glBomb

09-30-2006, 04:11 AM

The given blend eqn is

(RsSr+RdDr, GsSg+GdDg, BsSb+BdDb, AsSa+AdDa)

One way to draw a picture composed half of one image and half of another, equally blended, is to set the source factor to GL_ONE and the destination factor to GL_ZERO, and draw the first image. Then set the source factor to GL_SRC_ALPHA and destination factor to GL_ONE_MINUS_SRC_ALPHA, and draw the second image with alpha equal to 0.5. This pair of factors probably represents the most commonly used blending operation. If the picture is supposed to be blended with 0.75 of the first image and 0.25 of the second, draw the first image as before, and draw the second with an alpha of 0.25.

So when i draw 1st image

(Sr,Sg,Sb,Sa) = (1,1,1,1)

(Dr,Dg,Db,Da) = (0,0,0,0)

Assumin A1=1=As here for 1st pic & a black backround initially i.e. (Rd,Gd,Bd,Ad) = (0,0,0,0)

we get the followin pixel value after 1st picture as been drawn

(R1+0,G1+0,B1+0,A1+0) = (R1,G1,B1,1) i.e. the solid 1st picture pixels where 1 represents 1st pic

now on to second image we have

alpha = 0.5,so

(Sr,Sg,Sb,Sa) = GL_SRC_ALPHA = (0.5,0.5,0.5,0.5)

(Dr,Dg,Db,Da) = GL_ONE_MINUS_SRC_ALPHA = (0.5,0.5,0.5,0.5)

we also already have destination pixel in buffer as

(Rd,Gd,Bd,Ad) = (R1,G1,B1,1)

iven A2=0.5=As here

we get

( R2*0.5 + R1*0.5,G2*0.5 + G1*0.5,B2*0.5 + B1*0.5, 0.5*0.5 + 1*0.5) =

( R2*0.5 + R1*0.5,G2*0.5 + G1*0.5,B2*0.5 + B1*0.5, 0.75)

^

So we get what we want finally : the color intensities from both pics equally weighed for final pixel color

I would like someone to confirm that the above calculations are correct & this is in fact te way opengl alpa blends

ESP IS MY FINAL ALPHA VALUE OF 0.75 CORRECT? SHOULDNT IT TOTAL TO ONE ?

If this is the case then why is the problem that i asked about in this thread occurring ?

(RsSr+RdDr, GsSg+GdDg, BsSb+BdDb, AsSa+AdDa)

One way to draw a picture composed half of one image and half of another, equally blended, is to set the source factor to GL_ONE and the destination factor to GL_ZERO, and draw the first image. Then set the source factor to GL_SRC_ALPHA and destination factor to GL_ONE_MINUS_SRC_ALPHA, and draw the second image with alpha equal to 0.5. This pair of factors probably represents the most commonly used blending operation. If the picture is supposed to be blended with 0.75 of the first image and 0.25 of the second, draw the first image as before, and draw the second with an alpha of 0.25.

So when i draw 1st image

(Sr,Sg,Sb,Sa) = (1,1,1,1)

(Dr,Dg,Db,Da) = (0,0,0,0)

Assumin A1=1=As here for 1st pic & a black backround initially i.e. (Rd,Gd,Bd,Ad) = (0,0,0,0)

we get the followin pixel value after 1st picture as been drawn

(R1+0,G1+0,B1+0,A1+0) = (R1,G1,B1,1) i.e. the solid 1st picture pixels where 1 represents 1st pic

now on to second image we have

alpha = 0.5,so

(Sr,Sg,Sb,Sa) = GL_SRC_ALPHA = (0.5,0.5,0.5,0.5)

(Dr,Dg,Db,Da) = GL_ONE_MINUS_SRC_ALPHA = (0.5,0.5,0.5,0.5)

we also already have destination pixel in buffer as

(Rd,Gd,Bd,Ad) = (R1,G1,B1,1)

iven A2=0.5=As here

we get

( R2*0.5 + R1*0.5,G2*0.5 + G1*0.5,B2*0.5 + B1*0.5, 0.5*0.5 + 1*0.5) =

( R2*0.5 + R1*0.5,G2*0.5 + G1*0.5,B2*0.5 + B1*0.5, 0.75)

^

So we get what we want finally : the color intensities from both pics equally weighed for final pixel color

I would like someone to confirm that the above calculations are correct & this is in fact te way opengl alpa blends

ESP IS MY FINAL ALPHA VALUE OF 0.75 CORRECT? SHOULDNT IT TOTAL TO ONE ?

If this is the case then why is the problem that i asked about in this thread occurring ?