“立即模式"是什么意思?在 OpenGL 中是什么意思?

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，WP2