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Author Topic: Does this sound practical? High res scanning of over-sized prints.  (Read 12069 times)

bigben

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Hi All

We're currently digitising a collection of Piranesi prints in bound volumes (e.g. http://cat.lib.unimelb.edu.au/record=b2984624) and I've been looking at options for scanning large foldouts.  They're too big for our largest scanner (A1). I've considered using panoramic stitching software, but setting up the equipment to do that has its own challenges as the books have to be kept lying down, so the camera would have to be mounted from the ceiling.

As an experiment I thought I'd try creating a model of the foldouts in Photoscan and render that out at high resolution... but then I kicked myself. an orthophoto would be easier. I just need to get the pro version.  I'd appreciate some thoughts on the practicality of this and possible workflows.

  • Would there be any advantage to using a wider angle lens to help align higher resolution images?
  • Should I shoot much higher resolution than I need for the final output? Any suggestions for how much?
  • How low can i go with the dense point cloud and mesh given that I''m mainly interested in just the texture

I'll be testing these things out anyway but I'd appreciate any suggestions/ideas.

Wishgranter

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #1 on: June 03, 2014, 09:32:36 AM »
pano stitching is a no way, its destroy info on lens and etc. Its depend on what you need to get as results.... if hires mesh + texture there are better ways to get great results.... contact me if interested....

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bigben

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #2 on: June 03, 2014, 10:52:12 AM »
Just to clarify for anyone else as well, I've ruled out panoramic stitching for a number of reasons for now and was looking at using Photoscan as an alternative.
Camera (Canon 5D) on tripod with fixed height, fixed focal distance moved along a "flight path" around the print.

The screen grab shows the result of a medium quality dense cloud using a70mm lens as a test, with control points placed around the print (if that will help). For the higher resolution version we'd be using a 200mm lens, crossing my fingers that we pick up enough paper texture for alignment as appears to have happened with the 70mm series.

The mesh is not required in the end result. Only output will be a high resolution TIFF orthophoto. Using the demo Pro version at the moment but I've convinced my boss already to buy it soon as it also has benefits for our other work. 

JMR

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #3 on: June 03, 2014, 12:26:26 PM »
I would recommend using a macro lens better than a tele lens (much better for camera alignment). On the other hand you should try to keep camera as perpendicular to the drawings as if it was an aircraft photo-flight.
Long time ago I built a setup for a mummy photo scan using a linear guidance system http://medias.ina.de/medias/en!hp.ec.br/LFS?#LFS for a carriage http://medias.ina.de/medias/en!hp.ec.br/LFL..-SF?#LFL..-SF mounted on an aluminium profile. http://www.boschrexroth.com/en/xc/products/product-groups/assembly-technology/basic-mechanic-elements/index

The rail was fixed on two trestles allowing smooth and regular motion for the camera.

The setup has been useful in lots of projects after the mummy, so it's an investment that is worth.

gEEvEE

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #4 on: June 03, 2014, 12:58:47 PM »
Hi BigBen,

I did this already many times to document paintings and mural frescoes.
  • Just print some PS markers and place them around you object to be digitized;
  • Measure the distance between 7-10 markers (the longer the distance, the better);
  • If the object is perfectly flat, place three markers so that they form a perfect right-angled triangle. If the object is not flat, place two rulers alongside the object in a right angle. Make sure that these rulers are flat;
  • Then, photograph the whole object with an overlap of 80 %, using a good macro lens (do not zoom or change the focus);
  • Import all images in PS and let PS look for the markers (verify afterwards as some markers might be place wrongly);
  • Align the images;
  • Indicate the distances between the markers and optimise the image alignment with radial and decentring distortion active (k1-k3, p1-p2). Before performing the optimisation, make sure that you indicate the accuracy of your markers and distances in the ground control panel;
  • Then, provide the coordinates for the markers that are in a perfect triangle (or indicate new ones on the rulers so that they form a perfect right-angled triangle. One of them you give coordinates (0, 0, 0). The other two you vary in X and in Y (e.g. 10, 0, 0 and 0, 10, 0). The exact coordinates can be obtained by measuring the distances between the markers or  by reading the distances you see on the rulers. Also set the coordinate reference system to “Local”. This allows you to make a perfect orthophotograph;
  • Build a dense point cloud (medium suffices) and mesh the surface;
  • Mask the outer parts of the images, as lenses loose sharpness there;
  • Export an orthophotograph;
   
