View Full Version : blend equation on opengl reference page

03-03-2011, 10:24 AM
hello everyone, i found some question when i did some blend test.

and i googled first then i found some blend formula they all said:

(if i set glBlendFunc(GL_SRC_ALPHA, GL_DST_ALPHA))

the sfactor and dfactor are alpha value of scr color and dst

color ((As,As,As,As) and (Ad,Ad,Ad,Ad))

but opengl reference page said they are alpha value divided by

kc, which is 2^mc-1(mc is the number of rgba bitplain). it said

GL_SRC_ALPHA is (As/kA,As/kA,As/kA,As/kA). if i have 8-bit rgba

bitplain, it makes color component very small([0,1]/(kA=255))

and the result will look too dark and GL_DST_ALPHA,

GL_ONE_MINUS_SRC etc. are in similiar case. the fact is test

result doesnt seems divided by kc. so what kc does mean??? thx

in advance.

03-03-2011, 04:25 PM
could you us this to make sections of a bitmap transparent?

Alfonse Reinheart
03-03-2011, 05:20 PM
All of the "divided by kc" stuff is talking about is the transformation from normalized values to floating-point values. The colors in a "8-bit rgba bitplain" are not stored as [0, 1] floating-point values. They are stored as [0, 255] integers. The division by kc (255) is what converts them to [0, 1] floating-point values.

To be honest, the inclusion of that conversion in the blending page just makes it more confusing.

03-03-2011, 09:58 PM
to blessman11: i haven't try it, but i think it involves the

order of rendering of opaque and transparent objects

to Alfonse Reinheart: thx ur response! and excuse me it strikes

on another question: i have 8-bit rgba bitplain

if i use glColor4s(assume 2 byte short), the value is in

[0,65535], does opengl linearly interpolate the value to

[0,255] and finally transforms it to [0,1]?

what if i use glcolor4f, which is in [0,1], doesnt opengl

do anything? thus means i should use glColor*f then other

glColor*[b,s,i,d,...] in most case?