A full screen effect is a way in which computer graphics applications can have different special effects added to a scene. Rather than actually rendering a scene with these effects applied to the objects and geometry within it, they are essentially applied after the render. Which means the graphics program creates an image that the user sees, and then applies an effect over this in a way that is seamless. A full screen effect can be used to accomplish numerous tasks, including the addition of motion blur, bloom lighting, and color filtering.
To understand how computer graphics can use a full screen effect, it is often the easiest method to first realize how a scene appears. Programs that use Computer Generated Imagery, video games in particular, often render scenes to a display in real time. This means that as a player navigates through a virtual environment, the various objects in a scene that have been created by the developers of that game appear in relation to the player’s position. When the player walks into a room with a box, the game renders the walls, floor and ceiling, and the box in the room as a series of frames or images about 30 times every second.
A full screen effect can then be added to these individually rendered images to create various results. Motion blur, for example, is a phenomenon that can be seen in the real world or on film; objects often appear distorted and blurry as someone moves quickly past them. While this effect can be applied to objects in a virtual scene, it is often easier and less resource-intensive for it to be done as a full screen effect. Multiple partial renders of the objects in a game are created and overlapped so that a blurred image appears to be moving very fast.
Bloom lighting makes lights in a game appear heavier, to make them stand out, to make a game appear more realistic, give shiny textures of light to bright objects or to provide the graphics with a stylized aesthetic. When different light sources are rendered, the game engine then creates additional renders of increased intensity for the lights and then proceeds to overlap them. A player in a game can then see these lights as being brighter, with a stronger glow. A good example of this effect in use would be Runescape:
There is also such thing as simulated bloom effects, these are instances where particles are utilized to simulate bloom lighting around certain points of light, the developers do this by registering the particle count output.
Again from Runescape, on the left is how the fire already is, on the right is what a simulated bloom lighting effect would be like:
Color filtering is similarly done. If a game developer wants someone to see a room in black and white part of the time, without having to create multiple textures for objects within it, then this can be achieved through a full screen effect. While the actual textures in a scene are rendered properly, a filtered layer is placed over each frame to change the colors of objects for a player.
Depth blur recreates the effect caused by the optics of a lens. Images formed through a lens are in correct focus only when the subject is directly at a certain distance (the focal plane). Objects nearer or farther blur.Notice in this picture how the landscape in the distance is blurred, just like how we would see it in real life:
Often recreated in games by blurring the frame buffer to a temporary texture, and drawing over the frame buffer with that blurred version, alpha blending based on the depth of the scene.
At the end of the day, full screen effects is a fantastic way to either make your games more stylized or realistic.