什么是“立即模式"?在OpenGL中意味着什么? [英] What does "immediate mode" mean in OpenGL?

问题描述

什么是即时模式"?给出一个代码示例.

What is "immediate mode"? Give a code example.

何时必须使用立即模式而不是保留模式? 使用每种方法的利弊是什么?

When do I have to use immediate mode instead of retained mode? What are pros and cons of using each method?

推荐答案

立即模式"的一个示例是使用glBegin和glEnd以及它们之间的glVertex. 立即模式"的另一个示例是将glDrawArrays与客户端顶点数组(即不是顶点缓冲区对象)一起使用.

One example of "immediate mode" is using glBegin and glEnd with glVertex in between them. Another example of "immediate mode" is to use glDrawArrays with a client vertex array (i.e. not a vertex buffer object).

通常，您永远都不想使用即时模式(也许您的第一个"hello world"程序除外)，因为它已被弃用，并且无法提供最佳性能.

You will usually never want to use immediate mode (except maybe for your first "hello world" program) because it is deprecated functionality and does not offer optimal performance.

立即模式之所以不理想，是因为图形卡与程序的流程直接关联.驱动程序无法告知GPU在glEnd之前开始渲染，因为它不知道何时完成提交数据，并且它也需要传输该数据(只能 在).
同样，对于客户端顶点数组，驱动程序只能在调用glDrawArrays时提取数组的副本，并且这样做必须阻塞应用程序.原因是否则，您可以在驱动程序捕获阵列之前修改(或释放)阵列的内存.它无法计划该操作的早晚，因为它只知道数据恰好在某个时间点是有效的.

The reason why immediate mode is not optimal is that the graphic card is linked directly with your program's flow. The driver cannot tell the GPU to start rendering before glEnd, because it does not know when you will be finished submitting data, and it needs to transfer that data too (which it can only do after glEnd).
Similarly, with a client vertex array, the driver can only pull a copy of your array the moment you call glDrawArrays, and it must block your application while doing so. The reason is that otherwise you could modify (or free) the array's memory before the driver has captured it. It cannot schedule that operation any earlier or later, because it only knows that the data is valid exactly at one point in time.

与此相反，例如，如果使用顶点缓冲区对象，则用数据填充缓冲区并将其交给OpenGL.您的进程不再拥有此数据，因此无法再对其进行修改.只要总线空闲，驱动程序就可以依靠这一事实，并且可以(甚至是推测性地)上传数据.
您以后的任何glDrawArrays或glDrawElements调用都将进入工作队列并立即返回(在实际完成之前！)，因此您的程序将继续提交命令，而驱动程序则一步一步地工作.他们也可能不需要等待数据到达，因为驱动程序早就可以这样做了.
因此，渲染线程和GPU异步运行，每个组件始终处于繁忙状态，从而产生更好的性能.

In contrast to that, if you use for example a vertex buffer object, you fill a buffer with data and hand it to OpenGL. Your process does no longer own this data and can therefore no longer modify it. The driver can rely on this fact and can (even speculatively) upload the data whenever the bus is free.
Any of your later glDrawArrays or glDrawElements calls will just go into a work queue and return immediately (before actually finishing!), so your program keeps submitting commands while at the same time the driver works off one by one. They also likely won't need to wait for the data to arrive, because the driver could already do that much earlier.
Thus, render thread and GPU run asynchronously, every component is busy at all times, which yields better performance.

立即模式确实具有使用简单易用的优势，但是再次以非弃用的方式正确使用OpenGL也不完全是火箭科学-它只需要很少的额外工作.

Immediate mode does have the advantage of being dead simple to use, but then again using OpenGL properly in a non-deprecated way is not precisely rocket science either -- it only takes very little extra work.

以下是即时模式下的典型OpenGL"Hello World"代码:

Here is the typical OpenGL "Hello World" code in immediate mode:

glBegin(GL_TRIANGLES);
    glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(0.0f,   1.0f);
    glColor3f(0.0f, 1.0f, 0.0f);   glVertex2f(0.87f,  -0.5f);
    glColor3f(0.0f, 0.0f, 1.0f);   glVertex2f(-0.87f, -0.5f);
glEnd();

修改:
根据常见的请求，在保留模式下的同一件事看起来像这样:

By common request, the same thing in retained mode would look somewhat like this:

float verts = {...};
float colors = {...};
static_assert(sizeof(verts) == sizeof(colors), "");

// not really needed for this example, but mandatory in core profile after GL 3.2
GLuint vao;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);

GLuint buf[2];
glGenBuffers(2, buf);

// assuming a layout(location = 0) for position and 
// layout(location = 1) for color in the vertex shader

// vertex positions
glBindBuffer(GL_ARRAY_BUFFER, buf[0]);
glBufferData(GL_ARRAY_BUFFER, sizeof(verts), verts, GL_STATIC_DRAW);
glEnableVertexAttribArray(0); 
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);

// copy/paste for color... same code as above. A real, non-trivial program would
// normally use a single buffer for both -- usually with stride (5th param) to
// glVertexAttribPointer -- that presumes interleaving the verts and colors arrays.
// It's somewhat uglier but has better cache performance (ugly does however not
// matter for a real program, since data is loaded from a modelling-tool generated
// binary file anyway).
glBindBuffer(GL_ARRAY_BUFFER, buf[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(colors), colors, GL_STATIC_DRAW);
glEnableVertexAttribArray(1); 
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, 0);

glDrawArrays(GL_TRIANGLES, 0, 3); 

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