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LMax99
Participating Frequently
Feb 15, 2022 1:52 PM UTC
Question

Recalculate line width with CTM

Good day,

 

Context: I am rewriting content acquiring mechanism in Adobe Acrobat Core API.

I am having trouble recalculating line width with CTM. How it should be done?

In PDF Refernce:

The effect produced in device space depends on the current transformation matrix (CTM) in effect at the time the path is stroked. If the CTM specifies scaling by different factors in the horizontal and vertical dimensions, the thickness of stroked lines in device space shall vary according to their orientation.

As I understand, when I encounter w operator, I should only set operands fixed value to graphic state attribute line width and just before setting current path's graphic state (PDEElementSetGState) I should recalculate line width?
I tried this approach first:

 

		ASFixedRect r;
		r.bottom = fixedZero;
		r.left = fixedZero;
		r.right = graphic_state.lineWidth;	
		r.top = graphic_state.lineWidth;		
		ASFixedMatrixTransformRect(&r, &CTM, &r);
		double x1 = ASFixedToFloat(r.right - r.left);
		double y1 = ASFixedToFloat(r.top - r.bottom);
		double width = sqrt((x1 * x1 + y1 * y1) / 2);
		graphic_state.lineWidth = ASFloatToFixed(width);

 

However, I am not getting the correct  (but pretty close) results when using this approach. Maybe anyone has any idea how can i approach this?
Thank you in advance,

Cheers

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10 replies

Test Screen Name
Legend
Feb 15, 2022 3:16 PM UTC

In general there is no line width in device space. There is only a line width in  the (common and normal) case where the CTM has equal X and Y scaling. In general a line width is used as follows:

- a stroked line is contructed in user space, of the required width

- the shape of the line is transformed to device space

- adjustments are made for subjective improvement (undocumented and outside the scope of the standard); this may include pixel snapping, thin line removal or widening, antialiasing and other effects

- the transformed shape is rendered to the device

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MikelKlink
MikelKlink
Participating Frequently
Feb 15, 2022 4:51 PM UTC

To illustrate what it means that there is no line width in device space as @Test Screen Name said - this is a curve (well, a pair of curves) drawn without changing the width, merely distorted by transformation:

 

The content stream:

5 0 0 1 25 25 cm
50 25 m
50 38.81 38.81 50 25 50 c
11.19 50 0 38.81 0 25 c
S

 

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LMax99Author
Participating Frequently
Feb 17, 2022 6:47 AM UTC

I think there was a misunderstanding. I am getting w operand (line width) in user space units. I must process all operators in order to collect all content correctly from page data stream. I am having problem in recalculating line with just before stroking a path and adding it into content.

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