Bulk editing PDF files
December 16, 2008 10:56 AM   Subscribe

What's the best bulk way to take scanned documents with black text on red paper and make them into normal black-and-white readable scans?

I recently bought a Scansnap S510M and I've been getting rid of paper as much as possible. It's great having everything as searchable PDFs.

I have several hundred pages of course notes which were prepared by a note taking service which I'd like to scan and throw away. To discourage students from photocopying them for their friends in the class, they photocopy these notes onto fairly dark (obnoxiously dark) red paper, so there's not enough contrast to get a good copy on a normal photocopier.

What is the best way in Photoshop to take a color scan of a document like this in (dark red background, black text) and turn it into a black-and-white image with black text and a white background? As in which "adjustments" option would be the right one for this job?

Then, to take it one step further, how could I automate the process if I had a PDF file with many pages like this?
posted by david06 to Computers & Internet (11 answers total) 1 user marked this as a favorite
In Photoshop, you can use the threshold function to transform an image into a two-color black and white image. This will have the effect of eliminating any grayscale content in the scan, which is probably what you want. It will also have the side effect of making the images very small after compression.
posted by demiurge at 11:06 AM on December 16, 2008

Here's what I do:

Choose Image > Adjustments > Replace Color.

In the resulting box, click the "Image" radio button below the preview image. Make sure "Preview" is checked.
Use the eyedropper tool to click on the red you want to delete. That color will show up in the swatch square in the lower right. Now move the Transform sliders all the way to the right. The red will change to white.
Use the Fuzziness slider at the top to control how much black text gets affected by the color replacement.

I'm not sure, but you may be able to record a macro for this and do the images in batch mode.
posted by Fuzzy Skinner at 11:42 AM on December 16, 2008

Image > Adjustments > Auto Levels
posted by bricoleur at 11:50 AM on December 16, 2008

OK, I just did a quick test, and the Replace Color command is recordable.

Also, you can record the action to open a PDF, but you have to manually choose each page. (There may be a better way, but I don't know what it is offhand.) Once the pages are all opened, you can apply the Replace Color command you recorded to all opened files.
posted by Fuzzy Skinner at 12:56 PM on December 16, 2008

You can apply an Action to all files in a folder. File --> Automate --> Batch. Just make sure your "source" is set to "folder". Oh, and have a backup of all the images elsewhere, so if your batch routine totally tanks, you don't have to rescan :)
posted by misterbrandt at 1:00 PM on December 16, 2008

You can apply an Action to all files in a folder. File --> Automate --> Batch. Just make sure your "source" is set to "folder".

True. However, if the folder contains a multiple page PDF, you have to choose which page to open. That's what I meant by "manually choose each page."

But here's an idea: If you have a multi-page PDF, open it in Acrobat, then select all the pages in the Pages pane. Right click any one of them and select "Extract Pages" and click the check box that says "Extract pages as separate files." Navigate or create a new folder and save them there. Then you can select that folder as your source in the batch process.

(By the way... auto levels will not do what you want. In my test, it made the red pink instead of white. You can then use Levels to make the set the pink as the white point, but the edges of the text get funky.)
posted by Fuzzy Skinner at 1:31 PM on December 16, 2008

I know you asked for Photoshop solutions, but would it work to scan them high contrast B&W through a piece of red cellophane?
posted by ottereroticist at 1:32 PM on December 16, 2008

Using hue/saturation (apple-u) make the red background yellow by pulling the hue to the right until it turns as bright a yellow as you can make it. You may need to do this twice depending on the red to get the right bright yellow. Then convert to BW and use highlights contrast to either make the yellow background completely drop out or to just lighten the yellow background which will now be a shade of light grey. Or just skip the highlights/contrast step if you don't mind a grey background. Assuming all the scans are about the same you can easily record an action to this.
posted by wherever, whatever at 1:50 PM on December 16, 2008

Sorry "command-u" if you are a PC user.
posted by wherever, whatever at 1:52 PM on December 16, 2008

Sorry "command-u" "control-u" if you are a PC user.

posted by Fuzzy Skinner at 3:53 PM on December 16, 2008

posted by wherever, whatever at 7:11 PM on December 16, 2008

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