Provided by: libsvga1-dev_1.4.3-27ubuntu1_i386
vga_accel - calls the graphics accelerator
int vga_accel(unsigned operation, ...);
This is the major function of the new accelerator interface which was
sketched in version 1.2.3 (Michael: Hmm, it must have been later) but
was implemented much later.
The main goal is to define functions that can be used as part of
certain kinds of interesting graphical operations (not necessarily
interesting primitives on their own). Obvious useful primitives in
their own are FillBox, ScreenCopy, DrawHLineList (solid polygon),
An interesting purpose is the fast drawing of color bitmaps, both
straight and transparent (masked, certain color not written). For
masked bitmaps ("sprites"), there is a number of possible methods, the
availability of which depends on the chips. Caching in non-visible
video memory is often useful. One way is to use a transparency color
compare feature of a BITBLT chip, either transferring the image from
system memory or cached in video memory. If transparency compare is
not available, it may be possible to first clear (zeroe) the mask in
the destination area, and then use BITBLT raster-operation to OR the
image into the destination (this requires the mask color to be 0). A
higher level (library) interface should control this kind of operation.
vga.h contains several macros which may be used for operation. Most of
them accept several optional arguments which you may specify after
them. The accel(6) svgalib demo shows basic usage of this function. The
function returns -1 if the operation is not available and 0 if it is
(or better: wasi and could be performed).
Currently the following parameters for vga_accel() are defined:
vga_accel(ACCEL_FILLBOX, int x, int y, int w, int h)
Simple solid fill of a box at pixels x, y with width w and
height h in the current foreground color
vga_accel(ACCEL_SCREENCOPY, int x1, int y1, int x2, int y2, int w, int
Simple screen-to-screen blit. It copies a box of width w and
height h pixels from position x1, y1 to position x2, y2. You
may assume that the copy is non corrupting in case of
overlapping source and destination areas.
vga_accel(ACCEL_SCREENCOPYMONO, int x1, int y1, int x2, int y2, int w,
Monochrome screen-to-screen blit. It copies a box of width w and
height h pixels from position x1, y1 to position x2, y2.
However, each pixel will all bits set to 0 is drawn in the
background color, each pixel with all bits set to 1 is drawn in
the foreground color. To allow many different architectures
supporting this routine, behaviour is undefined for other
values. Bitmap transparency might be supported as well.
You should not expect ACCEL_SCREENCOPYBITMAP handling
overlapping screen areas gracefully.
vga_accel(ACCEL_PUTIMAGE, int x, int y, int w, int h, void *p)
Straight image transfer. It fills the given box with the data in
memory area p. The memory buffer must contain the pixels in the
same representation as used in the vga memory, starting at the
top left corner, from left to right, and then, line by line,
from up to down, without any gaps and interline spaces.
vga_accel(ACCEL_DRAWLINE, int x1, int y1, int x2, int y2))
General line draw. Draws a line from x1, y1 to position x2, y2
in the foreground color. You should not expect the reverse line
from x2, y2 to position x1, y1 to use the exact same pixels on
the screen. Several, esp. hardware, algorithms tend to yield to
vga_accel(ACCEL_SETFGCOLOR, int color)
Sets foreground color. It is used by most other draw commands.
vga_accel(ACCEL_SETBGCOLOR, int color)
Set background color. It is used by draw commands which might
vga_accel(ACCEL_SETTRANSPARENCY, int mode, ...)
Set transparency mode, see the table below for an explanation
vga_accel(ACCEL_SETRASTEROP, int mode)
Set raster-operation, see the table below for an explanation of
vga_accel(ACCEL_PUTBITMAP, int x, int y, int w, int h, void *p)
Color-expand bitmap. This works similar to ACCEL_PUTIMAGE but
the bitmap *p is a one bit bitmap. Each pixel related to a set
bit in *p is drawn in the foreground color, the other pixels are
drawn in the background color.
Each byte at *p contains 8 pixels. The lowest order bit of each
byte is leftmost on the screen (contrary to the VGA tradition),
irrespective of the bitmap bit order flag. Each scanline is
aligned to a multiple of 32-bits.
If the transparency mode is enabled (irrespective of the
transparency color), then bits that are zero in the bitmap are
not written (the background color is not used).
vga_accel(ACCEL_SCREENCOPYBITMAP, int x1, int y1, int x2, int y2, int
w, int h)
Color-expand from screen. This works similar to ACCEL_PUTBITMAP
but the bitmap lies at position x1, y1 and the destination box
at x2, y2.
