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GLBLENDFUNC() GLBLENDFUNC()
NAME
glBlendFunc - specify pixel arithmetic
C SPECIFICATION
void glBlendFunc( GLenum sfactor,
GLenum dfactor )
PARAMETERS
sfactor Specifies how the red, green, blue, and alpha source blending
factors are computed. Nine symbolic constants are accepted:
GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ONE_MINUS_DST_COLOR,
GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA,
GL_ONE_MINUS_DST_ALPHA, and GL_SRC_ALPHA_SATURATE. The initial
value is GL_ONE.
dfactor Specifies how the red, green, blue, and alpha destination
blending factors are computed. Eight symbolic constants are
accepted: GL_ZERO, GL_ONE, GL_SRC_COLOR,
GL_ONE_MINUS_SRC_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA,
GL_DST_ALPHA, and GL_ONE_MINUS_DST_ALPHA. The initial value is
GL_ZERO.
DESCRIPTION
In RGBA mode, pixels can be drawn using a function that blends the
incoming (source) RGBA values with the RGBA values that are already in
the frame buffer (the destination values). Blending is initially
disabled. Use glEnable and glDisable with argument GL_BLEND to enable
and disable blending.
glBlendFunc defines the operation of blending when it is enabled.
sfactor specifies which of nine methods is used to scale the source
color components. dfactor specifies which of eight methods is used to
scale the destination color components. The eleven possible methods
are described in the following table. Each method defines four scale
factors, one each for red, green, blue, and alpha.
In the table and in subsequent equations, source and destination color
components are referred to as (R_s, G_s, B_s, A_s) and (R_d, G_d, B_d,
A_d). They are understood to have integer values between 0 and (k_R,
k_G, k_B, k_A), where
k_c = 2^(m_c) - 1
and (m_R, m_G, m_B, m_A) is the number of red, green, blue, and alpha
bitplanes.
Source and destination scale factors are referred to as (s_R, s_G, s_B,
s_A) and (d_R, d_G, d_B, d_A). The scale factors described in the
table, denoted (f_R, f_G, f_B, f_A), represent either source or
destination factors. All scale factors have range [0,1].
+-----------------------+------------------------------------------------------------------------------------------------------------+
| parameter | $(f sub R , ~~ f sub G , ~~ f sub B , ~~ f sub A )$ |
+=======================+============================================================================================================+
| GL_ZERO | $(0, ~0, ~0, ~0 )$ |
| GL_ONE | $(1, ~1, ~1, ~1 )$ |
| GL_SRC_COLOR | $(R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$ |
|GL_ONE_MINUS_SRC_COLOR | $(1, ~1, ~1, ~1 ) ~-~ (R sub s / k sub R , ~G sub s / k sub G , ~B sub s / k sub B , ~A sub s / k sub A )$ |
| GL_DST_COLOR | $(R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$ |
|GL_ONE_MINUS_DST_COLOR | $(1, ~1, ~1, ~1 ) ~-~ (R sub d / k sub R , ~G sub d / k sub G , ~B sub d / k sub B , ~A sub d / k sub A )$ |
| GL_SRC_ALPHA | $(A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$ |
|GL_ONE_MINUS_SRC_ALPHA | $(1, ~1, ~1, ~1 ) ~-~ (A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A , ~A sub s / k sub A )$ |
| GL_DST_ALPHA | $(A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$ |
|GL_ONE_MINUS_DST_ALPHA | $(1, ~1, ~1, ~1 ) ~-~ (A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A , ~A sub d / k sub A )$ |
|GL_SRC_ALPHA_SATURATE | $(i, ~i, ~i, ~1 )$ |
+-----------------------+------------------------------------------------------------------------------------------------------------+
In the table,
i = min (A_s, k_A - A_d) / k_A
To determine the blended RGBA values of a pixel when drawing in RGBA
mode, the system uses the following equations:
R_d = min (k_R, R_s s_R + R_d d_R)
G_d = min (k_G, G_s s_G + G_d d_G)
B_d = min (k_B, B_s s_B + B_d d_B)
A_d = min (k_A, A_s s_A + A_d d_A)
Despite the apparent precision of the above equations, blending
arithmetic is not exactly specified, because blending operates with
imprecise integer color values. However, a blend factor that should be
equal to 1 is guaranteed not to modify its multiplicand, and a blend
factor equal to 0 reduces its multiplicand to 0. For example, when
sfactor is GL_SRC_ALPHA, dfactor is GL_ONE_MINUS_SRC_ALPHA, and A_s is
equal to k_A, the equations reduce to simple replacement:
R_d = R_s
G_d = G_s
B_d = B_s
A_d = A_s
EXAMPLES
Transparency is best implemented using blend function (GL_SRC_ALPHA,
GL_ONE_MINUS_SRC_ALPHA) with primitives sorted from farthest to
nearest. Note that this transparency calculation does not require the
presence of alpha bitplanes in the frame buffer.
Blend function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) is also useful
for rendering antialiased points and lines in arbitrary order.
Polygon antialiasing is optimized using blend function
(GL_SRC_ALPHA_SATURATE, GL_ONE) with polygons sorted from nearest to
farthest. (See the glEnable, glDisable reference page and the
GL_POLYGON_SMOOTH argument for information on polygon antialiasing.)
Destination alpha bitplanes, which must be present for this blend
function to operate correctly, store the accumulated coverage.
NOTES
Incoming (source) alpha is correctly thought of as a material opacity,
ranging from 1.0 (K_A), representing complete opacity, to 0.0 (0),
representing complete
transparency.
When more than one color buffer is enabled for drawing, the GL performs
blending separately for each enabled buffer, using the contents of that
buffer for destination color. (See glDrawBuffer.)
Blending affects only RGBA rendering. It is ignored by color index
renderers.
ERRORS
GL_INVALID_ENUM is generated if either sfactor or dfactor is not an
accepted value.
GL_INVALID_OPERATION is generated if glBlendFunc is executed between
the execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_BLEND_SRC
glGet with argument GL_BLEND_DST
glIsEnabled with argument GL_BLEND
SEE ALSO
glAlphaFunc, glClear, glDrawBuffer, glEnable, glLogicOp, glStencilFunc
GLBLENDFUNC()