==================================
Not uniform stretching of the background
==================================
x=thisComp.width; 
y=thisComp.height; 
[x,y]

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Uniform stretching of the background
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// created by: VIDEOLANCER.net // 

userX = 1920; userY = 1080; 
// Width and Height of the Comp in which the Scene was created // 

x = thisComp.width; 
y = thisComp.height; 
ratio = userX/userY; 
if (x/y >= ratio){[x,x]} 
else {[y*ratio,y*ratio]}

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Exp for Group Null, which is parent to Comp Size null
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// created by: VIDEOLANCER.net // 

userX = 1920; userY = 1080; 
// Width and Height of the Comp in which the Scene was created // 

x = thisComp.width/userX; 
y = thisComp.height/userY; 
if(x>=y){[value[0]*y/x,value[1]]} 
else{[value[0],value[1]*x/y]}