How can a I make a palette colour transparent?

[Vascillious]

I have an indexed colour-mode picture. I am trying to rotate lots of little pieces of it. They all need to be rotated by 180 then returned to their original locations. It is very laborious. 

The process I am using it to elliptically highlight each little image inside the circle it appears in, then rotate it on the spot.

I have many problems causing to take far longer than I'd like.

One of them is that when I rotate an image on the spot it is sometimes no longer exactly where I'd like after it's rotated, so after the 180 degree rotate I then have to Move transform.

This is fine but hard to arrange each little symbol inside it's circle when it was nearly touching the sides before, any white I have selected overwrites the background. It would be better if I could select, rotate, move and paste with transparency so if I move the symbol close to its boundary the white area I have picked up with it doesn't overwrite the black background it's in.

I tried to set the white colour to transparent. When I do this is works until I have finished one manipulation and then want to flatten the image. When I do the transparent white becomes white again and I have to start the process again.

Can I just swap white in the palette for transparent so this doesn't happen? Or can I stop Gimp turning the transparent area back to white when I flatten it? Thank you.

[tmanni]

How do you rotate each pieces ?
Could you post a sample of your picture ?

[Vascillious]

(04-24-2020, 06:32 PM)tmanni Wrote: How do you rotate each pieces ?
Could you post a sample of your picture ?

Yes, happily. Somebody else may have a far more efficient idea of how to do this than I do.

The image appears below. My job is to change the order of the symbols. The order they have to be in is the same as rotating the image through 180 degrees, but of course if I do that then the symbols are rotated too but I need them to be in the new order but the right way up. So the image in the lower right appears instead at the top left and so forth for all 64 symbols. Rotating it 180 degrees seemed like the obvious first move, then they are all in the right place, but they are also upside down. I need them to be in the order they appear in after rotating the whole image 180 degrees, but up right so they can be read. 

   

[Ofnuts]

Seriously, given the image, I would redo it from scratch, making a cell with a circle and 4 lines. Replicate the cell and lay out the copies in a matrix (there are script for this). You can put vertical/horizontal guides that intersect in the middle of the circles to easily center the labels.

   

[tmanni]

I don't see any fast method to do this. Rotating the image is imo not the solution because you have to re-rotate each symbol.
My way would be to create a new layer for each symbol (by selection and copy/paste in a new layer) and then move them to their new location.
A copy of the original layer would be used as background, where each symbol painted with white to clear them.

[rich2005]

Quote:  How can a I make a palette colour transparent?

Not possible with Gimp, there are utilities such as gifsicle which can assign a colormap colour as transparency but it will screw up your image anyway.

A re-make as Ofnuts post will give a better result, however it might be possible:

   

I did need the gimp_gmic_qt plugin http://www.gmic.eu and it went something like this:
Use the quickmask to select all the individual icons
Copy then paste as a new image
Use gmic to (1) extract the icons to individual layers (2) rotate the layers 180 deg.
Put the new image back in original and rotate 180 deg 

The inevitable demo video, four and a half minutes: https://youtu.be/TfGIncp0pJg




Hope you do not have hundreds of these to process