How to quickly clear a large TCanvas

Problem/Question/Abstract:

How to quickly clear a large TCanvas

Answer:

You can use the PatBlt API call for this purpose. The function takes six parameters:

HDC: hdc - The handle of the canvas to be cleared
nXleft: integer - X coordinate of the upper left corner of canvas to be cleared
nYleft: integer - Y coordinate of the upper left corner of canvas to be cleared
nWidth: integer - The width of the canvas to be cleared
nHeight: integer - The height of the canvas to be cleared
dwRop: dWord - Raster operation code (WHITENESS in our case, for clearing the canvas)


To be used like this:


{ ... }
PatBlt(Image1.Canvas.Handle, 0, 0, Image1.Width, Image1.Height, WHITENESS);
Image1.Refresh;
{ ... }


Instead of WHITENESS you could also use:

PATCOPY - Copies the specified pattern into the destination bitmap
PATINVERT - Combines the colors of the specified pattern with the colors of the destination rectangle by using the Boolean OR operator
DSTINVERT - Inverts the destination rectangle
BLACKNESS - Fills the destination rectangle using the color associated with index 0 in the physical palette (This color is black for the default physical palette)
WHITENESS - Fills the destination rectangle using the color associated with index 1 in the physical palette (This color is white for the default physical palette)