Since a lot of people are searching for somebody who made a seamless piston door where you can't see the pistons even when it is opened... These are the first designs of a door like this. Basically the way it works is replacing the pistons with building blocks while the door is opened and pushing back the pistons before closing.

The designs are 1x2 doors.
Design 1:
Only for houses where you have to go to the side after you got in to reach the room itself. The fastest - opens and closes almost instantly.
This design also will not hide the pistons from the inside so when you close the door from the inside of the room you'll see a bunch of pistons, This design was more like a piston-timing test.
Design 2:
For more comfortable house - you can open the door and you get in directly into the room, everything is powered from behind one wall. Works a little bit slower but much more comfortable to use.
There is a NAND gate above the room, this one is to be able to use 2 different levers to open/close the door from inside and outside. You can move the levers however you want just make sure to connect them to the inputs of the NAND gate.
Design 3:
Works almost as fast as Design 1, it could work in maximal speed but I prefer to decrease the speed and make it to take less space on the roof. The main improvement is in the wall - The mechanism inside the wall was 3 blocks wide, now it is 1 block wide. This should solve the problem of the thick wall, but There is still a big mess above the roof.
Design 4:
Made the door to work in maximal speed possible for this size . (only 1 tick slower than Design 1) the mechanism takes 1 block height above the roof so you can make a 3-height roof and the mechanism inside it will be completely hidden. The sad thing is that there is still a 3x3 mechanism above the wall (not going out of the wall!) that ruins the awesomeness of this compact design =/ But it shouldn't be a problem to make a special shape for the roof that will hide it too.

Important: Both designs are not compacted yet, so you can take a challenge and compact it for yourself or just wait for me to release the compacted version of these designs.
I upload it for any use in Minecraft but not for sharing any version of it! This means that you are not allowed to upload to PMC or anywhere else my designs or compacted/improved version of them.