A post-process is, very basically, an effect that is applied to all or parts of the image of the screen at some part of the rendering process. Try to see it as a layer in Photoshop to which you apply a filter. You can have this filter apply to part of the image or all of it.
In Piczle Colors, for example, the title screen showed Score-chan painting a trail of Piczle Paint 3000 in the air changing everything behind it to black & white. This was done with a post process material.
The paint trail is a 3D object, which is rendered only in the depth buffer. Using a stencil that area of the screen is turned monochrome (and adds some contrast). I also subtract the depth buffer stencils of the characters so that the parts that appear in front of the characters turn them black and white, but if the paint trail is behind them it won’t.
(NOTE: To give a little definition to the paint trail there is a second copy of the model as well with a material that adds some pre-rendered occlusion to the curves and shapes.)
Piczle Cross Adventure uses post processes sparingly. There are only a few. In a previous post I explained how I had to physically place the entire interface in the world to make sure the CRT effect applied to everything, including the UI. That CRT effect is in fact 2 post processes.
To give the players the option of turning off parts of the CRT overlay to their own liking, the scanlines and the slight curved monitor distortion are done as two separate post processes. If the player wants no scanlines or distortion at all both are turned off. With only scanlines one of the processes is turned on, and with maximum retro-stylings both are turned on.
One more example of a post-process in the game is in the spooky area of the world. It is an absolute requirement that spooky areas have random thunder and lightning. At irregular intervals the screen will flash and a rolling thunder can be heard. The flashing effect is achieved by a post process as well, cranking up the contrast and recolouring the scene for an instant. As post processes affect everything in the world, I had to make sure to add a little failsafe to stop this random process from happening if you’re playing a puzzle or are looking at the in-game menu while the player is in this area.
(NOTE: For people sensitive to flashing images, the “reduce flashing” options in the game will, amongst other little things, stop the screen from flashing with lightning in this area and only play the rolling thunder sound instead)
Other special colouring effects in the game are done merely with materials that use Scene Color nodes to change part of the screen black & white. The unsolved puzzles’ distortion field, for example, or any Piczle Dust 3000 laser gun effect that turns the immediate area black & white.
Post-processes are a powerful tool with which to add specific effects or image adjustments to your game. They can be quite expensive, though, in terms of screen updates and memory. Sometimes, though, the specific effect you want to achieve cannot quite be done through materials alone, at which point post-processes can be a handy tool.