“立即模式"是什么意思?在 OpenGL 中是什么意思? [英] What does "immediate mode" mean in OpenGL?

问题描述

什么是立即模式"?给出一个代码示例.

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

我什么时候必须使用立即模式而不是保留模式?使用每种方法的优缺点是什么?

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"的一个例子是使用 glBegin 和 glEnd 以及它们之间的 glVertex.立即模式"的另一个示例是将 glDrawArrays 与客户端顶点数组一起使用(即不是顶点缓冲区对象).

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).

您通常永远不想使用立即模式(可能除了您的第一个hello world"程序)，因为它是不推荐使用的功能并且不能提供最佳性能.

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.

立即模式不是最佳的原因是图形卡直接与您的程序流程相关联.驱动程序不能告诉 GPU 在 glEnd 之前开始渲染，因为它不知道你什么时候会完成提交数据，而且它也需要传输这些数据(它只能em> 在 glEnd 之后做.
类似地，对于客户端顶点数组，驱动程序只能在您调用 glDrawArrays 时提取数组的副本，并且在执行此操作时它必须阻止您的应用程序.原因是否则您可以在驱动程序捕获它之前修改(或释放)阵列的内存.它不能早晚安排该操作，因为它只知道数据恰好在一个时间点有效.

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，如果您使用顶点缓冲区对象，则将数据填充到缓冲区并将其传递给 OpenGL.您的进程不再拥有此数据，因此不能再修改它.司机可以依赖这个事实，并且可以(甚至推测)在公交车空闲时上传数据.
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！)，因此您的程序同时不断提交命令司机一一下班.他们也可能不需要等待数据到达，因为驱动程序可以更早地做到这一点.
因此，渲染线程和 GPU 异步运行，每个组件始终处于忙碌状态，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起来非常简单的优点，但是再次以非弃用的方式正确使用 OpenGL 也不是精确的火箭科学——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.

这是立即模式下典型的OpenGLHello 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();

根据一般要求，保留模式下的相同内容看起来有点像这样:

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); 

这篇关于“立即模式"是什么意思?在 OpenGL 中是什么意思?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！