1
2This is a first start for some documentation about frame buffer device
3internals.
4
5Geert Uytterhoeven <geert@linux-m68k.org>, 21 July 1998
6
7--------------------------------------------------------------------------------
8
9	    ***  STRUCTURES USED BY THE FRAME BUFFER DEVICE API  ***
10
11The following structures play a role in the game of frame buffer devices. They
12are defined in <linux/fb.h>.
13
141. Outside the kernel (user space)
15
16  - struct fb_fix_screeninfo
17
18    Device independent unchangeable information about a frame buffer device and
19    a specific video mode. This can be obtained using the FBIOGET_FSCREENINFO
20    ioctl.
21
22  - struct fb_var_screeninfo
23
24    Device independent changeable information about a frame buffer device and a
25    specific video mode. This can be obtained using the FBIOGET_VSCREENINFO
26    ioctl, and updated with the FBIOPUT_VSCREENINFO ioctl. If you want to pan
27    the screen only, you can use the FBIOPAN_DISPLAY ioctl.
28
29  - struct fb_cmap
30
31    Device independent colormap information. You can get and set the colormap
32    using the FBIOGETCMAP and FBIOPUTCMAP ioctls.
33
34
352. Inside the kernel
36
37  - struct fb_info
38
39    Generic information, API and low level information about a specific frame
40    buffer device instance (slot number, board address, ...).
41
42  - struct `par'
43
44    Device dependent information that uniquely defines the video mode for this
45    particular piece of hardware.
46
47  - struct display
48
49    Interface between the frame buffer device and the console driver.
50
51
52--------------------------------------------------------------------------------
53
54	    ***  VISUALS USED BY THE FRAME BUFFER DEVICE API  ***
55
56
57Monochrome (FB_VISUAL_MONO01 and FB_VISUAL_MONO10)
58-------------------------------------------------
59Each pixel is either black or white.
60
61
62Pseudo color (FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR)
63---------------------------------------------------------------------
64The whole pixel value is fed through a programmable lookup table that has one
65color (including red, green, and blue intensities) for each possible pixel
66value, and that color is displayed.
67
68
69True color (FB_VISUAL_TRUECOLOR)
70--------------------------------
71The pixel value is broken up into red, green, and blue fields.
72
73
74Direct color (FB_VISUAL_DIRECTCOLOR)
75------------------------------------
76The pixel value is broken up into red, green, and blue fields, each of which 
77are looked up in separate red, green, and blue lookup tables.
78
79
80Grayscale displays
81------------------
82Grayscale and static grayscale are special variants of pseudo color and static
83pseudo color, where the red, green and blue components are always equal to
84each other.
85
86