Resolution changes for better transformation results 1

[anesone]

I scan black and white line drawings (ink, pencil, charcoal) at 600ppi for archiving and reprint at 600dpi. Recently, friends have asked for part of the drawing to be enlarged, rotated or otherwise changed (transformation comand). This causes the black edges all along the image to take on a "jaggie" appearance which I have to paint over.

Can I eliminate the jaggies by doing the original scan at 1000 to 1600 ppi, then then converting to 600 dpi via Image - Image Size, change Resolution to 600 pixels per inch, check boxes for Constrain Proportions and Resample Image with Nearest Neighbor (remember, black and white artwork with no antialiasing)?

My understanding is Photoshop drops pixels when you lower resolution, so couldn't this also cause jaggies? If doing the original scan at higher than the 600dpi output gives a better result, does it erode as you do the original scan at a higher resolution? I've tried scanning the same image at 600 1200 1600, doing transformations, checking them for jaggies; doing the transformations before and after changing resolution to 600, but my eyes have gone buggy on me!

 

[blueark]

If you have a 600dpi image, and you enlarge it by a factor of 10, it becomes a 60dpi image. If you know how the maximum enlargement you need, you can work out the resolution you should scan it at. Simply multiply 600 by the enlargement amount.

Examples:
Enlarge by 2: scan at 600 x 2 = 1200dpi.
Enlarge by 150%: scan at 600 x 150% = 600 x 1.5 = 900dpi.

 

[anesone]

Thank you. The other part of the question was, are you better off scanning at 600 dpi, doing transformations, then printing at 600 dpi, or will scanning at 1000 dpi or higher, then transforming, then changing resolution to 600 dpi and printing give better results?

 

[blueark]

Starting with a higher resolution and scaling down as a last step will usually give you better results.

 

[anesone]

Thank you for the input. I did scan same drawing at 600 - 1000 1600 dpi and compared at each step - after scaling down, after transformations. It seemed intuitively correct, but I wanted to see the proof - 1000 was definitely better than 600, 1600 a bit better, esp. if no transformations, so issue then is storage space. Thanks again.

 

[Eggles]

For lineart, it is ALWAYS better to scan at much higher resolutions - min of 1200 dpi - no matter what the output device is capable of. Even if the printer is rated at 600 dpi - this doesn't mean the B&W images should be changed to 600 dpi before printing them. Leave them at the higher resolution and then print. The amount of data in a B&W image is very very small compared to colour or even greyscale, so file size at 1200 dpi will not be a problem.

It is NOT a 1 to 1 ratio between file resolution and printer resolution. When you print pure text to a printer, its resolution is not at 600 dpi - if it was, even it would look jaggy. Same with lineart, because it is just B&W.