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    Alpha overlap

    rrraptorrrraptor Member
    in General questions
    I need to achieve this effect:


    Two objects is overlapping on a layer that has alpha < 1, and the intersection part is twice darker then the original. Is there a way to fix it? I tried to apply "setBlendMode" to the layer with all combinations of src and dst modes, but no result.

    Problem representasion:
    application:setBackgroundColor(0x323232)
 
local layer = Sprite.new()
stage:addChild(layer)
 
local shadow1 = Pixel.new(0, 1, 300, 50)
shadow1:setPosition(100, 225)
layer:addChild(shadow1)
 
local shadow2 = Pixel.new(0, 1, 50, 300)
shadow2:setPosition(225, 100)
layer:addChild(shadow2)
 
layer:setAlpha(0.3)
    SuperShape | Nice blured shapes with shadows | Simple button pad | Simple layer class | Simple UI library | 2D camera lib | Color picker | FPS graph | Dear ImGui | cute_c2
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    Comments

    • MoKaLuxMoKaLux Member
      @rrraptor sorry I cannot answer your question but I have a dirty hack, create a third shadow?!
      application:setBackgroundColor(0x323232)
 
local layer = Sprite.new()
layer:setAlpha(0.3)
 
local shadow1 = Pixel.new(0, 1, 300, 50)
shadow1:setPosition(100, 225)
layer:addChild(shadow1)
 
local shadow2 = Pixel.new(0, 1, 50, 125)
shadow2:setPosition(225, 100)
layer:addChild(shadow2)
 
local shadow3 = Pixel.new(0, 1, 50, 150)
shadow3:setPosition(225, 275)
layer:addChild(shadow3)
 
stage:addChild(layer)

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    • olegoleg Member
      @rrraptor
      Set alpha = 1 -On dark stripes
      Move dark stripes under layer background
      Apply blending mode to the background

      Likes: MoKaLux

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    • antixantix Member
      basically there is no way to fix this using two rectangles because of the way alpha blending actually works.

      The only solution I can think of right now is using 5 rectangles but that's kind of a performance hit.

      Dislikes: Yan

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    • keszeghkeszegh Member
      edited August 2019 Accepted Answer
      render both to a rendertarget with alpha=1 and then put on screen a bmp with this rendertarget as texture with alpha=0.3. i think there is no other real solution.
      see the following discussion about the same question:
      http://forum.giderosmobile.com/discussion/6972/question-about-setting-alpha-of-a-mesh-uniformly-an-open-problem-for-you-to-solve#latest
      there actually is a way using stencils which may work in your case.
      Longtitle Productions, creator of
      Fragmenter - animated loop machine and IKONOMIKON - the memory game
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    • rrraptorrrraptor Member
      @keszegh yeah, i though about that, but will it cause performance issues with a large number of objects, because they can move?
      Hmm..or i could render static shadows into a different render target once (some sort of baking shadows xD), and dynamic shadows every frame (?) into a another render target buffer, then draw static shadows on it.
      SuperShape | Nice blured shapes with shadows | Simple button pad | Simple layer class | Simple UI library | 2D camera lib | Color picker | FPS graph | Dear ImGui | cute_c2
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    • keszeghkeszegh Member
      constant many rendertargets maintained per frame should be fine i guess but you better try it on the weakest machine you target.
      Longtitle Productions, creator of
      Fragmenter - animated loop machine and IKONOMIKON - the memory game
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    • antixantix Member
      If all of the crosses are going to be the same you would only need to maintain one RenderTarget.
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    • hgy29hgy29 Maintainer
      I just recalled about this ‘problem’ and I think it could be solved with stencil easily. I’ll try to post a solution tomorrow, unless someone else does it meanwhile :)

      Likes: MoKaLux, keszegh

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    • hgy29hgy29 Maintainer
      Accepted Answer
      It works!
      I just added a few stencil commands at the bottom of the OP original code.
      application:setBackgroundColor(0x323232)
 
local layer = Sprite.new()
stage:addChild(layer)
 
local shadow1 = Pixel.new(0, 1, 300, 50)
shadow1:setPosition(100, 225)
layer:addChild(shadow1)
 
local shadow2 = Pixel.new(0, 1, 50, 300)
shadow2:setPosition(225, 100)
layer:addChild(shadow2)
 
layer:setAlpha(0.3)
 
--Use stencil for drawing every shadow pixel only once:
--1) Clear stencil on layer (before drawing children)
layer:setStencilOperation{ stencilClear=true, stencilWriteMask=1 }
--2) Only draw pixels where stencil is 0, and increment stencil when drawn
shadow1:setStencilOperation{ stencilClear=false, stencilMask=1, stencilWriteMask=1, 
	stencilRef=0, stencilFunc=Sprite.STENCIL_EQUAL,
	depthPass=Sprite.STENCIL_INCR,
	stencilFail=Sprite.STENCIL_KEEP}
shadow2:setStencilOperation{ stencilClear=false, stencilMask=1, stencilWriteMask=1, 
	stencilRef=0, stencilFunc=Sprite.STENCIL_EQUAL,
	depthPass=Sprite.STENCIL_INCR,
	stencilFail=Sprite.STENCIL_KEEP}

      Likes: keszegh, oleg, rrraptor, MoKaLux, antix

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