fb.modes - Online Linux Manual Page

Section : 5
Updated : 2003-08-07
Source : 2.1
Note : Linux frame buffer utils

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 .RI

geometry

xres

yres

vxres

vyres

depth

timings

pixclock

left

right

upper

lower

hslen

vslen

options

value

rgba

red

green

blue

alpha

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)

rgba options (only valid with truecolor):

red

red color bitfields (in length or length/offset)

green

green color bitfields (in length or length/offset)

blue

blue color bitfields (in length or length/offset)

alpha

alpha color bitfields (in length or length/offset)

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

gsync

{

low

high

}

the sync on green polarity

bcast

{

false

true

}

enable or disable broadcast modes. If enabled the frame buffer generates the exact timings fot several broadcast modes (e.g. PAL or NTSC). Note that this option may not be supported by every frame buffer

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

sync

number

custom sync flags (frame buffer driver dependent)

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

nostd

number

select nonstandard video mode

accel

{

false

true

}

enable or disable hardware text acceleration

grayscale

{

false

true

}

enable or disable graylevels instead of colors

INTERNALS

Generally a frame buffer display is organized as follows: +−−−+−−−−−−−−−−−−−−−+−−−+−−−+

| |

| | |

| |

| | |

| |

| | |

+−−−#################−−−+−−−+

| #

# | |

| #

# | |

| #

# | |

<−>

<−−−−−−+−−−−−−>

<−>

| #

# | |

| #

# | |

| #

# | |

| #

# | |

+−−−#################−−−+−−−+

| |

| | |

| |

| | |

| |

| | |

+−−−+−−−−−−−−−−−−−−−+−−−+−−−+

| |

| | |

| |

| | |

| |

| | |

+−−−+−−−−−−−−−−−−−−−+−−−+−−−+ .ad n

left margin

xres

right margin

horizontal sync len

upper margin

yres

lower margin

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