Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!oliveb!sun!pepper!cmcmanis
From: cmcmanis%pepper@Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga.tech
Subject: Re: bitmaps with intuition...
Message-ID: <56215@sun.uucp>
Date: 11 Jun 88 11:37:09 GMT
References: <4370@gryphon.CTS.COM> <3170@charon.unm.edu>
Sender: news@sun.uucp
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
Organization: Sun Microsystems, Mountain View
Lines: 65

In article <3170@charon.unm.edu> hansb@ariel.unm.edu.UUCP (Hans Bechtel) writes:
>Questions:
>
>how do I convert planeptrs to window's bitmap to common x,y 
>coordinates?

Bitmaps are *always* an even multiple of 16 bits wide even when the image
they contain or are displaying is not. So the Address of a pixel in the 
plane pointer is y * the width in bytes of the bitmap. The width can be
calculated with the formula :
	ByteWidth = (BitMap->Width + 15)/8;
So the byte containing the pixel of interest is x + y * ByteWidth. The 
actual bit that is the pixel at X,Y is calculated using :
	PixelBit = x % 8;

>how do I get the correct offsets to the window's bitmaps?

Using the two formulas above you can get the bit using this formula
	MyBit = *(BitMap->PlanePtr[n] + ByteWidth*y + x) >> (7 - PixelBit);
This will return a value of one or zero depending on if the pixel has
a bit set in this bitplane.

>This may all seem easy, but I have read and read the manuals until
>the guru finally wins...

>Please post the code to do the following:
>
>read a x,y pixel on all bitplanes of a window (not the readpixel way)

I haven't tried this so take it with a grain of salt but here goes ...

To read a pixel, assuming the following defines and your bitmap is pointed
to by a BitMap structure named BitMap :

#define PIXEL(n,x,y) 	*(BitMap->PlanePtr[n]+ByteWidth*y+x)
#define ThisBit(n,x,y)	PIXEL(n,x,y) >> (7-(x % 8));

Assuming ByteWidth had already been calculated. Use the statement ...
  for (Pixel=0, i=0; i<BitMap->Depth; i++) 
	Pixel += ThisBit(i,x,y) * (1<<i);

to read a pixel.

>write a x,y pixel "      "

Assuming the same defines above, an additional define like the one below
and the value you are writing is already in the variable Pixel ...
#define MoveBit(from,to,byte) (((byte >> (7-from)) & 1) << to)

  for (i=0; i<BitMap->Depth; i++) {
	PIXEL(i,x,y) &= ~(MoveBit(7,(x%8),1)); /* Mask off old value */
 	PIXEL(i,x,y) |= MoveBit(i,(x%8),Pixel); /* Or in new value */
  }

>get the offsets

Offset = ByteWidth*y + x;

>etc....

Have fun ...

--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.