Alas, the sizes of the pixels in both bitmap are different. The
bitmap *p must have the format corresponding to ACCEL_PUTBITMAP
but will start at the screen memory location where the pixel
(x1, y1) would be (probably in off screen memory).
In modes where pixel will not start at byte boundaries
(typically those with less then 256 colors), the pixel (x1, y1)
must start at a byte boundary (for example in a 16 color mode
(4bpp rather than 8bpp for 256 colors) this means that x1 should
be an even number).
The easiest way to achieve this is probably to choose x1 == 0 in
You should not expect ACCEL_SCREENCOPYBITMAP handling
overlapping screen areas gracefully.
vga_accel(ACCEL_DRAWHLINELIST, int y, int n, int *x1, int *x2)
Draw horizontal spans. Each of the *x1 and *x2 arrays contains n
x-coordinate pairs. Starting with a horizontal line from *x1,y
to *x2,y consecutive horizontal lines (with increasing y values)
are drawn using the start and end points in *x1 and *x2. This
is usually a very quick operation and useful to draw arbitrary
polygons (when the accelerator cannot do an arbitrary polygon
vga_accel(ACCEL_POLYLINE, int flag, int n, unsigned short *coords)
draws a contiguous line through the n points listed in *coords.
*coords contains n pairs of shorts, the first is the x
coordinate, the second is the y coordinate.
Normally flag should have the value ACCEL_START | ACCEL_END.
However, if the evaluation of the points is costly, you can mix
calculations and drawings. Your first call to
vga_accel(ACCEL_POLYLINE, ...) must have ACCEL_START set. This
will initialize the accelerator. If you do not specify
ACCEL_END, you can (actually you have to) follow your call with
another vga_accel(ACCEL_POLYLINE, ...) call which will give
additional points to connect.
It is important that no other operations (even no color
settings) take place between a call with ACCEL_START and the one
with the corresponding ACCEL_END. Because of this, it is also
important that you lock the console with vga_lockvc(3) and
vga_unlockvc(3), s.t. you cannot be interrupted by a console
It is allowed not to set ACCEL_END for your last call to
...).Thiswillnotdrawthelastpixelofthelast line which is
important for some raster operations when drawing closed
polygons. The accelerator will automatically deinitialize when
called with another operation in this situation.
It is undefined what happens when you specify other values for
flag and when your polyline contains only a single point. The
line segments must also not be of length zero.
For implementors: In conjunction with raster operations
(ROP_XOR, ROP_INV) it is important that endpoints of inner line
section are only drawn once. If you cannot achieve that, you
must signal that this function cannot be used in conjunction
with raster operations. In this case it is valid to always draw
all points of the line segments including the endpoints
regardless of the existence of a ACCEL_END parameter.
vga_accel(ACCEL_POLYHLINE, int flag, int y, int n, unsigned short
This function combines the features of ACCEL_POLYLINE and
ACCEL_DRAWHLINELIST. Starting in row y horizontal lines are
drawn from top to bottom. For each horizontal scanline the
*coords array will contain a number m followed by m x
coordinates in left to right order. Horizontal lines are drawn
between the first and the second, the third and the fourth x
coordinates, and so on. If the m coordinates are exhausted, y is
increased, a new number m is read from the *coords array and
This procedure is done for n scan lines.
In addition there is a flag parameter which works similar to
ACCEL_POLYLINE. Your first call to ACCEL_DRAWHLINELIST must
have the ACCEL_START bit set for proper initialization. The y
parameter is ignored when ACCEL_START is not given.
On contrary to ACCEL_POLYLINE it is required that the last call
has the ACCEL_END bit set.
The function is intended for drawing complex filled polygons
using horizontal scanlines. By issuing small and fast calls for
few scanlines only it is possible to intermix drawing and
The operation of ACCEL_POLYHLINE is undefined if the x
coordinates are not sorted from left to right or there are zero
length segments in any scan line or if n or one of the m
counters are zero, or one of the m’s is not even.
Switches polygon fill mode on (onoff non-zero) or off.
