F14

Hi all,
I'm looking for informations about plans scan and I've found some messages about it but it seems that few people agreed about how many dpi must be used.
I'd like to know your experiences about plans scan.
Many thanks in advance!
Best regards

Angelo

VinceC

If you intend to view on screen, then use 72 dpi or nearest figure to that, as your screen only shows 72 dpi no matter what the file says.

To get a decent quality printed version you need to scan in 300 dpi or above

F14

Hi Vincent,
I need to scan in a very good quality because I want to import the bitmap into a cad system and trace over it...

oldcabmkr

My Feedback: (127)

If you scan it and save the image as a tiff file you can use a raster vector converter to convert the drawing and it makes the tracing in cad much faster. There are a few free converters on the net or email me and I can send one to you I downloaded, just can't remember from where.
Good luck
Doug

F14

Hi Doug,
yes I could convert from raster to vector (tried with everal programs, the last one is wintopo) but the results aren't very good even with a high dpi... now I'd like to try scanpro as raster_to_vector converter but the most of people who use cad systems told me that the best way is to import the bitmap then retrace on it on a different layer!
Yes, you're right, I need luck and a lot of patience! :-)

Rodrigo C

Angelo:

What is the priginal size of the plans you want to scan?

If it's from a magazine and you want to insert the image in Autocad to "redraw" the plans, it wont make much diference, so let's say you can use any range from 150 to 300 DPI.

About the raster vector converter, the experience I had with those is not much good, they usually add tons of lines and has no accuracy.

ekrcflyer

You might want to scan at highest resolution possible, especially if you will try to enlarge the image later. With proportional enlargement your loss of quality is also "proportional", meaning, if you enlarge original 600 DPI picture and make it twice the size, you will have 300 DPI picture. Reason for this - because your software have to "fill" the whitespace between existing dots, and it makes the "best estimate" - which is usually a weighted average for the colors around. That's why you black line on the white background start appearing like it has "gray-ish" shadows surrounding it.

F14

The original plan is the Avonds F-14 1:10 one, I want to put it in a cad system so I'll be able to rescale it... you're right about raster vector converter, this is the main reason I want to import a bitmap and redrawn on it.
In this way, if well vectorized, I should be able to enlarge drawings changing their scale without problems!

LesUyeda

The most satisfactory results that I have had, have been by importing the scanned image into Autocad (.jpg is good enough), tracing at whatever size it is, THEN scaling the traced image to whatever size I have wanted.

Les

F14

Hi Les,
this is exactly what I want to do and this is why I asked how many dpi are to use to scan the image!
Your suggestion ?

begun

General, use 300 DPI.

Very large drawing paper, 100~150 DPI.

F14

ok, thanks a lot!!!

toy264

I'm doing exactly what you are doing right now- using 8 1/2x11" 3 views to design a 70" warbird. Here's what I found works well: set your scan resolution to 1200 dpi and black and white. Hit the scan button and go get a cup of coffee! Creates a huge file, but so what- when you're done you'll delete it from you're drawing anyway? I've scaled some of the my scanned images in Autocad up by 50 to trace fine structural details, and the lines are still clear and thin.

Import the raster to one layer and trace on another so you can toggle them off and on to see how you're doing. Another tip: if you use splines, they look good but they're hard to edit. If you <save as> to a version R-12 file, splines get converted to polylines-- much eaiser to trim, etc.
John

F14

John,
the tip about splines is simply great!
Some colleagues of mine told me about them so I was thinking about splines specially on wing ribs but I don't know enough about cad to imagine that kind of tips... ok for 1200 dpi, I'm looking for the best solution so I don't care about file size!
Could you send me a small section of a scanned image with a small dxf to see how's your work ?
Many thanks again!!!

Angelo

toy264

Check your PM.
John

Rodrigo C

John, what is the diference to scan in 1200 and 300 DPI? let's say you are scanning a plan and the lines have 2mm width, at 300 or 1200 DPI it wont change, when you insert the image into autocad I assume you draw with the minimum thickness line available, that means you will trace your line in the middle of the original scaned line. The resolution wont change the thickness of the original line drastically to justify having to scan in a high resolution DPI and having to use a giant image file (if you have a power computer you will not have problems, but if you work with a computer like a P2 or O3 with 128 RAM you willl suffer when opening a 1200 DPI scanned image, even if it's in black and white.

Also if you want to scan with the max res possible, you have to consider your scanner optical resolution, I don;t know what is the optical scanners resolution nowadays, I have a HP 3300C which is 300x600 DPI optical and like 9600 Interpolated. Scanning with interpolated resolutions is worthless, it's just like using the digital zoom on your camera, or the same as scaling up the image in Photoshop.

And to finish, try not to use file formats that use compression, since there is always quality loss, a lower DPI scanned image saved as TIFF with no compression can have more accuracy than a higher DPI scanned image saved in JPG format.

That's my 2 cents

Forgot to mention something:
As stated before on this trhead your monitor wont show up more than 72 DPI, and when you scale an image in Autocad it is actually not scalling the image itself, it's scalling your drawing units, notice that when you scale the image in autocad it doesn't loose definition or interpolate when you make it bigger. So you want to set a resolution to get the most details as possible from the paper, and for a printed plan I wouldn't go for more than 300 DPI.

toy264

Rodrigo-
Thanks for your well-considered reply! Perhaps I should have been a bit more specific. I scan into Photoshop, fiddle with the image and save as a jpeg file. Since I’m drawing to scale in AutoCAD, I have to go from a 6 ½” paper original to a 70” raster image. So I import the image into AutoCAD and dimension it’s span. Then I compute the scale factor I have to use to get the 70” desired size, and scale the image. I’ve found from trial and error that if I use 300 dpi, I can’t enlarge (scale, not zoom) the file much before it goes fuzzy (admittedly I'd have to scale the small image up as much as 100 X to get my splines on the panel details spot-on).

