🚀 go-pugleaf

saxena@bohr.physics.purdue.edu (Virendra K. Saxena) writes:

>	Ok, I know that before the VGA came out, people actually programmed
>for the EGA card.  Does anyone have any code for fast bitmap writes in EGA.

    Not I.

>	Also, a couple of days ago, someone posted some DMA code for VGA
>bitmap routines(x by y array of colors, and then DMA the color value to VRAM).
>What is the difference between this and something like xblt?  

   They're basically the same... I'm not sure what you mean by 'xblt'
   but... There are a bunch of name for "putting a block of pixel values
   on the screen":  BitBlt, sprite, icon, BitBlock, Image... The various
   forms differ in:
    -Pixel data format (Run-length encoded, planar, packed, encoded, masked)
    -Type of image (A block of pixels, an odd-shaped hero...)
    -The video mode (ModeX, EGA, VGA 13h)
    -Collision detection

    The code for the (x by y array of colors) fell into this catagory:
     -Packed 8 bit per pixel row column order data.
     -Destination: VGA mode 13h
     -Assumes a rectangular block of pixels.

    I have code for ModeX planar sprites, which more or less means fairly
    fast, can handle non-rectangular sprites, can check for collisions.

>When I try to
>write many bitmaps to the screen one after the other, there is a lot of 
>flickering.  Why does this happen?

    Because of the horizontal retrace on the montitor... The electron
    gun sweeps down the monito while drawing the pixels, and then disappears
     and goes up to the top again.  It does the at 50-70 times a second.
     Lets says the gun is going down and is at row 30.  You draw a figure 
     on rows 10-20 and it won't appear until the gun goes all the way
     through the screen again (1/50 of a second later). Lets say you wait
     for a little bit of time, and then erase it, preparing to draw the
     figure in its next position... But, wonder if the gun doesn't pass
     through the figure again until after you erase it? Then the figure
     will not appear, and the next time it might, which will give the
     appearence of flicker... There are many different ways flicker can
     happen... just remember that slow electron gun sweeping down and
     redrawing what is *CURRENTLY* in memory.

     How to stop it?
     1. Wait for the electron gun to get to the bottom of the screen
	and then do all the redraws before it gets back up to the top.
     2. Or, use two pages... Draw on one while the gun shows the other,
	then flip 'em after waiting for one verticle retrace period.
	 (To make sure the page got displayed.)
     3. Or, use three pages... Then you don't need to wait for verticle
	retrace.

     Hope this helps.

-- 
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  John A. Slagel, email: jas37876@uxa.cso.uiuc.edu