Fixing scanned line drawings

[ferdly]

Hello. I have to clean up a bunch (>100) of line drawings that were scanned from old books at 300dpi grayscale and saved as PNGs. Here are two samples. These are quite large (1 and 2MB respectively) so you might want to right-click and save-as.

Freehand drawing

Engineering drawing

In cleaning them I need to do 2 things especially: getting rid of the oatmeally page texture, and in most cases reduce from 8bit (256 level) to 4-bit (16 level) grayscale. Hopefully this all becomes a photoshop Action but I need to work out what steps to use in what order.

I'm concerned especially with the engineering drawing, because when I use Levels to turn the light-gray bg to white, the diagonal lines thin down and get jaggy. I've fiddled with different kinds of blur first, but without much success.

I'd love to hear any ideas you experts have...

[mistermonday]

Ferdly, duplicate the background layer and change the blend mode to Overlay. The images were large so I just attached a small section of each.
Not sure what you mean by 4 bit. If you mean that you do not want more than 16 shades of grayscale, well you will get that. If you mean that you want to output your file with 4 bit words, that is not possible since the min word length of any file type is 8 bits.
Regards, Murray

[philbach]

Perhaps what you mean would go like this:
Like Murray above said was:
Copy the background layer and change the blending mode of the copied layer to overlay
Then use Image/Adjustments/Threshold
Change Image Mode to Bitmap.
That would get you down to 1 bit depth.

To go to 4 bit you would use Image/Adjustments/Posterize and select levels 16 (I think).

[chillin]

I have converted the scan in the Illustrator to B/W 128
Saved it as a PSD
Changed it to a grayscale
Reduced it's palette to indexed 16 colors (gray tones)
Saved as a palette-based png
I used PSP because of it's nice file saving options.

[ferdly]

Duplicate bg layer, blend mode to overlay -- that is brilliant! I'd'a never thought of that in a zillion years. Thank you so much!

[ferdly]

Playing around with this, I think actually blend of Linear Light is even more effective than Overlay.

[ferdly]

Not sure what you mean by 4 bit. If you mean that you do not want more than 16 shades of grayscale, well you will get that.

Yes, the drawings need at most 16 shades of gray. Turns out, you get that with Image> Adjustments> Posterize and set 16.

If you mean that you want to output your file with 4 bit words, that is not possible since the min word length of any file type is 8 bits.

Au contraire; in .png you have the option of representing each pixel of a grayscale file as 8 bits (256 shades), 4 bits (16 shades), 2 bits or 1 bit (monochrome). After posterizing, File > Save For Web does PNG-4 automatically.

[Dave.Cox]

The best option, would be to use live trace in Illustrator. If you don't have Illustrator, then try this method, that I did very quickly in Photoshop. The contrast is very good in both drawings. The mechanical drawing the lines are bold enough to color select the dark color of the line and copy the selection to a new layer. Add a white layer for the back ground you you have it. The hand drawing has some fine lines, so requires a bit more care. In this case I sampled the back ground, for the various gray levels. Select the first one, and then shift select and drag across the area for the rest. Invert to select the dark areas, copy to a new layer and add a background layer. Very quick, but works with something with good contrast like these.