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Sketch picture sizing

Question asked by Eric Snyder on Sep 14, 2009
Latest reply on Mar 13, 2011 by Paul Grothouse

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I am looking to solve one of the most basic and myterious questions that man has ever asked...when I import a sketch picture sizing seems totally random. I don't see any correlation to DPI and the originial size the picture starts out in Solidworks.

If I have a 1,200 x 1,200 DPI photo and place it in a sketch it turns out to be sized to something like 60" x 60". What's up with this?

All kidding aside...does anybody know how sketch pictures are sized? Is there a way to make sense of this so that I can get my starting picture sized properly before sketching? Currently I bring in the picture, sketch a box over a known element, measure and rescale the picture. Clumsy.

Any wisdom?

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Kelvin Lamport Sep 14, 2009 11:19 AM
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Re-sizing by DPI does affect pixels affects the image but why bother? Just resize to suit either by dragging a corner, or by using the size fields in the manager.

Message was edited by: Kelvin Lamport
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- Eric Snyder @ Kelvin Lamport on Sep 14, 2009 10:51 AM
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  Yes. That is what I do now. It seems inexact. I don't see any property that calls out DPI. Is there one?
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  - Kelvin Lamport @ Eric Snyder on Sep 14, 2009 11:10 AM
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    DPI is not set from within SW, just the physical dimensions.
    
    How exact do you need the picture? The Sketch Picture manager can be used to position and resize the picture to the same precision as regular geometry.
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    - Eric Snyder @ Kelvin Lamport on Sep 15, 2009 9:46 AM
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      Thanks for the answers. I think I was not specific enough.
      
      I am frequently working with images created from software like Adobe Illustrator. In Illustrator a person may create a feature like a box that is 1" x 3" and then output an image at 300dpi. Every image I have imported in SW ends up being way larger than the original. Actual examples:
      
      For instance. I have three different images:
      A JPG 1,601 pixels wide ended up importing at 66.22047244" wide. 1601 / 66.22047244 = 24.1768
      A JPG 222 pixels wide ended up importing at 11.14173228" wide.222 / 11.14173228 = 19.92509
      A BMP 804 pixels wide ended up importing at 31.65354331" wide.804 / 31.65354331 = 25.4
      
      Each one of these has a different ratio between the pixels and the final width. The last one looks like it was metric in some way. What I was hoping for was some consistent logic that I could make sense out of instead of guessing. I know that I can resize images by width or height and I can key in a percentage as well. When I am trying to match my sketching and features with something that was created by a different artist halfway around the world this guessing procedure creates problems and seems very not-detailed and un-engineering like.
      
      I supploe that I could take the DPI of the image, determine what width the image should be. Place the image and take the final measurement. Calculate the ratio and then rescale it from there. This is a pain when most graphics software respects the DPI data contained in the graphics file.
      
      Does this make more sense?
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      - Lenny Bucholz @ Eric Snyder on Sep 17, 2009 7:17 PM
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        If you are getting files from adobe...have the person save you a DWG file of it and it will come in as a drawing. file open the DWG either into an SW drawing at a 1 to 1 scale or you can open into a sketch of a part also.
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        Eric Snyder @ Lenny Bucholz on Sep 18, 2009 9:05 AM
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        Yep, agreed. Unfortunately... I don't always haver control of the source of the file.
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        Dale Dunn @ Lenny Bucholz on Sep 18, 2009 11:18 AM
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        That's a possible workaround, but the basic problem is that moving and sizing sketch pictures is painfully primitive. If I'm not mistaken, Inventor has the ability to apply dimensions to the border of a sketch image, maybe more. Myself, I'd like to be able to dimension to pixels (and apply coincident constraints) in the image to position and size it (and sketch over it!). That would make it dead easy. I believe I submitted an enhancement request for this quite a while ago. Half a decade ago, I think. Apparently not enough users complain about this problem.
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        Lenny Bucholz @ Dale Dunn on Sep 18, 2009 11:45 AM
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        all the picture in a sketch was really for was to get a quick, size, shape and form of what you want to make. it was made so pictures can be put on sketches, if you take a pic of something it could be skewed and no mater what the pixels are it still woulded be right.
        
        what i do is draw a line to a know dimention, the i pull the corner to get close to the dim, the move the pic to the correct position and tweek the size.
        you must have some sort of print out with size from a part or customer, then your sketching hard lines it in. it was a tool for Industrial Designers not the engineer, you know close enough and pretty . i use this alot and it could be better, but i use correl draw to get the edges and save out dwgs and use the jpg's and get it done.
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Kelvin Lamport Sep 14, 2009 11:15 AM
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To answer your original question see http://auctionrepair.com/pixels.html
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Peter Hunstein Sep 24, 2009 9:51 AM
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This is the way I do it (not my video)

http://www.youtube.com/watch?v=Mt3o6aZnniA

I do a bit of this with photos of parts and have just accepted that the imported size is going to be random. I try to have the photo taken with a ruler in the image so that I can get a size reference just from sketching a couple of lines over the ruler markings, measuring and scale using the same method in this video. I find that using 3 point arcs sketched over hole features in the original image can produce surprisingly accurate scaling information. Hope this helps.

peter-
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- Eric Snyder @ Peter Hunstein on Sep 24, 2009 10:21 AM
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  Right on. That's currently is how I do it. I agree that the initial picture size is pretty much random. Judging from the responses I received on this post the best way to do this is likely the way we are currently doing it.
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Kieran Choy Sep 25, 2009 3:07 AM
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From the Help file:

"You insert the picture with its (0, 0) coordinate at the sketch origin, an initial size of 1 pixel per 1 mm, and locked aspect ratio."

...which is what I get when I insert a sketch picture too. That's 25.4 DPI, which is your BMP DPI.

Not sure why the other images you tried out were not exactly 25.4 DPI, but I believe it's possible to change the aspect ratio of pixels in an image - although i doubt that's the problem here.

For the record, DPI is independant of image resolution, and is stored in the metadata of the image file. You can change it in programs like Photoshop. This may only apply to JPG's - PNG files don't have a DPI entry in their metadata - this may be the same as BMP. Try saving your JPG's as BMP's and see if they come in at 25.4 DPI.

If that works, your best bet is to save or convert images to a DPI of 25.4 or 254 and then scaling by a factor of 10.
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- Paul Grothouse @ Kieran Choy on Mar 13, 2011 8:40 PM
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  Just a follow up for anyone else in this boat the saving ther image in photoshop at 25.4 DPI works like a charm. I used it to import some old paper hand drawn plan curves, that were very hard to measure.
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