Android VideoPlayer 调用的OpenGL ES

2019-11-25  本文已影响0人  小老鼠oo

这次主要是为了看我们的OGLES driver里为什么会malloc的比mali多，dumpheap发现排名靠前的都是与texture有关的，我记得是glTexImage2D和glTexSubImage。里面是为了Mipmap临时malloc了system memory，但是谁知道后面会不会再用？不敢轻易free。OGL ES API我也不熟，怎么也得系统的看看。

书籍参考OpenGL ES 3.0 Programming Guide, www.java.1234.com可以下载，电子版，带目录。

这本书的中文版有点问题，他翻译的太多了，且不给英文原文，违和感很强。比如在书里看了统一变量，上下文，片段着色器，绘图表面的知识，再看代码，却很难在看到Uniform，context，fragment shader，surface时找到共鸣。

直接看英文书又太难，还是先看了中文熟悉基本的概念，再看英文，好看一点。

EGL API序列：

前面的API都是给状态，状态保存在glContext，只有Draw才会真正让GPU做事情，所有的draw API都会走到DrawPrimitive()，后面的Begin() => ValidateAttribute()可以看到所有的过程：

开始是EGL调用，EGL提供了下面的机制：

eglGetDisplay(EGL_DEFAULT_DISPLAY)=display_0;

eglInitialize(display_0, NULL, NULL);

DEFINE POINTER name=attrib_list_0, type=egl_attrib_list, count=0, data=imm{EGL_RENDERABLE_TYPE=EGL_OPENGL_ES2_BIT, EGL_RED_SIZE=8, EGL_GREEN_SIZE=8, EGL_BLUE_SIZE=8, EGL_ALPHA_SIZE=8, EGL_DEPTH_SIZE=0, EGL_CONFIG_CAVEAT=EGL_NONE, EGL_STENCIL_SIZE=8, EGL_SURFACE_TYPE=EGL_WINDOW_BIT, EGL_NONE};
eglChooseConfig(display_0, attrib_list_0, configs_0, 1, pointer_0);

eglCreateContext(display_0, configs_0[0], EGL_NO_CONTEXT, attrib_list_1)=context_0; //0x3

DEFINE POINTER name=attrib_list_2, type=egl_attrib_list, count=0, data=imm{EGL_WIDTH=1, EGL_HEIGHT=1, EGL_NONE};
eglCreatePbufferSurface(display_0, configs_0[0], attrib_list_2)=surface_0;

eglMakeCurrent(display_0, surface_0, surface_0, context_0);

这里的代码现在看也觉得很乱，后面再补充。至此，EGL调用做完了，本地环境有了，开始调用GL API.

GL ES API序列：

Vertex：

DEFINE POINTER name=buffer_0, type=uint, count=1, data=imm{null};
glGenBuffers(1, buffer_0);
API : its about Vertex Buffer Object
通过vertex arrays指定的vertex data保存在client memory，在调用glDrawArrays/DrawElements时必须保证这些data从client memory复制到了graphics memory。但是，不需要每次draw call都copy vertex data的，一直cache data in graphics memory更好。这就是vertex buffer objects的目的。
Vertex buffer objects 允许AP从graphics memory去allocate和cache vertex data，并从这个memory 去render，从而避免每次画primitive时都要resending data。
不只是vertex data, 描述primitive vertex indices的element indices，作为glDrawElements的arg，也可以cached在这个memory。
OGLES 3.0支持两种buffer object：

glBindBuffer(GL_ARRAY_BUFFER, buffer_0[0]);

DEFINE POINTER name=buffer_1, type=ubyte, count=64, file_offset=0, data=file{combine_0.bin};
glBufferData(GL_ARRAY_BUFFER, 64, buffer_1, GL_STATIC_DRAW);

glEnableVertexAttribArray(0x0);

//Example 6-3 Using Constant and Vertex Array Attributes
int Init ( ESContext *esContext )
{
    UserData *userData = (UserData*) esContext->userData;
    const char vShaderStr[] = 
        "#version 300 es \n"
        "layout(location = 0) in vec4 a_color; \n"
        "layout(location = 1) in vec4 a_position; \n"
        "out vec4 v_color; \n"
        "void main() \n"
        "{ v_color = a_color; \n"
        " gl_Position = a_position; }\n" ;

    const char fShaderStr[] = 
        "#version 300 es \n"
        "precision mediump float; \n"
        "in vec4 v_color; \n"
        "out vec4 o_fragColor; \n"
        "void main() \n"
        "{ o_fragColor = v_color; \n } " ;

