Question

I’m new to the CGAffine world. Anyone know why my transformation doesn’t work/take ?

1. I have a valid PDFContext created with UIGraphicsBeginPDFContextToData and the following current matrix:

   CGAffineTransform curMat = CGContextGetCTM(context);
   NSLog (@"current context matrix: %f %f %f %f %f %f", curMat.a, curMat.b, curMat.c, curMat.d, curMat.tx, curMat.ty);
   

2. NSLog values:

   current matrix: 1.000000 0.000000 -0.000000 -1.000000 0.000000 792.000000

3. I create the transformation matrix with:

   CGAffineTransform xform = CGAffineTransformMake(a, b, c, d, tx, ty);
   

4. NSLog values:

   transform matrix: 0.062500 0.000000 0.000000 0.062500 0.000000 0.000000

5. I get the my rect from:

   CGRect pdfBounds = UIGraphicsGetPDFContextBounds();
   

6. NSLog values:

   pdfBounds: 0.000000 0.000000 612.000000 792.000000

7. I apply the transformation matrix with:

    CGRectApplyAffineTransform (pdfBounds, xform);
   

8. Then I re-check the current matrix value with:

   CGAffineTransform curMat2 = CGContextGetCTM(context);
   NSLog (@"current context matrix after transformation: %f %f %f %f %f %f", curMat2.a, curMat2.b, curMat2.c, curMat2.d, curMat2.tx, curMat2.ty)
   

9. and the NSLog values are the same as at the beginning.

   current context matrix after transformation: 1.000000 0.000000 -0.000000 -1.000000 0.000000 792.000000

What am I missing ? Why is the transformation not happening/taking ? Thank you.

Answer 1

You are ignoring the result of the applied transformation and you are also not applying any transformation to the context.

Step 7 should be:

CGContextConcatCTM(context, xform);