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Graphics Rendering

FrameBuffer Displaying


Buffer Mapping

  • Mapping buffers in user-space
  • Raw drawing
  • Example rectangle
  • Dirty fb

Configuring The Display

  • Mode settings
  • automatic configuration for the connected devices in preferred modes, no cloning

Setting the display mode

The following code shows a code that tries every available mode of a connector in sequence: each mode stays enabled for 4 seconds. To be allowed to change the display mode, we need to create an adapted framebuffer (i.e., a source of pixel data). You can safely ignore that for now, we will explain it in details in Section~ref{sec:framebuffer}.

-- codes/src/DisplayModes.hs
  • mode not supported by display device: garbage, black screen, error screen

Drawing On The Screen

  • Generic framebuffer
  • full drawing example
  • Example DPMS (power-off)

Multiple-Buffering And V-Blank Synchronization

Computer screens usually have a fixed refresh rate. For instance in Listing~ref{lst:display_connectors_result} the first mode of the connected display device has a (vertical) refresh rate of 60Hz. That is, 60 times per second, the display device:

  • copies the contents of the framebuffer on the screen line-by-line: scan-out period
  • waits for some time until the next scan-out: v-blank period

Single Frame Buffer

Suppose we use a single frame buffer that is displayed on the screen. If we modify its content during the scan-out period, some artifacts may appear on the screen. For instance if we repeatedly clear the screen, then draw a filled rectangle and finally draw a circle on top of it, the display may either show the cleared screen, the rectangle alone or the rectangle with the circle. The rectangle and the circle seem to flash: this is called a flicker effect.

  • time to render a frame vs refresh period vs v-blank period
  • Explanation scan-out (flikering?)
  • Explanation multi-buffering
  • Code framebuffer swap ("page flip")
  • Explanation v-blank (tearing?)
  • Code synchro v-blank (event v-blank)
  • Note "async page flip" flag and "page flip complete" event
  • Adaptive v-sync
  • Dithering (frame rate control, TN panels 6-bits per RGB)

Advanced Topics

Atomic Configuration

Some drivers support an atomic configuration operation. Instead of setting a property at a time, several properties are set at once in a transaction: if a proprety fails to be set, the whole transaction is reverted. It ensures that the display configuration is never left in a transition state between two configurations.footnote{At the time of writing, haskus-system doesn't support this feature.}

Gamma Table

  • theory, tests avec qemu non concluants

Gamma correction consists in altering the way colors are displayed on a monitor. For each possible value of each color channel (red, green, blue),we can define a gamma factor. Usually there are 256 possible values per channel, but the additional texttt{controllerGammaTableSize} field gives the actual number of values. Each factor is an unsigned 16-bit word.

  • Word16 or 8.8 fixed float?

The following code shows how to retrieve and show the gamma look-up table of a controller with getControllerGamma. Similarly you can set another gamma table with setControllerGamma.

-- codes/src/DisplayGamma

Sub-Pixel Rendering

  • Controller sub-pixel
  • Used for fonts
  • Vertical vs horizontal
  • rotation property!