Instancing
Instancing绘制我想很多童鞋都不陌生，这个技术主要用来快速渲染大量相同的几何体，可以大大提高绘制效率。每个instance在shader中都有一个独一无二的索引，可以用来访问每个instance对应的渲染参数。使用Instancing技术之所以能够大大提高效率，主要是因为它大大减少了dip（draw indexed primitive）的数量。
在实际应用中，我们可以将所有的渲染参数打包到一个buffer中，将它们一起送到GPU，然后，只需要调用一次绘制命令，就可以绘制大批不同的实体。同样，如果instance data没有改变的话（比如我们明确知道它们是用来绘制静态物体的），则不需要每一帧都将这些数据往GPU送一次，只需要在程序第一次初始化组织数据时传输一次就够了。
可能大家比较熟悉的，还是通过VBO进行Instancing绘制，但在实际的程序编写和优化过程中，还有很多buffer可以用来Instancing，OpenGL也有很多存储数据的方式，像VBO,UBO,TBO,SSBO等等。下面就详细讲解一下利用不同的数据传输方式进行Instancing绘制。
Texture instancing
这种方式是将所有渲染用到的数据都保存在一张纹理当中，然后通过PBO（Pixel Buffer Object）的方式进行数据更新，PBO通过异步的方式实现CPU到GPU之间的数据传输，从而不需要阻塞CPU等待数据传输完成。
纹理创建：
glGenBuffers(2, textureInstancingPBO); glBindBuffer(GL_PIXEL_UNPACK_BUFFER, textureInstancingPBO[0]);
//GL_STREAM_DRAW_ARB means that we will change data every frame glBufferData(GL_PIXEL_UNPACK_BUFFER, INSTANCES_DATA_SIZE, 0, GL_STREAM_DRAW_ARB); glBindBuffer(GL_PIXEL_UNPACK_BUFFER, textureInstancingPBO[1]); glBufferData(GL_PIXEL_UNPACK_BUFFER, INSTANCES_DATA_SIZE, 0, GL_STREAM_DRAW_ARB); glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
//create texture where we will store instances data on gpu glGenTextures(1, textureInstancingDataTex); glBindTexture(GL_TEXTURE_2D, textureInstancingDataTex); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_R, GL_REPEAT); //in each line we store NUM_INSTANCES_PER_LINE object's data. 128 in our case
//for each object we store PER_INSTANCE_DATA_VECTORS data-vectors. 2 in our case //GL_RGBA32F, we have float32 data //complex_mesh_instances_data source data of instances, if we are not going to update data in the texture glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, NUM_INSTANCES_PER_LINE * PER_INSTANCE_DATA_VECTORS, MAX_INSTANCES / NUM_INSTANCES_PER_LINE, 0, GL_RGBA, GL_FLOAT, &complex_mesh_instances_data[0]); glBindTexture(GL_TEXTURE_2D, 0);
纹理更新：
glBindTexture(GL_TEXTURE_2D, textureInstancingDataTex); glBindBufferARB(GL_PIXEL_UNPACK_BUFFER, textureInstancingPBO[current_frame_index]); // copy pixels from PBO to texture object glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, NUM_INSTANCES_PER_LINE * PER_INSTANCE_DATA_VECTORS, MAX_INSTANCES / NUM_INSTANCES_PER_LINE, GL_RGBA, GL_FLOAT, 0); // bind PBO to update pixel values glBindBufferARB(GL_PIXEL_UNPACK_BUFFER, textureInstancingPBO[next_frame_index]);
//http://www.songho.ca/opengl/gl_pbo.html // Note that glMapBufferARB() causes sync issue. // If GPU is working with this buffer, glMapBufferARB() will wait(stall) // until GPU to finish its job. To avoid waiting (idle), you can call // first glBufferDataARB() with NULL pointer before glMapBufferARB(). // If you do that, the previous data in PBO will be discarded and // glMapBufferARB() returns a new allocated pointer immediately // even if GPU is still working with the previous data. glBufferData(GL_PIXEL_UNPACK_BUFFER, INSTANCES_DATA_SIZE, 0, GL_STREAM_DRAW_ARB); gpu_data = (float*)glMapBuffer(GL_PIXEL_UNPACK_BUFFER, GL_WRITE_ONLY_ARB); if (gpu_data) { memcpy(gpu_data, complex_mesh_instances_data[0], INSTANCES_DATA_SIZE); // update data glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER); //release pointer to mapping buffer }
使用texture instancing渲染：
//bind texture with instances data glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, textureInstancingDataTex); glBindSampler(0, Sampler_nearest); glBindVertexArray(geometry_vao_id); //what geometry to render tex_instancing_shader.bind(); //with what shader
//tell shader texture with data located, what name it has static GLint location = glGetUniformLocation(tex_instancing_shader.programm_id, s_texture_0); if (location >= 0) glUniform1i(location, 0); //render group of objects glDrawElementsInstanced(GL_TRIANGLES, BOX_NUM_INDICES, GL_UNSIGNED_INT, NULL, CURRENT_NUM_INSTANCES);
