Resizing B&W Line Art Images

[arnymars]

Anyone can suggest, how to scale down & resample B&W Line Art Images (preserved as GIF or PNG), like a handwriting image, without visible loss of image quality, while preserving its original color depth?

Current IV Resample feature rases color depth, when scaling down low res lineart B&W images, making them unfaxable by a B&W fax. If you than decrease Color Depth to B&W, image quality vanishes.

Is it a missing feature in IV - Image - Make Black and White - i.e. replace ALL color pixels, including those resulted from Scaling, except whites, with Black pixels? Combined with Sharpen (on each Resample stage), this technique allows to preserve color depth of a scaled up or down B&W image without visible loss of image quality (clarity) - am I right? And there seems to be no further need for pre/post manipulation with GIMP, etc.

How to select optimal resolution, when Resizing Images? How this selection depends on the Image Type, File Format, original and desired size, etc?

[midora]

Anyone can suggest, how to scale down & resample B&W Line Art Images (preserved as GIF or PNG),

There are to many questions, but please respect you are talking about black and white images. There is not a lot you can do. If you are downsampling you have to switch on dithering (to simulate a halftone effect if there are enough pixels left).

Still the best is to keep as much pixel as possible and use a good packing algorithm like CCITT in TIFF. There are newer ones (like jb2) which generate vectors from black and white, but not a lot of tools support them.

[Sam_Zen]

Many questions indeed.
I think there's no single solution here. It depends on the actual material, thickness of lines, etc.

Is it a missing feature in IV - Image - Make Black and White - i.e. replace ALL color pixels, including those resulted from Scaling, except whites, with Black pixels?

This is already how it works by decreasing to a 2 color depth.

Combined with Sharpen (on each Resample stage), this technique allows to preserve color depth of a scaled up or down B&W image without visible loss of image quality (clarity) - am I right?

No. With multicolor images you would, but not with B&W.
Both Resample and Sharpen turn the image into a fullcolor version, and are causing all kind of gray pixels at edges.
Then you are forced to decrease again to B&W, and it can cause loss of quality because of quite unpredictable artifacts.
So in this case, don't use Sharpen, and disable Resample and the filters at the Size method by choosing Resize.
You will see that, after that scaling down, the color depth stays 1 bit.

(This is nice about having IrfanPaint now. I wanted to test different methods without having a B&W picture available.
Now I could make an appropriate test image by writing my signature with a black brush on white)

[arnymars]

Thanks, Sam Zen.

Tried resizing, however, it sharply decreases B&W image quality. There is NOTHING WE CAN DO - or there is another approach & soft & algorithm to scale B&W LineArt Images?

[Bhikkhu Pesala]

I think I found a way to get acceptable results, at least it worked better with the attached image (reduced to 25% of the original size) than just resize and reduce colours. Originally a 2 colour PCX, I have saved it as 2 colour PNG. GIF or PCX would be fine too.

• Resize
• Enhance colours (reduce Gamma to minimum)
• Reduce colours

I attach the original as a PNG too.

[matera]

Yes, that's how I do my B&W scans, lots of gamma and contrast tweaking. It takes some testing to get the hang of it. Converting to grayscale after the resize is a good thing to do too. Don't try to get it perfect in one shot, take small gamma and contrast steps, maybe save an extra copy to revert to when it's almost done - just in case, you don't lose all the first steps.