Quality Loss after re-opening

[BushidoVR]

I've been using the same format for editing/making art in GIMP forever. 4500 x 5400 pixels, 300 PPI.

Recently I saved and closed a project, and upon reopening it, my brush lines are now jagged and pixelated. I can draw a new line with same brush settings and the lines come out smooth and fine. I am 100% positive my image was still smooth and fine when I saved the .xcf fie and closed GIMP. Something happened and I can't figure it out for the life of me.

Any help with this would be greatly appreciated, as I am essentially losing 8-10 hours of work here.
-Thanks

[Ofnuts]

Are the lines on transparent layers or painted directly on a background?

[BushidoVR]

(06-17-2023, 09:24 PM)Ofnuts Wrote: Are the lines on transparent layers or painted directly on a background?

They are on transparent layers.

[Ofnuts]

(06-17-2023, 09:30 PM)BushidoVR Wrote:

(06-17-2023, 09:24 PM)Ofnuts Wrote: Are the lines on transparent layers or painted directly on a background?

They are on transparent layers.

My only explanation for this is that your layer transparency got thresholded, and if this happened in all layers my only explanation is that you set the image to color-indexed mode, knowingly or not, because in color-indexed mode, to conform to GIF, the opacity of layers is binary.

You can restore most pf the antialiasing bydoing a poor man's supersampling.

• Scale up 4 times using Linear interpolation, which is going to produce lines with fuzzy borders
  
• Threshold the alpha (at 50%)
  
• Scale down to original size using NoHalo interpolation
  

So instance starting with this smooth line:

   

We "accidentally" threshold it:

   

And after applying the technique above you get:

   

For a closer look the difference of the thresholded and restored image with the initial image:

With thresholded:

   

With restored:

   

[PixLab]

(06-17-2023, 09:46 PM)Ofnuts Wrote:

• Scale up 4 times using Linear interpolation, which is going to produce lines with fuzzy borders
  
• Threshold the alpha (at 50%)
  
• Scale down to original size using NoHalo interpolation
  

I'm not really sure if you speak about Layer > Transparency > Threshold Alpha..., as you don't tell the path
If so, still you will have pixelated line, because the 0 to 100% is about the thickness/extend of the threshold.

Instead I would try Filters > Generic > GEGL Graph... and input something like "gegl:opacity value=1.5" or a bigger value

   

But in this case I would certainly use the Colors > Curves and on the alpha channel do a "vertical curve" because of better control of thickness AND smoothness

   

The scale-up/linear - scale-down/nohalo thing is clever, it does the thing nicely/smoothly

[BushidoVR]

Thank you both. This helped me to salvage a useable version of my piece. I'll certainly be farrrrr more diligent in my saving/creating various saved iterations of my larger projects going forward.