顶点着色器示例：
version 150 core
in vec3 s_pos;
in vec3 s_normal;
in vec2 s_uv;
uniform mat4 ModelViewProjectionMatrix;
uniform sampler2D s_texture_0;
out vec2 uv;
out vec3 instance_color;
void main()
{
const vec2 texel_size = vec2(1.0 / 256.0, 1.0 / 16.0);
const int objects_per_row = 128;
const vec2 half_texel = vec2(0.5, 0.5);
//calc texture coordinates - where our instance data located
//gl_InstanceID % objects_per_row - index of object in the line
//multiple by 2 as each object has 2 vectors of data
//gl_InstanceID / objects_per_row - in what line our data located
//multiple by texel_size gieves us 0..1 uv to sample from texture from interer texel id
vec2 texel_uv = (vec2((gl_InstanceID % objects_per_row) * 2, floor(gl_InstanceID / objects_per_row)) + half_texel) * texel_size;
vec4 instance_pos = textureLod(s_texture_0, texel_uv, 0);
instance_color = textureLod(s_texture_0, texel_uv + vec2(texel_size.x, 0.0), 0).xyz;
uv = s_uv;
gl_Position = ModelViewProjectionMatrix * vec4(s_pos + instance_pos.xyz, 1.0);
}
vertex buffer Instancing
通过将数据存储在vbo，然后通过制定不同的数据布局，来制定不同的渲染参数。
buffer创建：
//...code of base vertex declaration creation //special atributes binding glBindBuffer(GL_ARRAY_BUFFER, all_instances_data_vbo); //size of per instance data (PER_INSTANCE_DATA_VECTORS = 2 - so we have to create 2 additional attributes to transfer data) const int per_instance_data_size = sizeof(vec4) * PER_INSTANCE_DATA_VECTORS; glEnableVertexAttribArray(4); glVertexAttribPointer((GLuint)4, 4, GL_FLOAT, GL_FALSE, per_instance_data_size, (GLvoid*)(0)); //tell that we will change this attribute per instance, not per vertex glVertexAttribDivisor(4, 1); glEnableVertexAttribArray(5); //5th vertex attribute, has 4 floats, sizeof(vec4) data offset glVertexAttribPointer((GLuint)5, 4, GL_FLOAT, GL_FALSE, per_instance_data_size, (GLvoid*)(sizeof(vec4)));
//tell that we will change this attribute per instance, not per vertex glVertexAttribDivisor(5, 1);
渲染：
vbo_instancing_shader.bind(); //our vertex buffer wit modified vertex declaration (vdecl) glBindVertexArray(geometry_vao_vbo_instancing_id); glDrawElementsInstanced(GL_TRIANGLES, BOX_NUM_INDICES, GL_UNSIGNED_INT, NULL, CURRENT_NUM_INSTANCES);
顶点着色器：
#version 150 core
in vec3 s_pos;
in vec3 s_normal;
in vec2 s_uv;
in vec4 s_attribute_3; //some_data;
in vec4 s_attribute_4; //instance pos
in vec4 s_attribute_5; //instance color
uniform mat4 ModelViewProjectionMatrix;
out vec3 instance_color;
void main()
{
instance_color = s_attribute_5.xyz;
gl_Position = ModelViewProjectionMatrix * vec4(s_pos + s_attribute_4.xyz, 1.0);
}
Uniform Buffer Instancing:
buffer创建：
glGenBuffers(1, dips_uniform_buffer); glBindBuffer(GL_UNIFORM_BUFFER, dips_uniform_buffer); glBufferData(GL_UNIFORM_BUFFER, INSTANCES_DATA_SIZE, &complex_mesh_instances_data[0], GL_STATIC_DRAW); //uniform_buffer_data glBindBuffer(GL_UNIFORM_BUFFER, 0); //bind iniform buffer with instances data to shader ubo_instancing_shader.bind(true); GLint instanceData_location3 = glGetUniformLocation(ubo_instancing_shader.programm_id, "instance_data"); //link to shader glUniformBufferEXT(ubo_instancing_shader.programm_id, instanceData_location3, dips_uniform_buffer); //actually binding
着色器数据访问：
#version 150 core
#extension GL_ARB_bindable_uniform : enable
#extension GL_EXT_gpu_shader4 : enable
in vec3 s_pos;
in vec3 s_normal;
in vec2 s_uv;
uniform mat4 ModelViewProjectionMatrix;
bindable uniform vec4 instance_data[4096]; //our uniform with instances data
out vec3 instance_color;
void main()
{
vec4 instance_pos = instance_data[gl_InstanceID*2];
