Core mode and immediate rendering mode of OpenGL

Render now mode

In the early OpenGL Use immediate rendering mode （Immediate mode, That is, fixed rendering pipeline ）, Drawing graphics in this mode is very convenient .
OpenGL Most of the functions of are hidden by the library , Developers have little control OpenGL Freedom of how to calculate . And developers are eager for more flexibility . Over time , Norms are becoming more flexible , Developers have more control over drawing details .

The core model

Immediate rendering mode is really easy to use and understand , But it's too inefficient . So from OpenGL3.2 Start , The specification document begins to discard the immediate rendering mode , And encourage developers to OpenGL The core model of (Core-profile) Under development , The specification of this branch completely removes the old features .
When using OpenGL The core mode of ,OpenGL Force us to use modern functions . When we try to use an obsolete function ,OpenGL Will throw an error and terminate the drawing . The advantage of modern functions is higher flexibility and efficiency , However, it is also more difficult to learn . Immediate rendering mode from OpenGL Many details have been abstracted from the actual operation , So while it's easy to learn , It's also hard for people to grasp OpenGL How does it work . Modern functions require users to really understand OpenGL And graphic programming , It has some difficulty , However, it provides more flexibility , Higher efficiency , More importantly, we can have a deeper understanding of graphic programming .

That's why our tutorial is for OpenGL3.3 The core model of . Although it's more difficult to get started , But the effort is worth it .

The latest version opengl

Today, , Higher version OpenGL Has been released （ The latest version at the time of writing is 4.5）, You may ask ： since OpenGL 4.5 All out , Why should we learn OpenGL 3.3？ The answer is simple , all OpenGL Higher versions of are in 3.3 On the basis of , Introduced additional features , No changes to the core architecture . The new version just introduces some more efficient or useful ways to accomplish the same function . therefore , All the concepts and technologies in modern times OpenGL Consistent in the version . When you have enough experience , You can easily use from a later version OpenGL New features .

opengl An extension of

OpenGL A major feature of is the extension (Extension) Support for , When a graphics card company comes up with a new feature or a big optimization in rendering , It is usually implemented in the driver in an extended way . If a program runs on a graphics card that supports this extension , Developers can use this extension to provide some more advanced and effective graphics functions . In this way , Developers don't have to wait for a new OpenGL Specifications available , You can use these new rendering features , Simply check whether the graphics card supports this extension . Usually , When an extension is very popular or useful , It will eventually become the future OpenGL Part of the norm .

opengl State machine

OpenGL It's a huge state machine (State Machine)： A series of variable descriptions OpenGL How to run at this moment .OpenGL The state of is usually called OpenGL Context (Context). We usually use the following ways to change OpenGL state ： set an option , Operation buffer . Last , We use the current OpenGL Context to render .

Suppose when we want to tell OpenGL When drawing line segments instead of triangles , We change... By changing some context variables OpenGL state , To tell OpenGL How to draw . Once we change OpenGL The state of is to draw line segments , The next draw command will draw line segments instead of triangles .

When using OpenGL When , We will encounter some state setting functions (State-changing Function), This kind of function will change the context . And state usage functions (State-using Function), Such functions will be based on the current OpenGL State to perform some operations . As long as you remember OpenGL It's essentially a big state machine , It's easier to understand most of its features .

opengl object

OpenGL The library uses C Written language , It also supports the derivation of multiple languages , But its kernel is still a C library . because C Some language structures are not easy to translate into other high-level languages , therefore OpenGL Some abstraction layers are introduced during development .“ object (Object)” It's one of them .
stay OpenGL An object in is a collection of options , It represents OpenGL A subset of States . such as , We can use an object to represent the settings of the drawing window , Then we can set its size 、 Number of supported colors, etc . You can think of an object as C Style structure (Struct)：

struct object_name {
    
    float  option1;
    int    option2;
    char[] name;
};

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Text reference ：https://learnopengl-cn.github.io/01%20Getting%20started/01%20OpenGL/
Also recommend tutorials ：https://www.bootwiki.com/opengl/opengl-tutorial1.html