Jump to content
  • Advertisement
Sign in to follow this  

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

This topic is 1400 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

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;


#2# With specific pixel shader:

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

mask uv: (mask_pos - SV_Position) / mask resolution?

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

Share this post

Link to post
Share on other sites

#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

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

Participate in the game development conversation and more when you create an account on GameDev.net!

Sign me up!