I tried to check correlation between R, G, B, grayscale and IR channel, hoping to find something which could help me subtract better the image data leaked to the IR channel.
I run this script:
#load color and IR, no preview
i ${1},0,2,2
#cut out black borders
crop 20%,20%,80%,80%
div 257
+channels[0] 0
+channels[0] 1
+channels[0] 2
+rgb2srgb[0] luminance. srgb2rgb.
ap[1-5] "unroll x"
append[2] [1],y
append[3] [1],y
append[4] [1],y
append[5] [1],y
ap[2-5] "pointcloud 0,256,256"
ap[2-5] "-normalize 0,255"
o[2] {0,b}_cloud_R.tif,uchar
o[3] {0,b}_cloud_G.tif,uchar
o[4] {0,b}_cloud_B.tif,uchar
o[5] {0,b}_cloud_mono.tif,uchar"
rm
So what you get is on X axis the value of R, G,B,grayscale and on the Y axis the value of the IR channel. I have to point out that the grayscale is the “luminance”, which almost discards the blue channel, but it doesn’t matter much: the channels are not so different.
The stronger the leak, the more grouped together the values are.
Darker images have more points on the left, brighter ones more points on the right.
The results are best shown using the Windows thumbnails (each photo is followed by R, G, B, grayscale point clouds):
Overall, reddish photos work best with red channel, blue photos with blue channel and so on, so the grayscale is the best option.
I would be curious to see the same applied to Kodachrome to see how the clouds lok like!
But also the baseline is not a line, and a linear correlation would not work to remove effectively the leak. It is often good enough, but some photos have a noticeable curve.
At this point for my E6 slides I’m not sure how to proceed about subtraction of the leak.
I will try the blur as you suggested, at least to compare.