When in polygon fill mode, ACCEL_DRAWLINE and ACCEL_POLYLINE
will only draw a single point on each scanline of each line
segment. ACCEL_SCREENCOPYMONO will horizontally scan it’s
source area and start drawing in the foreground color when it
encounters a set pixel. When the next pixel is encountered, it
will start using the background color and so on.
This can be used for hardware filled polygons:
1. Enable polygon fill mode.
2. Fill an offscreen rectangular area with a the color with
all bits zero (usually black).
3. Draw a (usually closed) polygon outline in this offscreen
area in the color with all bits set (usually white). To
get the proper bits set for the polygon outline, it is
recommended to use ROP_XOR s.t. outlines intersecting in
a single point are handled correctly. To ensure that
polygon corners are handled right, both start and end
points must be drawn (in ROP_XOR mode). Thus it is best
to use ACCEL_DRAWLINE instead of ACCEL_POLYLINE.
Finally, skip drawing all horizontal lines (which would
4. Set fore- and background colors, raster operation, bitmap
transparency to those you want for your polygon.
5. Use ACCEL_SCREENCOPYMONO to copy the offscreen pattern to
The rasteroperations and transparency which are signalled to be
supported for ACCEL_POLYFILLMODE by vga_ext_set(3) are actually
meant to apply to the last ACCEL_SCREENCOPYMONO call.
Because this polygon drawing uses more screen read/write
operations it is probably slower than using ACCEL_DRAWHLINELIST
or ACCEL_POLYHLINE for drawing a polygon scanline by scanline.
However, it is easier to use and it will work mostly without
intervention of the CPU which can do other calculations then.
See BUGS below.
It is unspecified if the left or right end points of the
scanlines are drawn, and most probably some cards (like Mach32)
will omit them on one end, at least. Because of that you should
always draw the boundary line in the fill color (or another
color) after filling the polygon.
Set blit strategy. There are two choices for mode, namely
BLITS_SYNC and BLITS_IN_BACKGROUND. The first ensures that a
vga_accel() call only returns when the accelerator has finished
its operation. The second allows for an immediate return and
thus allows parallel operation of the CPU and the accelerator.
Consecutive accelerator operations will wait for each other to
complete (and block if necessary). However, direct screen memory
access (also when done implicitly by some call to an svgalib
function) may find any intermediate state in vga memory or even
corrupt the running accelerator operation.
Wait for accelerator to finish when in
vga_accel(ACCEL_SETOFFSET, int address)
set a screen offset as vga_setdisplaystart(3) does. The same
restrictions for this function as reported by vga_getmodeinfo(3)
apply to address.
Whenever the video screen offset is modified, the accelerator’s
offset will follow. However you can modify it later with this
The following mode values are defined for
vga_accel(ACCEL_SETTRANSPARENCY, int mode, ...)
vga_accel(ACCEL_SETTRANSPARENCY, ENABLE_TRANSPARENCY_COLOR, int color)
Whenever one of the vga_accel() operations would draw a pixel in
color color, no operation is performed and the destination pixel
is left unchanged. In fact that color is defined to be
disables the previous functionality.
in the bitmap expanding operations ACCEL_PUTBITMAP and
ACCEL_SCREENCOPYBITMAP whenever a non set bit is encountered, to
not perform any draw operation. The 0 bits do not draw in the
background color. Instead they are defined to be transparent.
disables the previous functionality.
The following mode values are defined for vga_accel(ACCEL_SETRASTEROP,
Straight copy. Pixels drawn by vga_accel() replace the
Logical or. Pixels drawn by vga_accel() are logical (bitwise)
ored to the destination.
Logical and. Pixels drawn by vga_accel() are logical (bitwise)
anded to the destination.
Logical exclusive or. Pixels drawn by vga_accel() are logical
(bitwise) exclusive ored to the destination (bits set in the
drawn pixels flip those pits in the destination).
Inversion. Pixels drawn by vga_accel() are inverted. Which color
is drawn is actually ignored. Any pixel which would be
overwritten is simply inverted (bitwise) instead.
IMPORTANT! Please note that a 0 returned by
vga_accel(ACCEL_SETTRANSPARENCY, int mode, ...) and
vga_accel(ACCEL_SETRASTEROP, int mode) simply means that the set
function is available (and thus probably some of above features) but
only partial functionality may be available. The VGA_AVAIL_ROPMODES
and VGA_AVAIL_TRANSMODES subfunctions of vga_ext_set(3) allow you to
check for valid parameters. The VGA_AVAIL_ROP and
VGA_AVAIL_TRANSPARENCY subfunctions return which of the vga_accel
operations are actually affected by these set functions.