My scanner’s hardware resolution is 600x1200 with enhanced hardware interpolation to 9600 (that I don’t use) but maybe I’m going about the Photoshop process wrong. I’m no expert, and I welcome your advise. The big scans do take three or four minutes and get as large as a couple of Mb, but I’m on a very fast workstation with enough disk space to archive the Genome Project, so it isn’t an issue for me. But if there’s better, faster way, I’m all for it : )
John

Rodrigo C

John,

are you really sure the image gets fuzzy in autocad when you scale it up?

From what I have been doing, I could scale the image like 1000x and it wouldn't change anything in the image, since I assumed that he isn't really changing the size of the image on Autocad but the scale of the lines you draw. That's the only possible explanation I could figure for the image not getting interpolated or losing resolution when scaling it up in Autocad. I actually just did some test with a plan I had scaned to redraw in autocad, it's a plan from RCM magazine, I think, of a Cessna 195 businessliner, and I scanned it with 300 DPI and saved the file in TIFF without compression. I scaled it up 1000x times and it didn't chage the resolution (note: scaled, I did not zoom). My real point about the DPI resolution is that it wil get to a point that you will start scaning the ink dots details of the print instead of really useable information to help further on the redrawing challenge. I believe scanning in higher DPI would be valuable if the source is a very high resolution print, or drawn with a .05 feather.

Anyways, try scanning same image with 600 or 1200 DPI and scan the same image with 300 DPI and do the test scaling up both images in autocad in the same file and do some comparision. If you get diference between the 2 files it will be odd, since I did the test here and both looked exactly the same to me.

I use Autocad 2004, maybe if you use an older version and the image is getting interpolated when scaling it up.

Hope it helps

F14

Rodrigo,
this is a really interesting question for me: you are talking about tracing the line in the middle of the original scanned line, right ?
Why not in the inner side or in the outer side?

Rodrigo C

F14,

Well that's realy up to the person doing the tracing, I believe that as long as you use 1 rule to trace the lines the result will be the same. So either trace all the lines in the middle of the original line, or all lines in the inner side or outer side.

I find it easier to trace the lines at the middle, since when you zoom up and the print quality can change from line to line, sometimes it's hard to know where to place your line when using the inner or outer side, since it gets blured most of the time and line thicknes change from line to line.

I think it will really depend on the source you are using, better the quality of the print, easier the job.

F14

thanks Rodrigo,
I understand what you mean... so this is another good reason to scan at 300 dpi (or above), is it right ?

Rodrigo C

F14,

Yes and no, you will get to a point where increasing the DPI wont change the final result because the factor limiting the resolution will be the source material you are scanning, like I said, i'ts no use if you can scan the ink dots of the printer on the source paper, usually 300 DPI works fine, if the source plans you are scanning are superp quality then it can be worth scanning with a bit more DPI, but not much more.

toy264

I’ve spent the morning looking into this subject, and it is a complicated one, or at least the technical end is. Without going into the details of how a scanner acquires an image, what sort of a file it generates, how the image editing software processes and presents the data in that file, and how it formats the data when you save it in whatever format you select (JPEG, GIF, TIF, etc), here’s what I think is pertinent to the discussion.

1. Whatever you start with, unless you are prepared to do a lot of tweaking in Photoshop or whatever, the result is not going to be much improved when you save your scan

2. The higher the resolution of your scan (up to the limit of the scanner’s optical resolution) the more faithfully the scanned image will reproduce the original.

3. Whatever sort of raster file you end up with, the more you enlarge it, the worse it’s going to look.


So, for discussion’s sake, a one inch line scanned at 300 dpi will be represented by a string of 300 dots. Ideally, if you zoom that by 30, you’ll see a string of 10 dots; however, as a result of the hardware and software processing, what you’ll actually see is a string of 30 blobs. When you look at your original under a magnifying glass, what you see may not look any better, in which case using a higher resolution is a waste—you’ll just get higher resolution blobs.

Since there are so many variables involved in this, here's my advice: start with the largest, clearest original you can find. Scan it at 300 dpi and see how much you can enlarge the image before it becomes unusable. If that isn’t good enough, rescan at higher resolutions until you’re happy or find a better original. Believe me, the time spent scanning and rescanning is going to be trivial compared to what you’ll spend tracing and fussing with your trace!
John

F14

ok,
this evening I'll scan some parts of my plan at 300 dpi to check the bitmap.
John, I don't understand what I've quoted: why should I enlarge the bitmap before re-tracing over it with a cad system ?
I need it to obtain the best resolution so it's more easy to re-drawn over it ?
Sorry if this question may be stupid.

Angelo

clscale-RCU

My Feedback: (7)

I just bought Design CAD 3-D Max program from www.outpost.com and it can import in Bitmaps and then it has an autotrace over the bitmap. It did a acceptable job converiting the bitmap to vectors, but I plan on trying some tricks to improve the quality. but at least I can import the bitmap and skip the bitmap to DXF conversion.

I think I can plot the import bitmap if all I wanted was a enlarged copy of the 3-view.

The program cost $70 and is easy to use.