Good luck, Geert

ozbigben

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #5 on: June 03, 2014, 01:40:27 PM »
Thanks guys, that's very helpful.  I had considered making a rig when we were looking ay pano stitching software but the fact that these aren't perfectly flat ruled out moving the camera in that instance. A rig would be more sensible in the longer term, but I'm looking at a proof of concept to get the ball rolling for now.  The angle of the camera at least does not cause problems with depth of field across the individual images. I exported an ortho image from the standard version and it looked really good, although I'll have a closer inspection when I get in tomorrow. Now I'm excited... again  ;D

I also don't have the RAM to handle the resolution I really want so our 100mm macro will suffice for now rather than my personal 70-200mm zoom. It's also our sharpest lens so we'd use that with a rig setup. One less thing to buy.  Looks like I'll be talking to our high performance computing guys for a virtual machine with lots of RAM.

Wishgranter

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #6 on: June 03, 2014, 03:27:49 PM »
If need can help with the processing...... RAM is not a problem here... if its a Academic work its for free.....
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Mfranquelo

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #7 on: June 03, 2014, 03:34:57 PM »
You can use a 45 degree mirror and shoot a panorama perpendicular :)
« Last Edit: June 03, 2014, 03:39:34 PM by Mfranquelo »

bigben

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #8 on: June 03, 2014, 04:15:14 PM »
If need can help with the processing...... RAM is not a problem here... if its a Academic work its for free.....
That's very generous. I've managed to squeeze out a 32K texture, although it slowed down at 85% where it was oscillating around the maximum RAM. I'll definitely keep it in mind while I fine tune things. Screengrab is the image after resampling to 50% and a bit of shapening which is looking very promising.

You can use a 45 degree mirror and shoot a panorama perpendicular :)
There were a range of issues that led me to rule out doing panos.  Not being flat ruled out moving the camera around as perspective changes cause nasty problems. Shooting from one point creates some practical issues around working distance and depth of field at the edges of the prints. I don't think our collection managers would be too impressed with the risk of suspending a mirror above 200-300 year old collection items  ;)

Marcel

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #9 on: June 03, 2014, 04:23:23 PM »
You can use a 45 degree mirror and shoot a panorama perpendicular :)

That's smart, I will remember that! (Easier to control the camera as well).

Looking at your action shot: My experience is that at that distance and angle you will have a slight blur from the depth of field at the top and bottom of the photo, even at f11. (If you go much higher than f11, your image quality will degrade because of diffraction). Shooting straight down will make for sharper images. The longer the lens, the more shallow the depth of field is (you can find depth of field calculators on the Internet or on your smartphone. The depth of field for a 200mm lens can be mere centimeters even with the subject far away), so using a 200mm might be harder than using a 100mm at closer range (or even 50mm). Normally the lens distortion of a wider lens would be a problem, but because you are using Photoscan this is not an issue.

Also, I can really recommend getting a wireless or wired shutter release to avoid moving the camera when pressing the shutter button (it might seem trivial, but if you want perfect pixel sharpness it's very important). Another option is the 2 sec self timer delay, but that is a bit slow to work with if you need to do many photos like this.

Be sure to disable Image Stabilization on your lens when you are using a tripod.

Another option would be to suspend the camera from the ceiling, and slide the prints on a table (if you want to make it really fancy, get some line lasers to show your exact field of view from the camera to make the positioning easier. It would be an easy step in between automated scanning and working on the floor :)

If you are only interested in the texture you can use the Sparse Cloud to generate the mesh. We have had good luck with that (you could align with more than the recommended 40k points to get a Sparse Cloud with more points if needed).

