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ship hull design

Apr 7, 2012, 14:15
Post: #1
ship hull design
Hello everyone,

As a scale modelist and a newcomer to GIMP, I am trying to obtain a profile, face and top view of a miniature ship - the usual way of representing ship hulls, both in scale and real design. In particular, the frames of the bow and aft sections should be shown in a single picture, as the attached image of a schooner shows. I first thought that a simple cut and paste from reproductions of original plans would do the job, but I soon gave up. Should separate layers for each section of the hull be used instead? As I have no experience with layers, I would very much appreciate any advice or assistance in this regard.

Thank you very much.


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Apr 7, 2012, 14:59
Post: #2
RE: ship hull design
Cut-and-paste or duplicating the layer and cropping out what you don't need is equivalent. What kind of problems did you come across with cut and paste? What is your original material (what are you starting with?)

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Apr 7, 2012, 18:01
Post: #3
RE: ship hull design
Thanks for your reply, oftnuts. After having cut one half of a frame section - in this case, it was the ship's bow (face) section - from a .jpg file and saved it to a new file, I cut the aft section from another .jpg file and tried to paste it directly opposite to the bow section, hoping the result would look like the top frames drawing shown in my attached sample. Instead, the second image simply was superimposed on the first one, fully hiding it. I was unable to line them up against one another. I surely must have missed some very basic command.

The .jpg files I used are scanned photographs of the original plans of the ship I am building. Scaling them with GIMP was not a problem. However, I didn't try duplicating layers and cropping out unnecessary parts of the images, as you suggest. That will be the first thing I'll look into and will let you know about the results.

Thank you very much for your help.
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Apr 7, 2012, 20:28
Post: #4
RE: ship hull design
Aw, yes, you would need to crop at least one of the picture along the keel axis.

The other solution is to convert the image to lines overs transparent background. More complicated (it depends a lot on scan quality). If you post a sample I can try to determine the best way to do it.

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Apr 8, 2012, 14:20
Post: #5
RE: ship hull design
Cutting and pasting worked at last, as you can see on the attached file (I am sending it in .jpg format, as the .xcf file exceeds 26 Mb). I cropped out unnecessary parts of the image with the Scissors Select Tool.

Both aft and bow sections are digital photos of the original plan. Could the resulting image be converted to lines over a transparent background or would you need a complete view of both sections?

I am still trying to get the same result with layers.


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Apr 8, 2012, 18:31
Post: #6
RE: ship hull design
Not too hard to do... here is what I get:
[attachment=1067]

(it will likely not display properly on GimpForums (black background added) click above to open 'raw' in your browser, or save and import in Gimp.

Will explain technique later (have to work on tomorrow's lunch cake Smile )

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Apr 8, 2012, 20:10
Post: #7
RE: ship hull design
Thank you very much.
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Apr 9, 2012, 10:50
Post: #8
RE: ship hull design
Your photo is pretty good, so the processing is quite easy:

- Use the Threshold tool to transform all the gray paper to pure white, and the lines to pure black. The trick is to find the value that makes all the gray patches disappear, but doesn't erode out the lines.
- this may make the lines slightly thinner. You can use "Filters/Generic/Erode" to enlarge the lines a bit (yes, "Erode" because the names are given from a "whites" point of view).
- You can also improve visual quality a bit by adding a slight blur to the lines "Filters/Blur/Gaussian blur" and use 1 or 2px.

So you should now have a nice black and white drawing. To make it transparent:
- "Layers/Transparency/Add alpha channel" to allow it to be transparent
- "Layers/Mask/Add layer mask", initialize to "Graysscale copy of layer" and tick the "Invert mask" box. You should see the white change into the checkerboard pattern of the canvas (or the layer below if you have one)
- "Layers/Mask/Apply layer mask" and you are done.

Come to think of it, since you seem to be adjusting things over other things, there is a very useful script, to perfectly align two layers. I never remember which layer matches which points so it's bit of trial and error each time, but it does a perfect job (and does it in one pass, which minimizes the slight blur that rotation/scaling always imply).

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Apr 9, 2012, 13:54
Post: #9
RE: ship hull design
Here is what I get, hopefully the right way. The attached .jpg file only shows the lines on white background. The .xcf image actually shows the lines and whites over the background layer (I tried to attach a .png copy of it, but somehow it got stalled).

Thank you for the "exact-aligner" script, which should be very useful indeed. I surely will give it a try. Btw, I hope I didn't spoil your lunch cake...


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Apr 9, 2012, 16:20
Post: #10
RE: ship hull design
Don't worry about th cake, they loved it Smile

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Apr 11, 2012, 19:19
Post: #11
RE: ship hull design
The "exact-aligner" script seems to work very well on details of an image. However, trying to align larger pictures into a single one is more problematic. The attached image will show you the result of my unsuccessful attempt at aligning two sections in top view of the aft part of a ship. Both pictures do get aligned, but not quite as they should at their intersection points, even after several corrections of rotation and scaling. Moreover, the left part of the image, which belongs to the background picture, remains blurred, with a low contrast.

Is there any way of improving this?


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Apr 11, 2012, 19:59
Post: #12
RE: ship hull design
If the background picture doesn't look too sharp, it's likely because the front picture isn't completely transparent. Take the Histogram dialog (Log view), and select the Alpha channel. You should have mostly a high black peak and a medium white peak with little in between (50-200). Otherwise, use the Levels dialog (select the alpha channel) to fix this (change the grays to black).

For the geometry, don't expect miracles form a script that will only scale/rotate/move. Your picture are slightly distorted and this is hard to fix. There is software that can compensate for this (in the FOSS world, most use the lensfun library) if the camera+lens is known (or if you can calibrate them). But ideally you shoot your pictures in very controlled conditions (medium telephoto to shoot from a distance) and macro prime lens on a DSLR with very strict centering/alignment to minimize distortions. Or use a scanner.

If you are lucky, you could also get decent results using panorama software (in the FOSS world, Hugin).

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Apr 14, 2012, 00:29
Post: #13
RE: ship hull design
Just found this:

http://registry.gimp.org/node/24248

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Apr 14, 2012, 09:44
Post: #14
RE: ship hull design
Thanks for the link. Unfortunately, I had a problem loading the page, as I get a "link broken down" message. Would I get another chance?


(Apr 14, 2012 00:29)ofnuts Wrote:  Just found this:

http://registry.gimp.org/node/24248
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Apr 14, 2012, 13:09
Post: #15
RE: ship hull design
Finally managed to download and install the plug-in for image alignment, so please disregard my last message. Thank you very much for this link. Will let you know about possible results. Meanwhile, I got a full top view of the ship I am modeling with a simple cut and paste in GIMP, sufficient though for importing the image to a 3d modeling and rendering software. Also, first attempts with Hugin are giving promising results.
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