Blend function - please help (MATLAB 2020b)

Hi,

Could someone clarify to me what the function below is needed for when setting up a script to run a stroop task?

Screen('BlendFunction', window, 'GL_SRC_ALPHA', 'GL_ONE_MINUS_SRC_ALPHA');

Many thanks in advance.

Answer 1

[Walter Roberson]

http://psychtoolbox.org/docs/Screen-BlendFunction

"The most common alpha-blending factors are sourceFactorNew = GL_SRC_ALPHA and destinationFactorNew = GL_ONE_MINUS_SRC_ALPHA They are needed for proper anti-aliasing (smoothing) by Screen(‘DrawLines’), Screen(‘DrawDots’) and for drawing masked stimuli with the Screen(‘DrawTexture’) command."

Screen('BlendFunction', window, 'GL_SRC_ALPHA', 'GL_ONE_MINUS_SRC_ALPHA');

is telling Psychtoolbox to use compute:

information to store = new screen data * alpha + old screen data * (1-alpha)

so for example if alpha were .8 then it would be new*0.8 + old*0.2

[Thomas Pace]

So if, for example, I wanted to present 4 textures using Screen('DrawTexture') in the same location, and have them all end up with equal alpha, what is the formula I would use for dictating each textures globalAlpha values?

For example, if globalAlpha (GA) for the first texture drawn was set at 1, I assumed the second texture drawn on top would need to be presented with GA of 0.5. But then what would the third texture drawn GA be such that all 3 have equally presented energy? Is there a formula for this?

[Walter Roberson]

syms I1 I2 I3 I4 a1 a2 a3 
t1 = I1*(1-a1) + I2*a1

t1 = 

t2 = t1*(1-a2) + I3*a2

t2 = 

t3 = t2*(1-a3) + I4*a3

t3 = 

t4 = collect(expand(t3),[I1,I2,I3,I4])

t4 = 

c1 = coeffs(t4,I1)

c1 = 

c2 = coeffs(t4,I2)

c2 = 

c3 = coeffs(t4,I3)

c3 = 

c4 = coeffs(t4,I4)

c4 = 

sol = solve([c1(end),c2(end),c3(end)]==1/4,[a1,a2,a3])

sol = struct with fields:
    a1: [1×1 sym]
    a2: [1×1 sym]
    a3: [1×1 sym]

sol.a1

ans = 

sol.a2

ans = 

sol.a3

ans = 

%cross-check
subs([c1(end),c2(end),c3(end),c4(end)], sol)

ans = 

So use alpha = 1/2 for the first overlay, alpha = 1/3 for the second overlay, alpha = 1/4 for the third overlay.