Blackmagic HDLink & LUTs
by crcaillouet on Apr.16, 2009, under High Definition News
A few months ago, i discovered the HDLink Pro, a pretty impressive converter for changing SDI or HD SDI signals into DVI or HDMI signals to drive computer monitors. The HDLink does the basic job and sells for about $500, but the Pro model, for about $800, gives you an extra treat.
Both HDLink models contain look-up-tables (LUTs), which provide for control of red, green and blue signals to the monitor. Most LUTs are maps from input to output values for each of the three color signals, but the HDLink gives you a “3D” LUT capability; it allows you to map from any RGB value to any other RGB value, providing some colorimetry control beyond simple color balance.
Actually, there is some trickery going on in this amazing box, as there is in most 3D LUT devices. A full, 10-bit, RGB map from any input to any output would require 30-bits of fast RAM, or about a Gigabyte, plus the computational headache of calculating each of those entries for each change in the LUT.
The trick is to use 16 or 17 values for each of the colors and to interpolate using a fast DSP for the rest of the values. The HDLink uses 16 values, as does the Apple Color app; most of the others use 17. It turns out that 17 makes the values fall in binary increments of 64 levels, but either will work. It is just a bit of a nuisance to have different standards. Apparently, Blackmagic software can convert from 17 points now, so using other standard LUTs will work. It’s not clear to me how Color deals with the discrepancy.
The treat that you get with the HDLink Pro is that the SDI loop-through connector can be programmed to get the corrected signal, making the unit an inline SDI/HD SDI hardware color corrector. The HDLink Utility gives you curve-based correction capability and if you are patient enough, you can build your own LUTs with specific corrections.
Steve Shaw has a LUT builder application that allows you to create a 3D LUT from a grade that you have done to an image by comparing the original to a corrected frame. You can find Steve’s app (for Windows only) at Light Illusion. One problem that i have with that approach is that you need a sophisticated color corrector to get the grade before you can build the LUT.
I have built a prototype correction tool that allows me to map six vector color corrections directly into a 3D LUT and i hope to be able to make it supportable in the near future, but right now, it still requires pasting the LUT results into a template file. It seems to work, but i’ll know how well after a couple more outings with it.
If you have any experience working with LUTs, please comment here or email me. I can use all the help that i can get.
cr/30
