Huge increase in file size after rotation

[taidi]

Hi - I have a problem with the Custom/Fine rotation tool.

I'm trying to join two large files in a panorama, they're black & white scans of A3 size maps in compressed .TIF format. Both images are around 1.7MB disc size but one of them is slightly askew so I have to custom rotate it by 0.25 degrees to get it square to the other map and line up properly for the join.

The program handles this just fine but the resulting saved image after rotation, instead of being just 1.7MB is now 12.3MB !!!

Is this normal - or could I be doing something wrong ?

[j7n]

A program will inflate the picture to 8bit grayscale before rotation. This could be the explanation if your pictures are 1bit black&white.

It is best to work in full color or grayscale (depending on contents) and reduce to black and white after all edits are done (if necessary).

[taidi]

Thanks. These files came to me in Group 4 Fax Encoding at 400 dpi - scanned on an engineering drawing office scanner, and the original prints were black and white.

If I convert them to greyscale for manipulation which lossless format would you suggest ?

[Sam_Zen]

The value of dpi is not that important. As soon as it's a graphic file, it's about the color depth.
As j7n says, Irfan View increases the color depth before a fine rotation. If one want to use an effect, the increase even will be to 24bit full color.

If I convert them to greyscale for manipulation which lossless format would you suggest ?

Any uncompressed format I suppose. So depending of what you're familiar with, BMP or uncompressed TIF.
So first convert your files to greyscale or full color, and save them in such a format. Use those copies to do the manipulation.
After you have done the final panorama stitching, you then can decrease the color depth of the result back to 1bit, if you like, before saving it. In this way you can tryout different procedures without loosing the originals, so you can start over again if the quality or the size of the result is not what you want.