Now for my second problem. 

I'm working with large images, 14764 by 2953 pixels. Yes they do need to be 
this big. The machine used for the work has 1GB of RAM and a 1.9GHz Athlon 
processor. The number of undo levels in the gimp is set to three. Tile cache 
size is set to 512MB. 

The images are landscapes stitched together from a number of digital photos. 
Typically on these the sky has been blown out and has no contrast. I 
therefore have been using a correctly exposed sky scene on another layer and 
need to make the original sky transparent, so there is the landscape on the 
top layer, with the new sky underneath. Selecting the old sky is relatively 
easy to do with the magic wand tool, but this doesn't work where the sky is 
seen through things like the branches of trees as the regions are not 
contiguous. I have therefore been copying selections around trees etc to a 
channel mask and using the threshold tool on the channel mask to have only 
pixels near 255 as white and everything else as black. This would then be 
converted to a selection and used to cut the sky out of the landscape layer. 
The problem I get is that on these very large images, when I copy the 
selection to the channel mask it gets displaced by about 30 pixels in the Y 
direction. After doing the thresholding and making the channel mask a 
selection, the selection will therefore be displaced from where it should be. 
I have tried making both original layers the same size as the canvas, in case 
this was the problem, but this has no effect. On smaller images (2560 x 
1920pixels) this technique works perfectly, with no displacement of the 
channel mask. 

Is this a bug with the gimp, or am I doing something wrong?


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