vulkan中vertex buffer的用法

-Vulkan的资源(buffer或者image)都用descriptor表示，vertex buffer也是以descriptor的形式来分配和使用的

下面讲的是如果使用vertex buffer来向shader传递顶点数据

1.准备vertex数据
    const float vb[3][5] = {

        /*      position             texcoord */

        { -1.0f, -1.0f,  0.25f,     0.0f, 0.0f },

        {  1.0f, -1.0f,  0.25f,     1.0f, 0.0f },

        {  0.0f,  1.0f,  1.0f,      0.5f, 1.0f },

    };

2.调用vkCreateBuffer创建VkBuffer(buf)，然后用vkAllocateMemory来分配device内存(men)

3.vkMapMemory将device内存map到cpu端(data)

4.memcpy(data, vb, sizeof(vb));

5.调用vkBindBufferMemory(device,buf,mem)将device内存和vkbuffer绑定

6.创建vi，VkPipelineVertexInputStateCreateInfo vi

    demo->vertices.vi.vertexAttributeDescriptionCount = 2;

    demo->vertices.vi.pVertexAttributeDescriptions = demo->vertices.vi_attrs;//vi_attrs是关键

    demo->vertices.vi_attrs[0].binding = VERTEX_BUFFER_BIND_ID;

    demo->vertices.vi_attrs[0].location = 0;

    demo->vertices.vi_attrs[0].format = VK_FORMAT_R32G32B32_SFLOAT;

    demo->vertices.vi_attrs[0].offset = 0;

    demo->vertices.vi_attrs[1].binding = VERTEX_BUFFER_BIND_ID;

    demo->vertices.vi_attrs[1].location = 1;

    demo->vertices.vi_attrs[1].format = VK_FORMAT_R32G32_SFLOAT;

    demo->vertices.vi_attrs[1].offset = sizeof(float) * 3;

7.在create pipeline的时候传入刚创建的vi：pipeline.pVertexInputState = &vi;

8.build cmd的时候，bind之前创建的vertex buffer
    vkCmdBindVertexBuffers(demo->draw_cmd, VERTEX_BUFFER_BIND_ID, 1,&demo->vertices.buf, offsets);

    vkCmdDraw(demo->draw_cmd, 3, 1, 0, 0);

    vkCmdEndRenderPass(demo->draw_cmd);

9.在shader中使用vertex buffer

   vs
#version 400
  #extension GL_ARB_separate_shader_objects : enable
  #extension GL_ARB_shading_language_420pack : enable
  layout (location = 0) in vec4 pos;
  layout (location = 1) in vec2 attr;
  layout (location = 0) out vec2 texcoord;
  void main() {

     texcoord = attr;

     gl_Position = pos;
  }

  fs #version 400
  #extension GL_ARB_separate_shader_objects : enable
  #extension GL_ARB_shading_language_420pack : enable
  layout (binding = 0) uniform sampler2D tex;
  layout (location = 0) in vec2 texcoord;
  layout (location = 0) out vec4 uFragColor;
  void main() {

     uFragColor = texture(tex, texcoord);
  }  

  comment：layout(location = attribute index) in vec3 position;
  跟glBindAttribLocation的功能类似，设置position使用哪个属性:

 static const GLfloat verts[3][2] = {

                { -0.5, -0.5 },

                {  0.5, -0.5 },

                {  0,    0.5 }   };

"attribute vec4 pos;\n"//in shader

glBindAttribLocation(program, window->gl.pos, "pos");

glVertexAttribPointer(window->gl.pos, 2, GL_FLOAT, GL_FALSE, 0, verts);

glEnableVertexAttribArray(window->gl.pos);

glDrawArrays()