------+
>
> I'm totally puzzled, any clues?

Liz,

What's tripping you up here is that unlike in Postscript, where the
"fill" operator consumes and destroys the current path, NSBezierPath
doesn't lose any of its state after you fill it. So, the second
sequence of -moveToPoint:/-lineToPoint: messages that you're sending to
the path are just getting appended to the existing path.

>
> // top left bottom and right are args to the function
> // and represent the origin and origin + size in each direction
> NSColor *blue ;
> NSColor *yellow;
>
> NSBezierPath *thePath = [[ NSBezierPath alloc ] init];
> blue = [[NSColor blueColor] colorWithAlphaComponent: 0.5];
> yellow = [[NSColor yellowColor] colorWithAlphaComponent: 0.5];
>
> // down the center
> [ blue set];
> [thePath moveToPoint: NSMakePoint ((right / 2),top)];
> [thePath lineToPoint: NSMakePoint ((right / 2),bottom)];
> // bottom to left edge
> [thePath lineToPoint: NSMakePoint (right,bottom)];
> // bottom left to top left
> [thePath lineToPoint: NSMakePoint (right,top)];
> // top left to top center
> [thePath lineToPoint: NSMakePoint ((right / 2),top)];
> [thePath closePath];
> [thePath fill];

Try adding:

[thePath removeAllPoints]; //here.

>
> [ yellow set];
> // across the center
> [thePath moveToPoint: NSMakePoint (left, ((top - bottom) / 2))];
> [thePath lineToPoint: NSMakePoint (right,((top - bottom) / 2))];
> // center right to bottom right
> [thePath lineToPoint: NSMakePoint (right,bottom)];
> // bottom right to bottom left
> [thePath lineToPoint: NSMakePoint (left,bottom)];
> // up to center left
> [thePath lineToPoint: NSMakePoint (left, (top - bottom) / 2)];
> [thePath closePath];
> [thePath fill];

Also, if you just want to fill in some rectangles, you don't need the
overhead of creating an NSBezierPath. You can just call NSRectFill() or
NSRectFillList(), or send +fillRect: to the NSBezierPath class itself.

HTH,

-jcr

[Liz S. Reynolds]

In article <3F4BD291.32BB9A6D@nospam.idiom.com>,

>
>What's tripping you up here is that unlike in Postscript, where the
>"fill" operator consumes and destroys the current path, NSBezierPath
>doesn't lose any of its state after you fill it. So, the second
>sequence of -moveToPoint:/-lineToPoint: messages that you're sending to
>the path are just getting appended to the existing path.

>> [thePath fill];

>
>Try adding:
>
>[thePath removeAllPoints]; //here.
>

That did it - thanks!

>
>Also, if you just want to fill in some rectangles, you don't need the
>overhead of creating an NSBezierPath. You can just call NSRectFill() or
>NSRectFillList(), or send +fillRect: to the NSBezierPath class itself.

It's just rectangles for practice, more complex shapes later, but thanks
for the tip.

-Liz