IAP@psuvm.psu.edu (Kirk Hanawalt) (02/08/90)
I'm trying to write a program (TurboC 2.0, IBM PS/2 Model 50
with VGA adapter, HP LaserJet II) to dump a bitmap image of the
high resolution VGA screen to a LaserJet printer. Using the
TurboC getpixel function, the 640x480 graphics screen can be
dumped in about 35 seconds. I would like to access memory
directly, and dispense with getpixel() altogether.
Unfortunately, I can't figure out how the pixels are mapped in
memory.
Starting at address A000:0000, each byte in memory seems to
indicate the on/off state of 8 pixels on the screen. For
example, the statement:
*((unsigned char far *) 0xa0000000L) = 0x80;
turns the first pixel in the first row of the graphics screen
on. I can use this method to get monochrome graphics.
Obviously, 16 color graphics requires 4 bits to specify pixel
color.
The problem is this: when using this method to read pixels
from the screen, I only get the odd color pixels (for the
default EGA/VGA palette 1=blue, 3=cyan,...,15=white). The even
color pixels do not have a bit set. The following pseudo-code
works only for odd colors, the even colors aren't printed.
ptr=(unsigned char huge *) 0xa0000000L;
for (j=0; j<=MaxY; j++)
for (i=0; i < (MaxX+1)/8; i++)
Send_To_Printer(*ptr++); /* send byte = 8 pixels */
This method dumps the screen to the LaserJet in about 15
seconds, so I would like to use it. However, getting only
half of the displayed colors printed is a drawback :-).
My question is this: where in the heck are the pixel values
stored in memory for the VGA adapter in 640x480 16 color mode?
Every book I've seen covers only monochrome graphics and uses
the method above to write/read pixels.
-------
Kirk Hanawalt | |
Chem. Eng. Dept. | BITNET: IAP @ PSUVM |
Penn. St. Univ. | OTHER: ...!psuvax1!iap@psuvm.bitnet |zmact61@tsun4.doc.ic.ac.uk.doc.ic.ac.uk (D Spinellis) (02/09/90)
In article <90039.091705IAP@PSUVM.BITNET> IAP@psuvm.psu.edu (Kirk Hanawalt) writes: > > I'm trying to write a program (TurboC 2.0, IBM PS/2 Model 50 >with VGA adapter, HP LaserJet II) to dump a bitmap image of the >high resolution VGA screen to a LaserJet printer. [...] >Unfortunately, I can't figure out how the pixels are mapped in >memory. [...] The EGA and VGA cards have all the color planes mapped into the same memory address. You specify from which color plane you want to read or write by outputing appropriate values to the various card controller registers. Different read and write modes give you the ability to access the planes together; naturally with some restrictions. For example you can write a color value to a specified pixel, or you can set all color planes to the same value. The Technical Reference Manual for the PS/2 systems covers these things. > The problem is this: when using this method to read pixels >from the screen, I only get the odd color pixels >The following pseudo-code >works only for odd colors, the even colors aren't printed. > > ptr=(unsigned char huge *) 0xa0000000L; > for (j=0; j<=MaxY; j++) > for (i=0; i < (MaxX+1)/8; i++) > Send_To_Printer(*ptr++); /* send byte = 8 pixels */ > [...] You need to modify your code to access the four color planes. Register 4 in address 0x3cf can be set to specify which plane you want to read. You must set the index register in address 0x3ce to 4 to point to the correct register. The following code should work: outp(0x3c4, 4); /* Set index register */ ptr=(unsigned char huge *) 0xa0000000L; for (j=0; j<=MaxY; j++) for (i=0; i < (MaxX+1)/8; i++, ptr++) for (k = 0; k < 4; k++) { outp(0x3cf, k); /* Specify color plane */ Send_To_Printer(*ptr, k); /* send byte = 8 pixels */ } Diomidis -- Diomidis Spinellis Internet: dds@cc.ic.ac.uk Department of Computing BITNET: dds@cc.ic.ac.uk Imperial College UUCP: ...!cernvax!cc.imperial.ac.uk!dds London SW7 2BZ JANET: dds@uk.ac.ic.cc
scjones@sdrc.UUCP (Larry Jones) (02/10/90)
In article <90039.091705IAP@PSUVM.BITNET>, IAP@psuvm.psu.edu (Kirk Hanawalt) writes: > I'm trying to write a program (TurboC 2.0, IBM PS/2 Model 50 > with VGA adapter, HP LaserJet II) to dump a bitmap image of the > high resolution VGA screen to a LaserJet printer. Using the > TurboC getpixel function, the 640x480 graphics screen can be > dumped in about 35 seconds. I would like to access memory > directly, and dispense with getpixel() altogether. > Unfortunately, I can't figure out how the pixels are mapped in > memory. Both the EGA and VGA need more memory than there is room for in the reserved address space. To get around the problem, they have a number of different mapping schemes which allow access to some of the bits at a time. Precisely which bits and where in memory they are depends on the particular mapping scheme which has been invoked. I suggest you get a good book such as Programmer's Guide to PC and PS/2 Video Systems (Microsoft Press, if I remember correctly) and study it. The mapping schemes are incredibly complex. ---- Larry Jones UUCP: uunet!sdrc!scjones SDRC scjones@SDRC.UU.NET 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 "You know how Einstein got bad grades as a kid? Well MINE are even WORSE!" -Calvin
lorena@infmx.UUCP (Lon Anderson) (02/13/90)
The VGA adapter uses a concept knows as memory planes. These memory planes map th the same location in memory and hold the "other" bits associated with a pixel in the color modes. You need to toggle the planes on and off to read the individual bits that make up the 4 bits of the pixel. This is done by manipulating the VGA registers. The code to do this is available in a number of books on VGA programming. Two I would recomend are Programmers guide to the EGA and VGA and Programming the VGA adapter by Brady. These are available at most good computer book stores.
greg@bluemtn.uucp (Greg Richter (2XS)) (02/14/90)
I recommend you take a look at "Programmer's Guide to the EGA/VGA" it explains these matters clearly, and in sufficient detail to be useful. At most bookstores, about 15 bucks. -GR
news@nems.dt.navy.mil (USENET News System) (02/26/90)
From: flitter@dtrc.dt.navy.mil (Flitter) Path: dtrc!flitter > My question is this: where in the heck are the pixel values >stored in memory for the VGA adapter in 640x480 16 color mode? >Every book I've seen covers only monochrome graphics and uses >the method above to write/read pixels. In 16 color mode there are four bit planes each of which are mapped into the same address space: A0000 - AFFFF. In order to get the value of a pixel you have to use the segment registers so you get all four bit planes. You will want to AND them together to find whether the monochrome pixel is on or off. I don't remember the proper settings off the top of my head. If you still need them write me a message and I'll look it up.