AntcuFaalb said:

You might not have write-access to the directory Avisynth is running from.

Try changing "hist1.txt" and "hist2.txt" to something like "C:\<...>\hist1.txt" and "C:\<...>\hist2.txt".

Replace the "<...>" with a directory you know for certain that you can write to as the current user.

Alternatively, you can try running whatever program is executing the script as an administrator.

I followed your instruction and, despite the fact that this time the two histogram files were written (but are small, 5Kb, it's normal?) I continue to have the same error messages...

Turisu said:

Forgive my ignorance but can chroma from an SD source really be combined with luma from an HD source without any perceivable loss in image quality?

Chroma is less "noted" by the human ey ethan luma... for example, a full HD luma resolution is 1920x1080, while chroma is 960x540 (1/4th) while VHS is 350x480 luma (NTSC) and 30x480 chroma(1/12th)!

Consider that PAL DVD has only 720x576 luma and 360x288 chroma, but when upscaled it will reach the same HD resolution, and, as chroma is less "important" (to the human eye) than chroma, the eventual artifacts given by the upscaling process will pass unnoticed.