[Drawkit] Transforms / ScaleBy

[Michael Caron]

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Hm. Seems like doing it that way yields the same second bounding box.  
Doesn't matter if I'm using the origin or not:

	NSRect boundingBox = [currentLayer unionOfAllObjectBounds];
	// x=16.0239811, y=15.9680157
         // h=18.1520519, w=18.1520519

	
	// Get the identity transform
	NSAffineTransform* txfm = [NSAffineTransform transform];
	
	// translate to origin
	[txfm translateXBy:-(boundingBox.origin.x) yBy:- 
(boundingBox.origin.y)];
	
	// scale by 1000
	[txfm scaleBy: [[self metadataObjectForKey:@"scaleFactor"]  
doubleValue]];
	
	[currentLayer applyTransformToObjects:txfm];

	NSRect boundingBox2 = [currentLayer unionOfAllObjectBounds];
	// x=25086.7695, y=25030.8047
	// h=26.4765625, w=26.4765625

Any thoughts?

On Apr 7, 2009, at 6:35 PM, Graham Cox wrote:

>
> On 08/04/2009, at 8:45 AM, Michael Caron wrote:
>
>> Hi all,
>>
>> I'm trying to understand the use of Affine Transforms through the  
>> use of DKLayer::applyTransformToObjects. Let me give some context.
>>
>> So I have a Drawing with a grid layer, a guide layer and a drawing  
>> layer. The grid layer is setup to use a centimeter span. I  
>> programatically add some DK objects like a few DKDrawablePaths, a  
>> few DKArcPaths. At the end of adding objects, I get the bounding  
>> box for the layer. Then I create an Affine Transform to scale  
>> everything by 1000 (to go to cm to mm). I'm a bit surprised to find  
>> it doesn't work as I thought.
>>
>> Bounding Box Before Transform:
>> 	x = 16.023.9811
>> 	y = 15.9680157
>> 	w = 18.1520519
>> 	h = 18.1520519
>>
>> Transform:
>> 	NSAffineTransform* txfm = [NSAffineTransform transform];
>> 	[txfm scaleBy: 1000];
>> 	[currentLayer applyTransformToObjects:txfm];
>>
>> Bounding Box After Transform:
>> 	x = 25086.7695
>> 	y = 25030.8047
>> 	w = 26.4765625
>> 	h = 26.4765625
>>
>> I would have expected the size of the bounding box to scale by 1000  
>> if the shapes contained in it scale by 1000. Thus yielding a  
>> bounding box like this:
>>
>> 	x = 16023.9811
>> 	y = 15968.0157
>> 	w = 18152.0519
>> 	h = 18152.0519
>>
>> Am I missing something here? Did the transform take the grid into  
>> consideration when scaling? I expected that it would simply rescale  
>> in quartz.
>>
>
> Firstly, there are 10mm in 1 cm, so surely you only want to scale by  
> 10?
>
> Secondly, transforms affect the coordinate system as a whole, so you  
> must always be mindful of where the origin is and how objects are  
> located with respect to it. Just scaling without taking that into  
> account is rarely enough. In this case you probably would need to  
> treat the top, left of your original bounding box as the origin so  
> that the objects are all repositioned and sized relative to that  
> point. Then you should find that the resulting bounding box ends up  
> as you expected. This involves:
>
> 1. create an empty (identity) transform
> 2. translate by -x, -y to the origin.
> 3. scale by 10 (or whatever)
> 4. translate by x, y to undo step 1.
> 5. apply the transform.
>
> The -applyTransformToObjects: method doesn't take into account the  
> grid. In fact, the grid really has very little involvement with the  
> internal structure of DK. What it does is provide a reference grid  
> that is meaningful to the user, so that sizes and positions can be  
> reported/displayed relative to that grid. Internally all drawing is  
> done using native Quartz units, always. Therefore you could simply  
> recalibrate the grid to convert objects from mm to cm. They wouldn't  
> need to move or change size, you just call them something else. The  
> view zoom factor can be changed to make them appear larger.
>
> --Graham
>
>
>
>
>
>
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Michael
- ------------------------------------------------
michael.r.caron at gmail.com
http://techrad.wordpress.com
http://mrcaron.wordpress.com

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