Gammon Forum

You can do it with a mask. As a demo, make a main window:

-- main result window
win = GetPluginID ()  
WindowCreate (win, 0, 0, 300, 200, 7, 0, ColourNameToRGB("silver"))  -- create window
WindowShow (win,  true)  -- show it 

Now draw whatever gradient (or background image) you want into a second window:

-- gradient window
win2 = GetPluginID ()  .. ".2"
WindowCreate (win2, 0, 0, 200, 80, 7, 0, ColourNameToRGB("white"))  -- create window
WindowShow (win2,  true)  -- show it  (for testing purposes only)

WindowGradient (win2, 0, 0, 0, 0, 
                ColourNameToRGB ("red"), 
                ColourNameToRGB ("yellow"), 
                1)  -- left to right

Now make a mask in a third window:

-- mask window
win3 = GetPluginID ()  .. ".3"
WindowCreate (win3, 0, 0, 200, 80, 7, 0, ColourNameToRGB("black"))  -- create window
WindowShow (win3,  true)  -- show it (for testing purposes only)

-- round rectangle
WindowCircleOp (win3, 3, 20, 20, 180, 60, 
                ColourNameToRGB("black"), 5, 0,  -- no pen
                ColourNameToRGB("white"), 0,  -- brush
                25, 25)

Convert both temporary windows into images, and merge them into the main window:

-- convert to images
WindowImageFromWindow (win, "gradient", win2)
WindowImageFromWindow (win, "mask", win3)

-- do the merge
WindowMergeImageAlpha (win, "gradient", "mask", 20, 20, 200, 100, 0, 1, 0, 0, 0, 0)

Normally you wouldn't show win2 and win3, I have just done that so you can see what is happening under the hood. By tweaking coordinates, colours etc., you can have pretty-much any effect you want.

One effect you can experiment with is blurring the mask. To do that add this line after creating the round rectangle:

WindowFilter (win3, 0, 0, 0, 0, 3, 0) -- blur both directions

This gives this mask:

Now the resulting image looks like this, which you may or may not like: