Looks awesome!, but..
4K is not a bed of roses, everything is (obviously) bigger and heavier to handle, previz, space on disk, moving files on the server, and so on. Blender suffers this internally as well.
Rendering
On the render side (not compositing), the Blender internal did great, you’d expect twice the render time, but some shots did much less than that, I didn’t feel much difference locally, neither our farm did (bunch of quad i7 and some Xeon Octacores), but compositing is another story…
Compositing
The biggest bottlenecks when rendering in Blender are in the compositor, even in 2K, in some cases calculating the Defocus node takes up to 50% of the render time, if you combine that with FSA (Full Sample Antialias, calculates compositing for each one of the samples, so if your OSA/FSA is in 8, it will do it 8 times..), it can go up to 80 or even 90% of the total render time. Ridiculous 
Possible solution? OpenCL! Tiled Compositing!, all the Blenderheads around the globe need you!, where do I have to donate?
Previsualization
Guess what we used for previz? Blender 2.49′s animation player! 
On my computer I even have set the ‘jpg’ and ‘png’ files to open with “blender -a” (where the 2.49b Blender binary is plus the ‘-a’ tag), so just opening one image from the sequence will load it.
But this player doesn’t scale down, so for that Blender 2.5 Video Sequencer was used. Since it frees memory nicely, not the case of the Image editor..
Some Needed Fixes
- Shadow Buffers!
- – Increase them, is veeery noticeable otherwise :/ takes a while, but is worth it.
- Nodes!
- – Some nodes were having issues on high resolutions, like FastGaussian’s Blur, it have been fixed in both Render Branch and Trunk, also the Defocus node wasn’t working properly on high res, didn’t scale well..
- Exploding Pixels!
- – Couldn’t really find out why, but has been fixed, some pixesl were extremely bright and when passed by nodes like glare, would explode in a nice rainbow effect, sadly not what the director was looking for 
- More super bright pixels
- – This ultrabright pixels (with float values of more than 10x!) sometimes need to be pulled down, otherwise they go over the range of colors and end up being a blueish spot in your render, there are many ways to do this but when you’re in a hurry the best thing is to just clamp them, so they are inside 0 and 1.0 again.
You can get this job done by using a “Separate RGBA” node, pass each channel (except alpha of course) through a “Map Value” node (enable Max and Min), then combine again with the “Combine RGBA”. This clamps the rgb values and they’re back to normal.
- Desert Shot
- – Missing lensflare and smoke sim :/
- Vignettes
- – Most of us used vignettes in the movie (dark corners and sometimes color correction), there are many ways to achieve this, some used a 4k image scaled down, some a 1k image scaled up, in my case I used an image at the beginning but switched to a Blend texture, since it scales automagically to the render-size, so is always right, and reliable.
But the images didn’t scale well before, had to be done manually and was a bit annoying, so I asked Diego Borghetti (bdiego) for a feature on the Scale node to scale whatever input to render-size, it worked out of the box and made its way to Render Branch and Trunk! thanks Diego 
- Workarounds..
- – There are many workarounds for bugs or non-bugs but weird behavior on some features, most of them are already reported and fixed but some old files in Sintel had this extra stuff that was messing with the final result, so these needed fixing..
One that is left is for example some times when you pass an image texture node through the Scale node, it outputs black.. no image, just black, so you need to convert this image first by passing it through a colorramp first.
Changes?
Not really I got tempted and fixed some alpha issues here and there, and the floating collar dilemma on Proog’s shot (turns out the Ishtarian girl had her neck-thingie controlled by a constraint, and its influence set to 0.5!, just moving it up to 1.0 fixed the issue) but that’s all, the movie will be same as 2k, perhaps if we scale this version down we get better AA, but still, same classic Sintel you saw months ago online
OK I *really* have to go to the airport now, catching a plane back home!
I never thought I would get involved in Durian, now Sintel, it was an amazing project and extremely fruitful experience.
Thanks to the community for making this an even more awesome Open Project, without your support and cheering all the time this kind of projects wouldn’t be close as fun as they are.
Gracias!
(now you know how to thank in Spanish!)
Pablo, venomgfx, the Pinkifier as they called me on the team ♥
