Partial transparency/alpha in mesh?

Questions
1

Hi all, I’m trying to simulate a 2d spotlight underwater. The water is a semi transparent mesh which gets less transparent the deeper you go. The water mesh is drawn over the ground mesh, and everything else that is underwater. The spotlight is a triangular mesh and it’s two end points are completely transparent to simulate falloff.

I tried drawing the spotlight with additive blending but I guess it only works with bitmaps, I’d like to avoid drawing the mesh to a bitmap every frame as it can completely fill the screen.

Is there any way to set the transparency of a mesh (acording to the spotlight position and alpha values), or even better somehow draw it with additive blending? Any other suggestions to simulate lighting?

Thanks

2

Additive blending does work with meshes. Could you post your code?

blendMode(ADDITIVE)
myMesh:draw()
blendMode(NORMAL)

One thing to remember with transparency and additive drawing, is that black (RGB 0) renders as transparent, because you’re adding 0 to what’s already there. Alpha then is in a sense redundant with additive drawing (though it is applied, I believe as + R * A, + G * A, + B * A). So, instead of having your spotlight mesh fade to alpha zero, experiment with having it fade from a bright colour (near white) to black, with additive blending.

Also, with additive blending, draw order is important. Draw the spotlight after everything else that you want the spotlight to brighten.

3

Ah yes, upon closer inspection I noticed it did actualy work but the result wasn’t at all what I expected. I’m not sure what is wrong exactly, I’m going to experiment some more with the transparency of the water. Thanks!

4

You could use a shader to calculate distance from the spotlight and adjust transparency accordingly

5

@Kirl Not sure if this is what you’re after. Move your finger around the screen to move the cone.

supportedOrientations(PORTRAIT_ANY)
displayMode(FULLSCREEN)

function setup()
    dx,dy=0,0
    m=mesh()
    for z=1,255 do
        m:addRect(WIDTH/2,900-z*2,z*2,2)
        m:setRectColor(z,color(255,255,255,z))
    end
    m:addRect(WIDTH/2,-110,WIDTH*2,1000)
    m:setRectColor(256,color(255,255,255,255))
    m:addRect(-312,550,1100,3000,math.rad(-18.5))
    m:setRectColor(257,color(255,255,255,255))
    m:addRect(1080,550,1100,3000,math.rad(18.5))
    m:setRectColor(258,color(255,255,255,255))    
end

function draw()
    sprite("Cargo Bot:Startup Screen",WIDTH/2,HEIGHT/2)
    pushMatrix()
    translate(dx,dy)
    m:draw()
    fill(0)
    ellipse(384,900,20)
    popMatrix()
end

function touched(t)
    dx=dx+t.deltaX
    dy=dy+t.deltaY
    if dx<-380 then
        dx=-380
    elseif dx>380 then
        dx=380
    end
    if dy>450 then
        dy=450
    elseif dy<-800 then
        dy=-800
    end    
end
6

Thanks dave, I’m going to see how that looks in my setup! :slight_smile: