[Solved] Scaling a TIFF

[hauke]

I am working on partly re-drawing the scale of an analogue voltmeter.

Apparently, the scanner's dpi value is not to be trusted, so CADintosh grew a scaling feature. There are two issues with it:

The fine handbook does not really explain how the scaling factor is to be calculated (what CI thinks divided by the real size), how CI will apply it, and to what. You want to re-scale before drawing around the TIFF.

CI assumes it can scale both dimensions by the same factor (i.e. square pixels), which turns out not to be true. The scanner's horizontal scale is determined by the ccd, the vertical scale by the stepper motor, and the two may well be different. Specifically, for the Epson V100 I used, the TIFF's horizontal size is roughly 2% larger and its vertical size is 2% smaller than the physical size. I guess I need to re-scale both dimensions of the TIFF in GC before importing?

As a side note, would it be possible to detect edges in the TIFF, to make measuring easier? And how about making the scaling option sticky (tool-like), so I don't have to re-select it every time during my repeated attempts to get it right?

Cheerio,
Hauke

[forum_adm]

Dear Hauke,

CADintosh does read the ppi/dpi value from the TIF.

The value is used to calculate the real size the following way:
image width / ppi = width in inch
image height / ppi = height in inch

image width / ppi / 25.4 = width in mm
image height / ppi / 25.4 = height in mm

Thorsten

[hauke]

I understand that CI uses the TIFF's dpi parameter.

It's just that your average scanner (I have an old HP Scanjet 4c here, maybe that is better behaved) appears to scale rather generously. For my application - a label that matches the original scale's size and mounting holes - 2% tolerance is too much.

And since CI does offer setting a scaling factor (which needs explaining in the fine manual), why not offer a per-dimension factor?

[forum_adm]

We add that to the todo list.

[hauke]

FTR: I did fire up the old HP ScanJet 4c, scanned two orthogonally laid out steel rulers, and imported the scan into CI.

For a ruler reading of 120mm the y dimension (stepper) gave me 120,0 mm, the x dimension (ccd) gave me 119,9mm - well within tolerance for freehand positioning, I'd say.

So there are scanners, and there are good scanners, and you need to check what you are using.

[forum_adm]

Yes, I suggest to use a current scanner with square pixels.
You can use apps like our GraphicConverter for non-proportional scaling of images. So, the pixels get square.

Also, we fixed an issue with the Edit > Calibrate TIFF dialog in CADintosh.

Just download the latest BETA:
http://www.lemkesoft.org/beta.html