instance_color = instance_data[gl_InstanceID*2+1].xyz;
gl_Position = ModelViewProjectionMatrix * vec4(s_pos + instance_pos.xyz, 1.0);
}
Texture Buffer Instancing:
buffer创建：
tbo_instancing_shader.bind(); //bind to shader as special texture glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_BUFFER, dips_texture_buffer_tex); glTexBuffer(GL_TEXTURE_BUFFER, GL_RGBA32F, dips_texture_buffer); glBindVertexArray(geometry_vao_id); glDrawElementsInstanced(GL_TRIANGLES, BOX_NUM_INDICES, GL_UNSIGNED_INT, NULL, CURRENT_NUM_INSTANCES);
顶点着色器数据访问：
#version 150 core
#extension GL_EXT_bindable_uniform : enable
#extension GL_EXT_gpu_shader4 : enable
in vec3 s_pos;
in vec3 s_normal;
in vec2 s_uv;
uniform mat4 ModelViewProjectionMatrix;
uniform samplerBuffer s_texture_0; //our TBO texture bufer
out vec3 instance_color;
void main()
{
//sample data from TBO
vec4 instance_pos = texelFetch(s_texture_0, gl_InstanceID*2);
instance_color = texelFetch(s_texture_0, gl_InstanceID*2+1).xyz;
gl_Position = ModelViewProjectionMatrix * vec4(s_pos + instance_pos.xyz, 1.0);
}
SSBO Instancing:
Buffer创建
glGenBuffers(1, ssbo); glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo); glBufferData(GL_SHADER_STORAGE_BUFFER, INSTANCES_DATA_SIZE, complex_mesh_instances_data[0], GL_STATIC_DRAW); glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, ssbo); glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0); // unbind
渲染代码：
//bind ssbo_instances_data, link to shader at 0 binding point glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo_instances_data); glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, ssbo_instances_data); glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0); ssbo_instancing_shader.bind(); glBindVertexArray(geometry_vao_id); glDrawElementsInstanced(GL_TRIANGLES, BOX_NUM_INDICES, GL_UNSIGNED_INT, NULL, CURRENT_NUM_INSTANCES); glBindVertexArray(0);
顶点着色器：
#version 430 #extension GL_ARB_shader_storage_buffer_object : require in vec3 s_pos; in vec3 s_normal; in vec2 s_uv; uniform mat4 ModelViewProjectionMatrix;
//ssbo should be binded to 0 binding point layout(std430, binding = 0) buffer ssboData { vec4 instance_data[4096]; }; out vec3 instance_color; void main() { //gl_InstanceID is unique for each instance. So we able to set per instance data vec4 instance_pos = instance_data[gl_InstanceID*2]; instance_color = instance_data[gl_InstanceID*2+1].xyz; gl_Position = ModelViewProjectionMatrix * vec4(s_pos + instance_pos.xyz, 1.0); }
Multi draw indirect：
这是一个非常有用的绘制命令，它可以达到跟instancing一样的效果，一次函数调用，多次dips。它对绘制一批Instance，不同geometry非常有用。
代码实例如下：
//fill indirect buffer with dips information. Just simple array
for (int i = 0; i < CURRENT_NUM_INSTANCES; i++)
{
multi_draw_indirect_buffer.vertexCount = BOX_NUM_INDICES;
multi_draw_indirect_buffer.instanceCount = 1;
multi_draw_indirect_buffer.firstVertex = i*BOX_NUM_INDICES;
multi_draw_indirect_buffer.baseVertex = 0;
multi_draw_indirect_buffer.baseInstance = 0;
}
glBindVertexArray(ws_complex_geometry_vao_id);
simple_geometry_shader.bind();
glMultiDrawElementsIndirect(GL_TRIANGLES, GL_UNSIGNED_INT, (GLvoid*)multi_draw_indirect_buffer[0], //our information about dips
CURRENT_NUM_INSTANCES, //number of dips 0);
glMultiDrawElementsIndirect在一次调用中，进行了多次glDrawElementsInstancedIndirect的绘制，但需要注意的是，这条指令有个很烦人的特点，就是每次glDrawElementsInstancedIndirect绘制对应对立的gl_InstanceID,每一次调用，gl_InstanceID都会从0重新开始。这在实际使用过程中一定要注意。
以上就是在实际instancing绘制中，主要会用到的数据组织方式，每一种方式都有自己的使用场景，需要根据实际的项目需要以及平台架构来决定哪种方式的效率最高。
引用link:
https://www.gamedev.net/articles/programming/graphics/opengl-api-overhead-r4614/
https://www.g-truc.net/post-0518.html
http://sol.gfxile.net/instancing.html