Help with Sprite:setBlendingMode "multiply"

Comments

• hgy29 Maintainer

  July 2015

  Hmm, to me it works as it should: you draw a moving white spot on your scene then multiply the scene by a back or white square. It renders what I would expect.
  
  What did you want to achieve ?

  +1 -1Share on Facebook
• pie Member

  July 2015

  @hgy29 I'd like to see the background "under" the black shape where the white spot overlaps it.
  Maybe I misunderstood the usage of "multiply" blending mode..
  thank you

  +1 -1Share on Facebook
• hgy29 Maintainer

  July 2015 Accepted Answer

  Still not really sure I understood correctly, but I suppose it doesn't work because the spotlight is drawn before the 'background', while the background doesn't take up full screen.
  Original idea was:
  1°) you draw a white spotlight above a black background
  2°) the (full screen) logo was then multiplied by what was on the before it screen, making it appear where the spotlight was (white(1) * white -> white, white * black (0) -> black)
  
  It could have been written has:
  1°) draw anything you want in the background
  2°) draw the spotlight (with multiply) above
  
  but in that case the spotlight sprite must cover all screen, so that all pixels are blended.

  +1 -1Share on Facebook
• pie Member

  July 2015

  @hgy29 my goal was to use multiply blending mode and "spotlights" to draw a kind of "fog of war" over a tilemap. I'm doing it in the "wrong" way now, and I was looking for a better (more performing) solution: now I am checking for each tile and setting an alpha on a black layer "above the tilemap". But this was a workaround, and it's time to fix it
  
  I thought I misunderstood how "multiply" works.
  
  So if there is no-pixel (outside the spotlight sprite) it can't blend because it's missing the "black" pixels? (and this is why you say that the spotlight sprite must cover all screen?)
  
  Thank you very much
  

  +1 -1Share on Facebook
• hgy29 Maintainer

  July 2015 Accepted Answer

  Yes, think that way: every time gideros draws a sprite, it actually draws a series of triangles. A bitmap sprite for instance is comprised of two triangles forming a rectangle. When the GPU render those triangles, it determine the color and alpha value or each pixel of the drawn shape, then each pixel is blended with the pixel it would otherwise replace on screen.
  So blending applies only to areas where the GPU actually want to draw something.

  +1 -1Share on Facebook
• pie Member

  July 2015

  Allright, this makes makes sense even to me
  
  thanks

  +1 -1Share on Facebook