Chapter 8 primitive processing of OpenGL super classic (version 7)

One 、 Surface subdivision

Put a triangle （ Or quadrilateral, etc ）, Meshing generates more internal triangles

1. Surface subdivision primitive mode

gl_TessLevelInner[]： Inside the graph
gl_TessLevelOuter[]： Outside the graph
(1) Use four view surface tessellation
Four views ： quadrilateral
gl_TessLevelInner[]： The length is 2, Inside the graph , Separately control the long 、 The number of wide segments
gl_TessLevelOuter[]： The length is 4, Outside the graph , Respectively control the number of segments divided into four sides
(2) Surface tessellation using triangles
gl_TessLevelInner[]： The length is 1
gl_TessLevelOuter[]： The length is 4
(3) Surface tessellation using isolines
gl_TessLevelOuter[]： Only this , The length is 2, Respectively control the number of lines and the number of segments divided by lines
(4) Surface subdivision point mode
Render as points

2. Surface subdivision mode

(1) Surface subdivision three sub segmentation modes ：
equal_spacing： Segmented equidistant mode , shortcoming ： The surface tessellation factor is rounded up to the nearest integer , The level may jump , The following two are the solutions to this problem
fractional_even_spacing： Segmented even spacing mode , The surface tessellation factor is rounded down to the nearest even number
fractional_odd_spacing:： Segmented odd spacing mode , The surface tessellation factor is rounded down to the nearest odd number
(2) Control the wrapping order
cw： Clockwise
ccw： Anti-clockwise

3. Data transfer between surface subdivision shaders

(1) The surface tessellation evaluation shader is the same as the surface tessellation control shader , Run once for each generated vertex , You can access all vertex data in the patch
(2) Vertex data is passed in addition to the array form , It can also directly transfer the data between the constant stages on the whole patch （Patch keyword ）
(3) No surface tessellation controls the rendering of shaders , The default value for the subdivision level of internal and external surfaces is 1.0, It can be done by void glPatchParameterfv() change

4. Shader call communication

(1) Each output in the surface tessellation control shader is an array , The length is equal to the number of control points in the output patch
(2) The surface tessellation control shader can read its output variables , To confirm whether the output variable is actually written , Added barrier() function , Flow control barrier

5. Example ： terrain render

6. Example ： Three Bezier patches

Two 、 Geometry shaders

sketch ： Shaders that handle primitives , A primitive runs once , A primitive can be a dot 、 Line segments or triangles .

1. Transfer geometry shaders

The output can only be points、line_strip and triangle_strip, have access to EndPrimitive() Indirectly become points、lines and triangles

2. Use geometry shaders in your application

The vertex shader output to the geometry shader becomes an array , Because the geometric shader is based on primitives

3. Delete geometry in geometry shader

sketch ： Reduce the number of vertices , It can eliminate objects from different angles

4. Modify the geometry in the geometry shader

sketch ： Modify the primitive of Geometry , Model decomposition and model explosion can be realized

5. Generate geometry in geometry shaders

sketch ： Generate more objects by increasing the number of vertices

6. Modify primitive types in geometry shaders

sketch ： Change primitive type , It can realize the visualization of normal vector

7. Multiple storage streams

sketch ： You can configure the 4 Multiple output streams , Render geometry at the same time and store data in the transform cache
Limit ：1. The output primitive pattern must be a dot 2. Only the first stream can be rendered , Other streams are used to store

8. Introducing new primitive types through geometry shaders

GL_LINES_ADJACENCY
GL_LINE_STRIP_ADJACENCY
GL_TRIANGLES_ADJACENCY
GL_TRIANGLE_STRIP_ADJACENCY
Do not understand , I'll see it later .

9. Multiple viewport conversions

opengl At least support 16 Three views
void glViewportIndexedf() Set viewport size and boundary
void glViewportIndexedfv() Set viewport binding
void glDepthRangeIndexed() Set the viewport depth range
If you set up multiple viewports at once , You can consider using it glViewportArrayv() and glDepthRangeArrayv()
Built-in variables gl_ViewportIndex You can choose which viewport to render to