debug error

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Floyd

In the main loop you repeatedly do:

Draw to back buffer
Flip
Grab from back buffer

This has unpredictable results. You can't be sure what the back buffer holds after a flip.

So your main loop should do:

Draw to back buffer
Grab from back buffer
Flip

Here's an example.

Open in online IDE

Graphics 640, 480
SetBuffer BackBuffer()

Shift = CreateImage (640,55)
Image = CreateImage (64,55)

Rect 0,0,64,55,False
Rect 1,1,62,53,False
GrabImage Image,0,0

Repeat

	Cls
	
	DrawImage Shift,0,20
	DrawImage Image,640 - Count,20
	GrabImage Shift,3,20
	Flip True
	
	Count = Count + 3
	If Count > 65 Then Count = 3

Until KeyDown(1)