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  #11  
Old 03-17-2016, 03:13 PM
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zubie zubie is offline
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Hmmm, the thing about "upgrading" the scanner being at the root makes me wonder about whether the scanner is indeed the problem. Have you scanned some ruled graph paper at a specified dpi and then compared it against the measured file properties on GIMP: e.g. scan 1 inch square at 300 dpi sb a 300x300pixel square on the final image. If not, then you may have found a certain optical error in the way the scanner works (could be an aspect of newer "cheaper" scanners). I can't think of ever working from a scanner in a way that matters (such as parts) so I haven't noticed a particular problem from my scanner. Since scanners are not that sophisticated optically (a kind of "contact" print) I wouldn't be surprised that there may be accuracy issues on home scanners I haven't noticed before.

If the scanner is doing the distortion, is it repeatable? If so the fix might be to just re-scale the image by a corrective amount. Ugly fix, but probably workable.
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  #12  
Old 03-18-2016, 11:20 AM
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whulsey whulsey is offline
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Zubie probably has the 'best' solution. When I used to work in pre-press we found that most scanners has a slight distortion in one axis. After much discussion it was finally decided that it had to do with the optical properties of the surface glass and the lens getting into all kinds of big words like 'achromatic' and optical dispersion. So we as zubie said figured out the correction and used it. Like on my set up here at home at 100% width I need to set the width at around 98% to get it to print out close to correct. Pain in the butt, but having lenses corrected for it was why the lenses on our big cameras to shot the negs for printing plates (remember all that antique stuff) were around $4K.
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  #13  
Old 03-18-2016, 01:58 PM
Ted60 Ted60 is offline
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What I have resolved to do ... on my own drawings ... I'm just gonna draw a straight line 100mm long close to my image ... then I'll scan the drawing, save as required, and print out a test. Then measure the line and make adjustments (scale) as necessary.

I will revisit this issue down the road. Yeah ... its the poke and hope method for now. I was hoping for an easy fix but right now I am too frustrated and flat tired of messing around with these dang confusers ... and I am really wanting to build that Curtiss.

Thanks for all the help guys.

T
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  #14  
Old 03-18-2016, 04:13 PM
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whulsey whulsey is offline
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As a suggestion draw 2 lines in one corner from a zero point were you have the scale in both directions.
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  #15  
Old 03-20-2016, 12:12 PM
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zubie zubie is offline
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I would suggest you use a large sheet of graph paper instead as it allows you to easily correct for angular distortion, either because the sheet is not placed properly or due to an offset in the scanner. A large sheet also allows you to check over short or larger areas up to the order of 200mm where it would be more apparent and measurable with greater precision (A4 21.0 x 29.7cm, Letter 21.59 x 27.94cm). Recall that the error may differ along the scanned image's x and y axis.

Also if you are using GIMP, take advantage of the selection tool to check the image properties. With rectangular selection, as you select the rectangle the window indicates the size of the selection relative to the start point in the active ruler values (pixels, in, cm, etc). I forget if Gimp has an auto align feature or not. If not using the same rectangular selection tool over the longest vertical line so that it creates a diagonal, the angle of rotation is the arcTan of the short length over the long length (IIRC @=tan-1(opp/adj) also typically opp is the short length unless you just slapped the sheet any old way). Whether the angle of rotation is pos or neg depends which dir it's off.
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http://constantvariation.blogspot.com/
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