Instead of calling vga_accel() for each operation to find out if it is
supported, you can call:
int vga_ext_set(VGA_EXT_AVAILABLE, VGA_AVAIL_ACCEL)
When the logical bitwise and of the return value with one of the
following predefined (one bit set only) integer constants is non zero,
the corresponding operation is available: ACCELFLAG_FILLBOX,
ACCELFLAG_SCREENCOPY, ACCELFLAG_PUTIMAGE, ACCELFLAG_DRAWLINE,
ACCELFLAG_SETFGCOLOR, ACCELFLAG_SETBGCOLOR, ACCELFLAG_SETTRANSPARENCY,
ACCELFLAG_SETRASTEROP, ACCELFLAG_PUTBITMAP, ACCELFLAG_SCREENCOPYBITMAP,
ACCELFLAG_DRAWHLINELIST, ACCELFLAG_SETMODE and ACCELFLAG_SYNC.
In addition, calling
int vga_ext_set(VGA_EXT_AVAILABLE, VGA_AVAIL_TRANSPARENCY)
int vga_ext_set(VGA_EXT_AVAILABLE, VGA_AVAIL_ROP)
does not list the supported values for raster operations and
transparency but instead returns the ACCELFLAG_ values for the
accelerator operations which respond the raster operation resp.
The availability of the operations will usually depend on the current
video mode selected. You should not try to use them or check for
availability prior to selecting the mode you want to use with
I found the Mach32 buggy in that it occasionally omits drawing last
pixels of lines when in polygon fill modes (that means, a single point
for the last scanline touched by a line). Obviously this confuses the
polygon fill hardware. However, screen corruption will always be
restricted to a small area as ACCEL_SCREENCOPYMONO will work only on a
limited area. It is not clear if this is a driver error, but it seems
to be a hardware bug, and I don’t know a clutch to avoid it yet. In
case you experience problems with certain applications, try blit
nopolyfillmode in the configuration file or the SVGALIB_CONFIG
You must ensure that the given screen coordinates lie in screen memory.
Actually you may not really be sure how offscreen areas are handled,
you can only really trust that coordinates which are visible are
supported. For example, the Mach32 restricts the allowable x and y
coordinates to the range -512 .. 1535. However, even on a 1MB VGA
memory card, the offscreen point (0, 1599) would identify a valid
screen memory location (if you could use it).
Where supported, the vga_accel(ACCEL_SETOFFSET, ...) directive might
help to ease things a bit in such situations.
Svgalib’s accelerator support is a mess. Right now, only the Ark, the
Cirrus, the Chips&Technologies, and the Mach32 svga drivers really
support this function. The Mach32 still also supports the old style
accelerator functions vga_bitblt(3), vga_blitwait(3), vga_fillblt(3),
vga_hlinelistblt(3) and vga_imageblt(3) which were first designed for
the Cirrus cards and thus the Mach32 has its problems emulating them.
The gl_ functions use the accelerator to some extend. Currently the use
both the new and the old style accelerator. You should avoid mixing
calls of the new and the old style kinds.
These functions are not well tested. You should expect weird bugs. In
any case, the accelerator is of not much use in many typical svgalib
applications. Best if you are not using them.
BEWARE! You should not use the graphics accelerator together with the
background feature of vga_runinbackground(3). However, you can try
using vga_lockvc(3) to lock the vc prior to using the accelerator.
The Mach32 driver does this on it’s own, and even keeps the console
locked while background accelerator functions are in progress. Other
drivers might not be as graceful.
svgalib(7), vgagl(7), libvga.config(5), accel(6), vga_bitblt(3),
vga_blitwait(3), vga_ext_set(3), vga_fillblt(3), vga_getmodeinfo(3),
vga_hlinelistblt(3), vga_imageblt(3), vga_runinbackground(3),
This manual page was edited by Michael Weller <firstname.lastname@example.org-
essen.de>. The exact source of the referenced function as well as of
the original documentation is unknown.
It is very likely that both are at least to some extent are due to Harm
Occasionally this might be wrong. I hereby asked to be excused by the
original author and will happily accept any additions or corrections to
this first version of the svgalib manual.