fb.modes(8)                                         Linux frame buffer utils                                         fb.modes(8)



NAME
       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

              csync {low|high}
                     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 provided 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 displayed 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       |   |   |
                                                     +---+---------------+---+---+
                                                     |   |       ^       |   |   |
                                                     |   |       | 8     |   |   |
                                                     |   |       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 calculated,
       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 doubled. 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)



local                                                       Aug 1996                                                 fb.modes(8)