Alpha compositing with Marble Index



Let’s discuss patching techniques for Marble Index, the LZX Expedition series analog alpha compositor module.


Admin note: This question asked by Leslie Rollins on the former LZX Knowledgebase

With all settings set to the defaults as shown in the User Reference Card I’m experiencing behavior that I don’t fully understand. Using hard keyed ramps from Visual Cortex, through two separate Doorway modules, when I input the H ramp into Marble Index Ch. A red input, V ramp into MI Ch. B red input, and red output into Channel A red input on VC, I get this result with the Opacity knobs 1 & 5 each set to the middle position:

Then, with the Opacity knob 1 left in the middle position & Opacity knob 5 turned fully right, I get this result:

It seems like Opacity B takes precedence and removes A completely from view. If I swap the settings and turn knob 1 all the way right and leave knob 5 in the middle, the image doesn’t change from what is shown in the second image. In order to produce a similar effect in Channel A, I need to turn knob 1 all the way right, and knob 5 all the way down. Can someone please explain why it works this way?


Marble Index uses Alpha-channel based mixing to create layered composites from three different RGB images (Background, Channel A, and Channel B.) Here are some illustrations that show how this works using similar sources to what you’re doing. If you’re more used to thinking in terms of crossfaders than alpha channels, you can think of Opacity A as a crossfade between the Background and Channel A inputs, and Opacity B as a crossfade between the first fader’s output and Channel B. Another important thing to remember is that with no input, the image for the channel is simply a black/blank screen. The RGB offsets make it easy to dial in a background matte color if applied to the BG inputs only.


Here’s my basic patch for Marble Index exploration:

Marble learning Patch: MI RGB outs to left Cortex RGB In. All cortex controls default per MI Reference Card

Patch Horizontal lines to Red BG, a SHAPE to Red channel A, and Vertical lines to Red channel B (ONLY USE ONE COLOR CHANNEL). All switches off.

Use the Opacity controls to fade between BG/A/B. Return Opacity. Start throwing ONE switch at a time and then turning Opacity controls (and RGB Offsets for corresponding switches) and Opacity A/B and then returning each switch to OFF.

Do this until you have run through ALL the switches.

Repeat systematically until you have a feel for the controls, then try them in combinations. THEN add in other related or dissimilar signals into G and B of each section of MI. Repeat. Repeat. Repeat.


There’s GOLD in @wiatrob’s hills! The tips he shares above really, really helped me get to grips with Marble Index. :slight_smile:


Glad it’s helpful! If anyone has tips to add, please be sure to post 'em!


Still playing and experimenting. Hope to develop some tips and tricks to share.


More cross posted info from Facebook on how Marble Index works.

Lars Larsen - Marble Index is more about voltage controlled mixing between sets of RGB inputs. On top of this, Marble Index has a LOT more circuitry related to complex blending modes – so it’s kind of more like an analog computer designed to achieve compositor math typically found in computer graphcis. I’m not sure if anything like it really exists… trying to do something totally new. You can think of Marble Index more like a video mixer, except instead of A/B banks you have 3 layers, and instead of an A/B fade control, you have 2 opacity controls.

Lars Larsen - You can read more about the math behind blending modes here:
Most of the modes described on this page can be achieved (with analog circuitry) using different combinations of modes on Marble Index.

Lars Larsen - Likewise, most of the Porter-Duff compositing methodologies are achievable using a pair of Doorway keyers to manipulate alpha channels (which then get passed along to control the Opacity CV inputs on Marble Index.)

More reading

Lars Larsen - Marble Index, once available, is really the final piece of “glue” between all of the color mixer/manipulation modules we’ve designed for the system so far, and can function in harmony with Passage, Mapper, Color Chords, Cortex compositor, etc to enable a huge variety of different workflows that have complex hierarchies and full expandability without any “end points.” Cortex is designed to be the system’s primary RGB/color module in under 3U systems, but as systems expand, the color complexity can be relegated to other modules and Cortex then becomes like a final output processor, where it’s easy to do “fade out to black/color” or output effects (like Negative, really handy for external feedback loops)


@destroythings Thanks for posting these helpful notes over here it will be a great resource to search this forum once things have fully migrated over here.