Hi
I just add some small thing that's I'm using a lot(It's useful for isolated objects if you've got rendered image sequence from any 3d software like Maya, 3dsmax or other into *.tga or *.tif RGBA):
How to get a Grayscale Alpha "Lume" from image sequence(*.tga RGBA)
ffmpeg -i image_name.%d.tga -vf "[in] format=rgba, split [T1], fifo, lutrgb=r=minval:g=minval:b=minval, [T2] overlay [out]; [T1] fifo, lutrgb=r=maxval:g=maxval:b=maxval [T2]" image_name_alpha.%d.tga