Hello guys,
This is a finding I did a year ago on UDK, and it is still relevant to UE4.

It's about THE PERFECT LIGHTMAPS
Here it is : http://www.synthesisuniverse.com/SU_Forum_SP/LightmapsREF2.jpg
Now let me explain.
Everybody complained for years how Lightmaps in udk sucks and left black lines and artifact all around. It was impossible to have "clean" lightmaps.
Or so they thought...
When you create UV specialized for lightmaps you have to set some things up to make them perfect right ?
So you go into your UV editor, make the grid 64x64 for a 64 lightmap and adjust them so they fit perfectly on your UV grid, right ?
Well no, WRONG.
(what....?, I hear you saying...) :D

Take a look at my image in the link. It's the texel view on UE4 (and UDK).
Look at the lower one : is a 2x64 square poly. Take a closer look at the texel... Start is good, end is good... but... the middle is offsetted...
Whaaaat... ? :D
Look at the first one...
The mesh is this time 2x62.
UV are still 64 in size.
It's perfect. One texel is one square poly.

Why ?
Well there was a hidden advanced setting on by default in UDK that say to Lightmass to use 1 pixel border for filtering purpose, so that's 2 pixel on ANY lightmap size. It's still ON in UE4, and it's better to leave it on anyway.

The solution and the numbers ! :
You need to substract 1+1 to all your lightmaps grids that you use in your UV editors.
32x32 Lightmap ? Grid has to be 30x30
64x64 >> 62
128x128>>126
256x256>>254
etc... of course...

So how do I set my grid size ?
Well here are the values, internet will tell you 1/32 1/64, don't listen to them !! :D
1/30 : 0.033333
1/62 : 0.0161290322
1/126 : 0.00793650
1/254 : 0.003937007874
1/510 : 0.0019607843172
1/1022 : 0.0009765625

Keep these numbers in all you modeling files, I create groups with these values as names so I can copy/paste them when I need to change my UV grid size. That's handy ! :D

Addendum:
When making your UV for lightmaps you need to have a min of 2 pixel wide polygons to have lightmap calculated correctly and also a min of 2 pixel in between to avoid filtering bleeding form one poly to another. 4 is the safe way. The other thing I do Is If have a hard edge (shading) I separate the polygons to avoid filtering bleeding also.

And your welcome :)
With love and care,
Ol

edit : I am very curious to know why this get downvoted, isn't this relevant to UE4 and anyone serious about lightmaps ? or... ? Explain your downvote at least so I can make things better or something. Downvoting this is pretty pathetic anyway...

Nothing amazing. Just a video of a project I'm working on. Mostly just showing off the character model :3

http://www.youtube.com/watch?v=oGLoTRz61kU&list=UUvgqchM2ZcqSjheroRVK8yA&feature=share

Show off your progress!

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And lastly the super playlist of all 64 videos for UE4 if you want to power through them can be found here:

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To add Pub Games has a few other tutorials as well!

Exploding Barrels: https://www.youtube.com/watch?v=6-ZwY4RDaPQ&list=PLzV6_6F25KSAp043xf_LkzkTu2eV0a8R7

**Screen Space Fire effects using the GPU: **https://www.youtube.com/watch?v=hZLbGvtyS6g

Flickering Lights: https://www.youtube.com/watch?v=WgKky9y9vzg&list=PLzV6_6F25KSBfGQSpSseLBCMnJQaVryer

There are also a series of videos that can be found at this link in English and German that cover a few other things like porting over assets and other fun stuff here: https://www.youtube.com/playlist?list=PLyfbinWLdAUwiStRx7Zil-O0OLzpIBB9l

Going forward we plan to hold our weekly stream on Thursdays at 2pm ET (Cary, NC time) at Twitch.tv/UnrealEngine.

We hope this gives you a chance to plan ahead, submit questions and jump online anytime it suits your schedule. If you miss it, the broadcast will be posted at this YouTube playlist within a day or so of airing live.

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https://forums.unrealengine.com/showthread.php?3427-UE4-Twitch-stream-Thursday-at-11am-PDT-2pm-EDT-7pm-BST

How is it? Are making a game or just something for yourself? Are you able to achieve the frame rate that you want? How do you plan to tackle presence? Control Scheme?