BPPBMAP.TXT - Directly Accessing Memory Bitmaps with Borland PowerPack


Q:
I am using Borland PowerPack and trying to access a local memory grafMaps
mapTable and rowTables directly so that I can fool around with the bitmaps 
pixels myself. When I try to do this I get an exception. What's going on? 
This worked in real-mode.


A:
Ok, this is a bit complicated. When running under PowerPack, our code 
executes in a different address space than your program does.  We use a 
data segment that encompasses your data address space, as well as the 
graphics adapters bitmap address space.  When you allocate raster lines (or 
if we allocate raster lines) they will be in your address space.  In order 
to access them in our expanded address space, we need to add a delta to 
every address (32 bit offset) that you give us (or that malloc gives us). 
Once we have added this delta, we can access the data at your pointers. 
The drawback is that you can't anymore.  What you have to do is un-fixup 
what we have done in order to get the memory pointers back to their 
original values.  To do this, you need to get the value in the mapAltMgr 
field of the grafMap.  Subtract this value from all mapTable and rowtable 
values and they will be good pointers for your program again.  But don't do 
it directly or we will no longer be able to access the bitmap, so make a 
copy of the maptable and rowTable pointer that you modify.  For example:


  grafPort *myPortPtr; // pointer to grafPort
  grafMap  *myMapPtr;  // pointer to graMap
  Byte    **rowTable;  // pointer to rowTable array of raster line pointers
  Byte     *pixelPtr;  // pointer to start of 1st raster line data


  // create the local memory bitmap (return a port pointer)
  myPortPtr = CreateBitmap(...);

  // get the pointer to the grafMap structure for the bitmap
  myMapPtr  = myPortPtr->portMap;

  // get address to the bitmap rowTable (table of pointers to each raster)
  // (fixup mapTable value since it's relative to MW's private selector)
  rowTable  = (Byte **)( (long)myMapPtr->mapTable[0] -
                         (long)myMapPtr->mapAltMgr );

  // get address to the first raster line of pixels
  // (fixup rowTable value since it's relative to MW's private selector)
  pixelPtr  = (Byte *)( (long)*rowTable - (long)myMapPtr->mapAltMgr );


/* End of File - BPPBMAP.TXT */