Megamosaic method.

[ollypenrice]

After finishing the processing of this project initiated and captured by Yves Van den Broek I thought I'd write down some notes in case they might be helpful.

What I received from Yves:

- 32 separate OSC panels stacked and calibrated in APP.

- A 32 panel mosaic constructed in APP. This was a good template for a final image but was not at all satisfactory as an image, with gradients and joints visible and many of the panels badly black clipped.

So...

- The captures were done robotically using plate solving so the APP mosaic was almost oblong, barring the sawtooth edges. I used lens correction in Ps to make it perfectly oblong so as to reduce cropping at the end. The Crescent was in danger of exclusion! This would be my template for aligning the subs in Registar.

- Pixinsight. I edge cropped each sub and did a quick DBE on all of them, with a touch of SCNR green on some. Typically I used no more than 4 to 6 background markers. All the subs flattened easily.

- Photoshop CS3:  I recorded as an action an initial modest stretch (levels-curves routine) and applied it to all subs. This makes them consistent and warns you of any problems they might have - though these were excellent. It's also super-fast. (The odd sub doesn't come out like the rest and needs a levels tweak to look like the rest but the system proved sound.)

- Registar. The mosaic was 4 panels wide and 8 panels deep so I worked from the top down making strips of 4 across but as well as saving the combined strip of 4 I saved the 3 registered calibrated subs separately because...

...I took each strip of 4 into Ps to check it. Use the Image-Adjustments-Equalize function for these checks.  This is a top tip. Joints, corners, bad blends stand out when equalized. If you have two joined panels called Overlap and Underlap there are two possible joint problems, the edge of Overlap and the edge of Underlap. If you have a line where Overlap ends, open the registered calibrated Underlap and paste on top. Using a feathered eraser remove it from everywhere except the line you're wanting to hide.

Fortunately my super PC could do post processing routines from here onwards on the whole mosaic but the second and final stretch I also recorded as an action. That meant that if a problem appeared in the mosaic we could take a repair panel and give it the same exact stretch to speed up making a repair patch.

And there you have it. If there's a program into which you can chuck 32 panels and expect it to knock out a seamless mosaic with well matched levels, colours and joints... I haven't found it! On mega mosaics you have to roll up your sleeves and help them.

Olly