如何在 OpenGL 中以正确的方式进行实例化。

Question

我正在尝试以最有效的方式使用 VBO 和 Instancing 机制。我有一个基于体素的世界，我想用尽可能少的绘制调用来绘制它们。下面的代码为 VBO 准备了一个四边形：

void VoxelView::initVBOs() {
/*--------------------- Main OpenGL Program ---------------------*/
/* Vertices of a triangle (counter-clockwise winding) */
float data[6][3] = {
                                            // Left bottom triangle
                                            { -0.5f, 0.5f, 0.0f  },
                                            { -0.5f, -0.5f, 0.0f },
                                            { 0.5f, -0.5f, 0.0f  },
                                            // Right top triangle
                                            { 0.5f, -0.5f, 0.0f  },
                                            { 0.5f, 0.5f, 0.0f   },
                                            { -0.5f, 0.5f, 0.0f  }

                                       };

/*---------------------- Initialise VBO - (Note: do only once, at start of program) ---------------------*/
/* Create a new VBO and use the variable "triangleVBO" to store the VBO id */
glGenBuffers(1, &triangleVBO);

/* Make the new VBO active */
glBindBuffer(GL_ARRAY_BUFFER, triangleVBO);

/* Upload vertex data to the video device */
glBufferData(GL_ARRAY_BUFFER, 6 * 3 * sizeof(float), data, GL_STATIC_DRAW);

/* Specify that our coordinate data is going into attribute index 0(shaderAttribute), and contains three floats per vertex */
glVertexAttribPointer(shaderAttribute, 3, GL_FLOAT, GL_FALSE, 0, 0);

/* Enable attribute index 0(shaderAttribute) as being used */
glEnableVertexAttribArray(shaderAttribute);

/* Make the new VBO active. */
glBindBuffer(GL_ARRAY_BUFFER, triangleVBO);

/*-------------------------------------------------------------------------------------------------------*/

/*--------------------- Load Vertex and Fragment shaders from files and compile them --------------------*/
/* Read our shaders into the appropriate buffers */
vertexSource = filetobuf("Shaders/exampleVertexShader1.vert");
fragmentSource = filetobuf("Shaders/exampleFragmentShader1.frag");

/* Assign our handles a "name" to new shader objects */
vertexShader = glCreateShader(GL_VERTEX_SHADER);
fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);

/* Associate the source code buffers with each handle */
glShaderSource(vertexShader, 1, (const GLchar**)&vertexSource, 0);
glShaderSource(fragmentShader, 1, (const GLchar**)&fragmentSource, 0);

/* Free the temporary allocated memory */
free(vertexSource);
free(fragmentSource);

/* Compile our shader objects */
glCompileShader(vertexShader);
glCompileShader(fragmentShader);
/*-------------------------------------------------------------------------------------------------------*/

/*-------------------- Create shader program, attach shaders to it and then link it ---------------------*/
/* Assign our program handle a "name" */
shaderProgram = glCreateProgram();

/* Attach our shaders to our program */
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);

/* Bind attribute index 0 (shaderAttribute) to in_Position*/
/* "in_Position" will represent "data" array's contents in the vertex shader */
glBindAttribLocation(shaderProgram, shaderAttribute, "in_Position");

/* Link shader program*/
glLinkProgram(shaderProgram); 

我正在通过以下方式渲染四边形：

void VoxelView::renderVBO()
{
/* Set shader program as being actively used */
glUseProgram(shaderProgram);

/* Set background colour to BLACK */
glClearColor(0.0, 0.0, 0.0, 1.0);

/* Clear background with BLACK colour */
glClear(GL_COLOR_BUFFER_BIT);

/* Actually draw the triangle, giving the number of vertices provided by invoke glDrawArrays
   while telling that our data is a triangle and we want to draw 0-3 vertexes
*/
glDrawArrays(GL_TRIANGLES, 0, 6);
}

我想使用实例化机制多次绘制这个四边形（使用 VBO）。我希望它相当简单，因为我想为更复杂的代码实现它。我知道我应该使用 glDrawElementsInstanced 方法来使用实例化，但我不知道该怎么做。有人知道怎么做吗？

Answer 1

使用 glDrawElementsInstanced 时，您需要让着色器使用 gl_InstanceiD

#version 410

layout (location = 0) in vec3 Position; 
layout (location = 1) in vec2 TexCoord; 
layout (location = 2) in vec3 Normal; 
layout (location = 3) in mat4 WVP; 
layout (location = 7) in mat4 World; 

out vec2 TexCoord0; 
out vec3 Normal0; 
out vec3 WorldPos0; 
flat out int InstanceID; 

void main() 
{ 
    gl_Position = WVP * vec4(Position, 1.0); 
    TexCoord0 = TexCoord; 
    Normal0 = World * vec4(Normal, 0.0)).xyz; 
    WorldPos0 = World * vec4(Position, 1.0)).xyz; 
    InstanceID = gl_InstanceID; 
};

glDrawElementsInstanced 使用 gl_InstanceID 变量，就好像它是一个静态整数顶点属性。当顶点的第一个副本被发送到 OpenGL 时， gl_InstanceID 将为零。然后它将为每个几何副本增加一次，并最终达到 instanceCount - 1。

它的行为是这样的

for (int n = 0; n < instancecount; n++)
{
// Set the value of gl_InstanceID
glVertexAttrib1i(gl_InstanceID, n); // except this is internally incremented and not a real vertex attribute
// Make a normal call to glDrawElements
glDrawElements(mode, count, type, indices);
}

除了这种情况发生在内部，您只需调用一次glDrawArraysInstanced()。为了对每个实例使用不同的变换，您可以传递使用 gl_InstanceID 的统一矩阵和索引数组，或者您可以使用纹理缓冲区对象并将变换存储在那里。

所以你的绘图代码应该变成这样，但请注意，你仍然需要为数组中的每个实例传递统一矩阵或一次性传递一个纹理缓冲区对象。

glDrawElementsInstanced(primitivetype, indices, GL_UNSIGNED_INT, 0, instanceCount-1);

你的顶点着色器应该变成

uniform mat4 instancematrices[32];

void main()
{
  gl_Position = gl_ModelViewProjectionMatrix * instancematrices[gl_InstanceID] * gl_Vertex;
}