Graphic Erase

[Gary Beene]

You can put the graphic target into a memory bitmap, change all pixels of the line color, and then copy the memory bitmap back to the target.

This was given as an example in the help file.

' Change all red pixels to blue
 LOCAL PixelPtr AS LONG PTR
 GRAPHIC GET BITS TO bmp$
 xsize& = CVL(bmp$,1)
 ysize& = CVL(bmp$,5)
 PixelPtr = STRPTR(bmp$) + 8
 FOR i& = 1 TO xsize& * ysize&
   IF @PixelPtr = BGR(%red) THEN @PixelPtr = BGR(%blue)
   INCR PixelPtr
 NEXT
 GRAPHIC SET BITS bmp$

This would be useful if the pixels making up the line(s) to be erased are the only pixels with that color, or are bounded within a definable area.


On first guess, it's probably slower than redoing multiple lines but it has the advantage of not having to know anything about the lines except their color.

It also has the advantage that if your "bad" line overlap your "good" lines, you won't see wrongly colored pixels where the bad lines were drawn over good lines (necessitating a redraw of the good lines).

[Dominic Mitchell]

If you do not want a border, use a NULL_PEN. For example, the following SDK code draws a white rectangle.

 
  LOCAL trc           AS RECT     ' bounding or formatting rectangle
  LOCAL hDC           AS DWORD    ' handle of device context
  LOCAL hPenOld       AS DWORD    ' handle of original pen of device context
  LOCAL hBrushOld     AS DWORD    ' handle of original brush of device context
  
  hDC = GetDC(hWnd)
  ' Set the pen
  hPenOld = SelectObject(hDC, GetStockObject(%NULL_PEN))
  ' Set the brush
  hBrushOld = SelectObject(hDC, GetStockObject(%WHITE_BRUSH))
  trc.nLeft   = 105
  trc.nTop    = 85
  trc.nRight  = 246
  trc.nBottom = 181
  Rectangle hDC, trc.nLeft, trc.nTop, trc.nRight, trc.nBottom
  SelectObject hDC, hBrushOld
  SelectObject hDC, hPenOld 
  ReleaseDC hWnd, hDC

[Cliff Nichols]

Is there a "Graphic Erase" command that could be used to erase what I would call "Hidden lines"????

Some of the shapes I am making I need to hide these lines, and the best I can figure out is to redraw the shape with a color but border color the same as the fill color.

Is there a way to erase without painting over a line with another line of color to make it appear erased????