fb.modes - frame buffer modes file


DESCRIPTION

       /etc/fb.modes  contains  an unlimited number of video mode
       descriptions. The general format of a video mode is:

       mode
              geometry <xres> <yres> <vxres> <vyres> <depth>
              timings <pixclock> <left> <right>  <upper>  <lower>
              <hslen> <vslen>
              options <value>
       endmode


OPTIONS

       geometry options:

              xres   visible horizontal resolution (in pixels)

              yres   visible vertical resolution (in pixels)

              vxres  virtual horizontal resolution (in pixels)

              vyres  virtual vertical resolution (in pixels)

              depth  display depth (in bits per pixel)

       timing options:

              pixclock
                     length of one pixel (in picoseconds)

              left   left margin (in pixels)

              right  right margin (in pixels)

              upper  upper margin (in pixel lines)

              lower  lower margin (in pixel lines)

              hslen  horizontal sync length (in pixels)

              vslen  vertical sync length (in pixel lines)

       other options:
              the first value of this options is the default

              hsync {low|high}
                     the horizontal sync polarity

              vsync {low|high}
                     the vertical sync polarity

                     the composite sync polarity

              extsync {false|true}
                     enable   or   disable  external  resync.  If
                     enabled the sync timings are  not  generated
                     by  the frame buffer device and must be pro­
                     vided externally  instead.  Note  that  this
                     option  may  not be supported by every frame
                     buffer device

              laced {false|true}
                     enable or disable interlace. If enabled  the
                     display  will  be  split in two frames, each
                     frame  contains  only  even  and  odd  lines
                     respectively.  These two frames will be dis­
                     played alternating, this way twice the lines
                     can  be displayed and the vertical frequency
                     for monitor stays the same, but the  visible
                     vertical frequency gets halved

              double {false|true}
                     enable  or  disable  doublescan.  If enabled
                     every line will be displayed twice and  this
                     way  the  horizontal frequency can easily be
                     doubled, so that the same resolution can  be
                     displayed on different monitors, even if the
                     horizontal frequency specification  differs.
                     Note  that  this option may not be supported
                     by every frame buffer device


INTERNALS

       Generally a frame buffer display is organized as follows:

                     +---+---------------+---+---+
                     |   |       ^       |   |   |
                     |   |       | 5     |   |   |
                     |   |       v       |   |   |
                     +---#################---+---+
                     |   #       ^       #   |   |
                     |   #       |       #   |   |
                     |   #       |       #   |   |
                     | 1 #       |   2   # 3 | 4 |
                     |<->#<------+------>#<->|<->|
                     |   #       |       #   |   |
                     |   #       | 6     #   |   |
                     |   #       |       #   |   |
                     |   #       v       #   |   |
                     +---#################---+---+
                     |   |       ^       |   |   |
                     |   |       | 7     |   |   |
                     |   |       v       |   |   |
                     +---+---------------+---+---+
                     |   |       ^       |   |   |

                     |   |       v       |   |   |
                     +---+---------------+---+---+

              1  left margin
              2  xres
              3  right margin
              4  horizontal sync len
              5  upper margin
              6  yres
              7  lower margin
              8  vertical sync len

       The area bordered with `#' is the  visible  display  area.
       Horizontal and vertical frequencies can now easily be cal­
       culated, for this the sum of horizontal or vertical values
       are important

              htotal = left + xres + right + hslen
              vtotal = upper + yres + lower + vslen

       The length of one line can now be calculated with pixclock

              line = pixclock * htotal

       and we have the horizontal frequency

              hfreq = 1E12 / line = 1E12 / (pixclock * htotal)

       To get  the  vertical  frequency  vtotal  must  eventually
       adjusted.  If  the display is laced, vtotal must be halved
       or if the display is a doublescan one, vtotal must be dou­
       bled. Now we can calculate the length of one frame

              if (lace)   vtotal /= 2
              if (double) vtotal *= 2

              frame = vtotal * line

       and we get also the vertical frequency

              vfreq = 1E12 / frame = hfreq / vtotal


SEE ALSO

       fbset(8), fbdev(4)


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