[Tig] Re: LOG [was Re: DI workflow, capabilities and requirements ]

Steve Shaw digital.praxis
Thu Mar 31 08:30:43 BST 2005

All film transfer devices 'capture' the image in linear space, as all
electronic light sensitive devices (CCDs, etc,) output an electronic signal
that is a linear response to the illumination they see.

This analogue signal is then encoded into linear digital, and then mapped
into LOG if that is the required output.

The Spirit1 has a 12bit AtoD on the front end, which in reality is not
enough for a 10bit Log signal to be generated from, but only if you do the
math. Visually it works.

The dataMill has a 14bit AtoD, so is more accurate in lin to log
conversions, and actually to maintain the correct film density per sample in
linear space takes 14bits to match 10bit log.

In this way LOG encoding is a lossless form of compression.

16bit linear is way beyond the human eye's perception, taking up additional
space without image benefit.

I just asked the vfx team at one of my clients to 'show' me differences
between performing vfx work in 10bit LOG space vs. 16bit Linear, asking them
to define areas where things went wrong. They could find no obvious visual
differences, although the math says there will be some.

Also, some systems such as Quantel's iQ actually work in LOG space,
performing the math correctly without the need to move image data between
log and linear space, so preserving the image integrity better.

The reason for saying all this? There is no right and wrong way to do this
stuff. If the result looks good, it is good...


Steve Shaw
Digital Praxis Ltd
+44 (0)7765 400 908
steve at digitalpraxis.net

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