Blend function - please help (MATLAB 2020b)

5 vues (au cours des 30 derniers jours)
Barbara Fiorani
Barbara Fiorani le 22 Oct 2020
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.

Réponses (1)

Walter Roberson
Walter Roberson le 29 Oct 2020
"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
  2 commentaires
Thomas Pace
Thomas Pace le 16 Fév 2021
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
Walter Roberson le 16 Fév 2021
Modifié(e) : Walter Roberson le 16 Fév 2021
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.

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