3.2. Video displays basics

A video display usually shows a rectangle of pixels. When connected to a computer, it is managed by a graphics chipset which indicates to the video display the color of each pixel of the current image to show several times per second.

The algorithm used by the graphics chipset is roughly:

For each pixel in the current image
   Read its color from memory
   Convert the color into something intelligible for the video display
   Send the converted pixel color to the video display

Wait until it's time to send the next image (vertical blanking interval)

3.2.1. Refresh rates

The number of images (or frames) sent to the video display depends on its refresh rate. Most video displays have a fixed (configurable) refresh rate. For instance 60Hz (60 frames per second).

Newer video displays may also support variable refresh rates where the delay between two frames may depend on the next frame being ready (or a timeout if it isn’t even after some time).

3.2.2. Switching between frames

If we modify the memory containing the pixel colors while the graphics chipset is reading it, the video display may show incoherent pixels (e.g. pixels from two different frames). This is called tearing. We usually want to avoid it.

The idea is to modify the pixel colors only while the graphics chipset is waiting between two transfers. For historical reasons this period of time is called the vertical blanking interval (VBI or VBLANK).

If the software can’t render a frame fast enough during the VBI, we usually want the previous frame to be sent again. In order to do that, we use two buffers (double-buffering): one buffer contains the current frame that is sent repeatedly to the video display and the other contains the frame the software is currently rendering. Once the next frame is ready we only have to switch the buffer pointer (called page-flipping) during the VBI to inverse the roles of the two buffers.

If the software renders frames too fast we can either block it until page-flipping occurs or we can use triple-buffering: one buffer contains the current frame as before, the second contains a pending frame (if any) which is a frame ready to be displayed, and the third one contains the frame being rendered as before. When the rendering in the third buffer is done, there is a switch between the second and the third buffer (i.e. one of the rendered frame may not be displayed at all if there was already a pending frame). During the VBI, if there is a pending frame in the second buffer, there is a switch between the first and the second buffers.

3.2.3. Plane composition

Instead of using a single source for the pixel colors, some graphics chipsets allow the use of several pixel sources that are blended/composed together.

A plane describes a portion of the video display surface that uses a specific source of pixel colors. Basic graphics chipsets only have a single primary plane that occupies the whole video display surface; other graphics chipsets may have several other planes with different properties (pixel formats, dimensions, rotation, scaling, etc.).

This is particularly useful for what is called hardware cursor. A small cursor plane is dedicated to display the mouse cursor so when the mouse moves we only have to change the position of the cursor plane independently of the other planes. It makes the cursor much more responsive because this operation is very cheap.

Planes can also be used to render hardware decoded videos, overlays, etc.

3.2.4. Summary

The graphics chipset sends the pixel colors from the memory to the connected video display several times per second (depending on the refresh rate).

The graphics chipset supports at least one primary plane but it can also support additional planes (overlay, cursor, etc.) with additional properties (scaling, rotation, different pixel format, etc.).

The software is responsible of producing pixel colors for each plane. To avoid tearing, the switch from one frame to the other must be done during the vertical blanking interval (VBI or VBLANK). Double- or triple-buffering can be used for this purpose.