OK - I think the Matrix code is wrong in getting the shear values compared to
the description in the PDF spec.
eg. using a text matrix to produce an 'italic' text
AffineTransform transform = AffineTransform.getShearInstance(0, 0.5);
contentStream.setTextMatrix(transform);
produces the same result as
contentStream.setTextMatrix(1, 0, 0.5, 1, 0, 0);
Which is inline with the spec as c is the sy value.
Now getShearX, getShearY and createAffineTransform in Matrix are wrong IMHO as
they have the extraction flipped between sx and sy.
e.g. printing the values of
AffineTransform transform = AffineTransform.getShearInstance(0, 0.5);
produces
AffineTransform[[1.0, 0.0, 0.0], [0.5, 1.0, 0.0]]
where
printing the values of
Matrix matrix = new Matrix();
matrix.setValue(1, 0, 0.5f);
produces
AffineTransform[[1.0, 0.5, 0.0], [0.0, 1.0, 0.0]]
which is wrong as it should produce AffineTransform[[1.0, 0.0, 0.0], [0.5, 1.0,
0.0]]
BR
Maruan
Am 27.12.2014 um 23:47 schrieb John Hewson <j...@jahewson.com>:
> Yes, the pure matrix code seems to work fine. As do the examples which Tilman
> mentioned. I can’t find any issue with the code itself. However, we make use
> the following Matrix method:
>
> public AffineTransform createAffineTransform()
> {
> AffineTransform retval = new AffineTransform(
> single[0], single[1], // m00 m10 = scaleX shearY
> single[3], single[4], // m01 m11 = shearX scaleY
> single[6], single[7] ); // m02 m12 = tx ty
> return retval;
> }
>
> Recall that single[] is the array:
>
> | sx hy 0 |
> | hx sy 0 |
> | tx ty 1 |
>
> Which is fine, except that shear-x and shear-y are flipped vs. the wording in
> the PDF spec, namely:
>
> "Skew shall be specified by [1 tan a tan b 1 0 0], which skews the x axis by
> an angle a and the y axis by
> an angle b.”
>
> But the AffineTransform constructor uses the following parameters:
>
> "m00 - the X coordinate scaling element of the 3x3 matrix
> m10 - the Y coordinate shearing element of the 3x3 matrix
> m01 - the X coordinate shearing element of the 3x3 matrix
> m11 - the Y coordinate scaling element of the 3x3 matrix
> m02 - the X coordinate translation element of the 3x3 matrix
> m12 - the Y coordinate translation element of the 3x3 matrix"
>
> At this point I’m left thinking that Java and PDF have chosen different
> meanings of “x” and “y” when talking about skew, indeed this appears to be
> the case, as the PDF spec defines the x-coordinate transform on p120 as:
>
> x’ = a*x + c*y + e
>
> while the AffineTransform documentation defines the same transform as:
>
> x' = [ m00*x + m01*y + m02 ]
>
> which might seem different until one realises that m00 = a, m01 = c, m02 = e.
> The only difference appears to be the description that c "skews the y axis”
> in PDF and m01 is "the X coordinate shearing" in AffineTransform. It seems
> like PDF is weird in this regard?
>
> -- John
>
>> On 27 Dec 2014, at 16:51, Maruan Sahyoun <sahy...@fileaffairs.de> wrote:
>>
>> Hi,
>>
>> I tried the following with skew being [1 tan a tan b 1 0 0] according to the
>> spec.
>>
>> Matrix matrix = new Matrix();
>>
>> which gives me [1.0,0.0,0.0,1.0,0.0,0.0]
>>
>> then
>>
>> matrix.setValue(0, 1, 0.5f);
>> matrix.setValue(1, 0, 0.7f);
>>
>> which shall set the tan a and tan b values
>>
>> which gave me
>>
>> [1.0,0.5,0.7,1.0,0.0,0.0]
>>
>> then
>>
>> Matrix matrix2 = new Matrix()
>>
>> again being [1.0,0.0,0.0,1.0,0.0,0.0]
>>
>> then
>>
>> matrix.concatenate(matrix2)
>>
>> resulting in [1.0,0.5,0.7,1.0,0.0,0.0]
>>
>> To me that indicates that the pure matrix class is fine for that case. What
>> are the arguments to the Concatenate operator in your case? Could it be that
>> reading the values is not OK?
>>
>> BR
>>
>> Maruan
>>
>> Am 27.12.2014 um 14:16 schrieb John Hewson <j...@jahewson.com>:
>>
>>> Hi All,
>>>
>>> I’ve been looking at the Matrix class and trying to understand if it has
>>> some problems. The Matrix class
>>> in PDFBox uses the following matrix:
>>>
>>> | sx hy 0 |
>>> | hx sy 0 |
>>> | tx ty 1 |
>>>
>>> In a PDF file this matrix is represented as the 6-element array [a, b, c,
>>> d, e, f] where the meaning of
>>> the values comes from p118 and is [sx, hx, hy, sy, tx, ty]. Note that h is
>>> used to mean shear/skew.
>>>
>>> The Concatenate operator class populates the Matrix as follows:
>>> PDFBox PDF Spec
>>> newMatrix.setValue(0, 0, a.floatValue()); sx a = sx
>>> newMatrix.setValue(0, 1, b.floatValue()); hy c = hx <- Flipped?
>>> newMatrix.setValue(1, 0, c.floatValue()); hx b = hy <- Flipped?
>>> newMatrix.setValue(1, 1, d.floatValue()); sy d = sy
>>> newMatrix.setValue(2, 0, e.floatValue()); tx e = tx
>>> newMatrix.setValue(2, 1, f.floatValue()); ty f = ty
>>>
>>> I’ve annotated what PDFBox does on the left, compared to the PDF spec, the
>>> x and y skew elements
>>> (hx and hy) are flipped in PDFBox. This flip matches the order in which
>>> AWT’s AffineTransform’s
>>> constructor expects its elements but does not match the arrays used in the
>>> PDF spec.
>>>
>>> Are we accidentally flipping skew-x and skew-y in the Concatenate operator?
>>>
>>> -- John
>>>
>>
>