Mfranquelo

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #10 on: June 03, 2014, 04:43:20 PM »

Quote
There were a range of issues that led me to rule out doing panos.  Not being flat ruled out moving the camera around as perspective changes cause nasty problems. Shooting from one point creates some practical issues around working distance and depth of field at the edges of the prints. I don't think our collection managers would be too impressed with the risk of suspending a mirror above 200-300 year old collection items  ;)

The prints seems to be pretty flat on the photo you've shown arent they ?
Im using 600mm lens at 6 meters for painting documentation. DOF is OK if you use more than 60 percent of overlap and just use the center of the image... (it depends on the size of the prints of course...) but you can just do two panoramas from two positions... or even more.
 *If you use a crop sensor on a full frame lens then "you have more DOF" since youre just using the center of the image, and because of other variables.

- You can always do focus stacking ! its pretty easy using PTgui and helicon focus.
- There are many methods for documentating old books. There's one which interests me a lot, using a prism you dont have to suspend anything. The optical prism is not cheap however.


ozbigben

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #11 on: June 03, 2014, 05:13:34 PM »
@Marcel: I thought about moving the print as well. Both methods have their challenges.  Ultimately, moving an A0 foldout attached to a thick A2 book with binding falling apart would require a very sturdy platform to avoid any changes in the position of the foldout. I think moving the camera will be simpler for our purposes.

With the Canon 5D we'd be (hopefully) looking at imaging A4 sections of the print. At the angle used in this test I think you're right. The depth of field was pretty good but there was a bit of drop off in sharpness across the image. We have wired releases for our camera, and for this test I was using the self timer to avoid camera shake. Getting rails built shouldn't be too much of a problem but I will have to make a case for it and it will take a bit of time.  A counter-weighted pole might be a better interim method for positioning the camera while we sort out the exact specifications for a set of rails.

@Mfranquelo: I'd considered a 400mm lens and a motorised telescope mount run via Papywizard, but apart from that being a few extra pieces of equipment to purchase without a proof of concept, our experience with stitching A1 scans of large maps with folds in them (also using PTGui) is that the small perspective differences can be problematic. While there are workarounds for these I think the principle of photogrammetry is a better fit for this particular application.

Mfranquelo

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #12 on: June 03, 2014, 05:30:22 PM »
Im using this system:

http://www.dr-clauss.de/en/  works slightly better than papywizard. Its significantly more expensive tho.

If you place the lens to rotate around its optical axis you wont have perspective issues when aligning and stitching images on Ptgui. You can also autofocus on each image and check images to have a different FOV between each other to compensate when optimizing.
You're having problems because you're scanning from different positions. This can be avoided.

Photogrammetry is another approach, but you cannot export 16bit.
Hope what im saying helps :)

xM


ozbigben

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #13 on: June 03, 2014, 11:07:46 PM »
Don't worry, I'm well aware of the requirements for shooting panoramas. I was a beta tester of PTGui, and helped determine a practical order for optimising image parameters for auto-alignment  ;)

Yes it can be done with pano stitching (even though autofocus will fail at times), but in order to do it to the necessary standard requires a great deal of precision which complicates the equipment setup. I'd love to use a Rodeon but for this collection we're only looking at 0.5% of the images requiring this technique, and this drops dramatically when you look at all of the material that we scan. I couldn't even justify the expense and effort to my self let alone sell the idea to our execs.

With this method I can get a fully functional prototype for under $1K including the software upgrade using mostly available equipment. The usage of that can then establish whether there is a case for investing in additional equipment to streamline the process.  From what I've read here there may not be a 16bit workflow at the moment but there is a 32bit workflow.

Mfranquelo

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Re: Does this sound practical? High res scanning of over-sized prints.
« Reply #14 on: June 04, 2014, 02:19:25 PM »
I understand :)

I'd love to see results of this method. Are you planning on uploading everything to high res viewers ?