Blend in OpenGL)

Blend is a technology that combines the source and target colors to generate special effects in some way. A mixture is often used to draw transparent or translucent objects. The α value that plays a key role in blending is actually mixing the source and target colors at a given ratio to achieve different levels of transparency. If the α value is 0, it is completely transparent, and if the α value is 1, it is completely opaque. The hybrid operation can only be performed in rgba mode, and the α value cannot be specified in color index mode. The Rendering sequence of objects affects the mixed processing of OpenGL.

 

Glable (gl_blend); // enable Hybrid

Gldisable (gl_blend); // disable hybrid access

 

Obtain mixed information

Glget (gl_blend_src );

Glget (gl_blend_dst );

Glisenable (gl_blend );

 

Glblendfunc (glenum sfactor, glenum dfactor); // mixed function

Sfactor source mixing factor

Dfactor target hybrid factor

Mixed factor Enumeration

Gl_dst_alpha	(AD, AD)
Gl_dst_color	(Rd, Gd, BD, AD)
Gl_one	(1, 1, 1)
Gl_one_minus_dst_alpha	(1, 1, 1)-(AD, AD)
Gl_one_minus_dst_color	(1, 1, 1)-(Rd, Gd, BD, AD)
Gl_one_minus_src_alpha	(1, 1, 1)-(as,)
Gl_src_alpha	(As,)
Gl_src_alpha_saturate	(F, 1): F = min (AS, 1-AD)
Gl_zero	(0, 0, 0, 0)

 

Glblendfunc (gl_one, gl_zero); // The Source color overwrites the target color.

Glblendfunc (gl_zero, gl_one); // The target color will overwrite the source color.

Glblendfunc (gl_src_alpha, gl_one_minus_src_alpha ); //

If the source color is (1.0, 0.9, 0.7, 0.8)

Use gl_src_alpha as the source color

That is, 0.8*1.0, 0.8*0.9, 0.8*0.8, 0.8*0.7

Result 0.8, 0.72, 0.64, 0.56

 

The target color is (0.6, 0.5, 0.4, 0.3)

Use gl_one_minus_src_alpha for the target color

That is, 1-0.8 = 0.2

0.2*0.6, 0.2*0.5, 0.2*0.4, 0.2*0.3

Result 0.12, 0.1, 0.08, 0.06

Therefore, using this hybrid function, the α value of the source color determines the percentage of the result color.

Here, the Alpha value of the source color is 0.8, that is, the source color accounts for 80% of the result color, and the target color accounts for 20%.

 

Arrange the polygon from far to near, and use the following function

Glblendfunc (gl_src_alpha_saturate, gl_one );

Glable (gl_polygon_smooth );

The polygon anti-pattern can be optimized, but there must be an Alpha-bit plane to store the accumulated overwrite value.

 

Hybrid 3D object

When a 3D object is mixed, the basic principle is the same as that of a 2D object, but the deep detection must be disabled or set to read-only.

Because deep detection removes some blocked objects.

 

Glable (gl_depth_test); // enable deep Cache

Gldisable (gl_depth_test); // disable the deep cache.

 

Gldepthmask (gl_false); // The deep cache is read-only.

Gldepthmask (gl_true); // The deep cache is read/write.

 

To use hybrid and deep detection in 3D scenarios, follow these steps:

1. Use deep Detection

2. Draw opaque objects

3. Read-Only deep Detection

4. Draw a translucent object

5. Set read/write Depth Detection

 

The following describes how to test the hybrid algorithm and use the glblendfunc (gl_src_alpha, gl_one); mode.

Glclearcolor (0.0f, 1.0f, 0.0f, 0.5f); // green background (target), first put in Frame Buffer

Glcolor4f (1.0f, 0.0f, 0.0f, 0.5f); // rectangular color (source color), fragment

Calculation Result:

 

(0.5f, 1.0f, 0.0f, 0.75f)

Green background, red rectangle

Turn on the color when blending is disabled

 

Result:

1. the background color is also involved in the hybrid process;

2. The green background test shows that the value in rgba has no effect on a single color.

Transparency is calculated based on the value;

3. When the input color value is RGB, the default value of A is 0.0 for hybrid calculation.