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# How to apply Alpha Masks that move around and affect only specific objects (no fullscreen masks)

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### #1Icebone1000  Members

Posted 19 August 2014 - 08:19 PM

Imagine a tiled 2D game where your character goes behind a wall (found a secret spot). Say I want to apply an alpha mask (a circle with faded borders) in the same position the player is, but only for the tiles/sprites hiding the secret spot, not for surrounding environment tiles (where the mask will overlap (due its big size) but not affect, cause theres notting behind then, its a solid wall)

check how the mask affects only specific tiles

Whats the best way to accomplish that?

#1# With stencil buffer Id need to render in lots of passes. (I suppose, I dont remember have ever used the stencil buffer before)

Render unaffected tiles + player;

Render the mask in the player position to the stencil buffer;

Render the tiles hiding the secret spot;

Render unaffected tiles + player;

Render the tiles hiding the secret spot w/ a specific pixel shader, that also takes the mask and its screen position.

Which method is better?

The pixel shader method sounds better to me, but I dont know how/if it would work.

Id need to check if it overlap with the mask (comparing SV_Position witht the given mask screen position constant), and compute the equivalent uv on the mask..?

Computing the equivalent mask uv would be something like

What if the current pixel does not overlap (generating negative uvs, or greater than 1)? (if the mask is constrained to have black on the borders than clamping the computed uv to [0,1] would work I guess?)

Would #1 or #2 work?

Any better way of doing this?

Edited by Icebone1000, 19 August 2014 - 08:40 PM.

### #2L. Spiro  Members

Posted 19 August 2014 - 08:32 PM

#1 will not give you a smooth fall-off.

Using #2 is very simple.  The only extra input is the screen-space point from which the alpha should be increasing from 0 (closer to the [X,Y] a pixel is the lower the alpha modifier).

float fAlphaModifier = sin( saturate( length( SV_Position - vTargetPos ) / 256.0f /* Size if the fade area. */ ) * 1.570796 );

The final color’s standard w (the same w you would have otherwise) is multiplied by fAlphaModifier on output.

L. Spiro

Edited by L. Spiro, 19 August 2014 - 08:35 PM.

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