        GLuint programObject;
        //Create program object;
        programObject = esLoadProgram(vShaderStr, fShaderStr);
        if(programObject == 0)  return GL_FALSE;
        //Store program object
        userData->programObject = programObject;
        glClearColor(0.0f, 0.0f, .0.0f, 0.0f );
        return GL_TRUE;
}

void Draw(ESCOntext *esContext)
{
    UserData *userData = (UserData*)esContext->userData;
    GLfloat color[4] = {1.0f, 0.0f, 0.0f, 1.0f};
    //3 vertices, with(x,y,z)per-vertex
    GLfloat vertexPos[3*3] =
    { 0.0f,0.5f,0.0f, //v0
      -0.5f,-0.5f,0.0f, //v1
      0.5f,-0.5f,0.0f //v2
    };

    glViewport(0, 0, esContext->width, esContext->height);
    //
    glClear(GL_COLOR_BUFFER_BIT);
    glUseProgram(userData->programObject);
    glVertexAttrib4fb(0, color);
    glVettexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, vertexPos);
    glEnableVertexAttribArray(1);
    glDrawArrays(GL_TRANINGES, 0 ,3);
    glDisplayVertexAttribArray(1);
}

vertex attribute:color，用glVertexAttrib4fb指定一个常量，没有enable vertex attribute array 0。
vertexPos这个attribute,通过glVertexAttribPointer指定了使用vertex array，并enable array(by glEnableVertexAttribArray)
所以，color值对所有vertices of triangle都是一样的，但是每个vertices的vertexPos attribute是不同的。
上面的code用到了glVertexAttrib4fv(index, *values);用于指定Vertex Attribute Data，
Vertex attribute data，可以通过vertex array为每个vertex指定，也可以把一个constant value用于primitive的所有vertices。

void glVertexAttribPointer(index, size, type, normalized, stride, *ptr)
index : specifies generic vertex attibute index
size: 通过index引用的vertex attirbute的分量，1-4
stirde: as pitch(跨距) to get vertex data for next index.
ptr : 使用client-size vertex array时，指向持有vertex attibute data的buffer； 使用vertex buffer object时，表示offset in that buffer。

stirde: as pitch(跨距) to get vertex data for next index.
ptr : 使用client-size vertex array时，指向持有vertex attibute data的buffer； 使用vertex buffer object时，表示offset in that buffer。

先走着

texture：

glActiveTexture(GL_TEXTURE0);

//Vertex shader
#version 300 es
layout(location = 0) in vec4 a_position;
layout(location = 1) in vec2 a_texCoord;
out vec2 v_texCorrd;
void main()
{
    gl_Position = a_position;
     v_texCoord = a_texCoord;
}

//Fragment shader
#version 300 es
precision mediump float;
in vec2 v_texCoord;
layout(location = 0) out vec4 outColor;
uniform sampler2D s_texture;
void main()
{
    outColor = texture( s_texture, v_texCoord );
}

vertex shader以一个two-component texture coordinate作为vertex input, then passes it as output to framgnet shader.
fragment shader consumes the texture coordinate and used it for texture fetch(读取).
fragment shader声明了一个类型为sampler2D的uniform variable，called s_texture。

sampler是uniform变量的一个特殊类型，用来fetch from a texture map，读取纹理贴图。
sampler uniform will be loaded with a value specifying texture unit to which texture is bound;
比如，specifying a sampler with value of 0, says to fetch from unit GL_TEXTURE0，specifying a value of 1 says to fetch from GL_TEXTURE1, and so on.

Textures are bound to texture units by glActiveTexture(GLenum texture)
texture: texture unit to make active: GL_TEXTURE0/1/2/3/.../31/
glActiveTexture设置了current texture unit，以便后续的glBindTexutre()会Bind texture to currently active uint。
example show how sampler and texture are bound to texture unit:

//get the sampler locations
userData->samplerLoc = glGetUniformLocation(userData->programObject,"s_texture");

//bind texture
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, userData->textureId);

//set sampler texture unit to 0
glUniformli(userData->samplerLoc, 0);

到这里，我们已经load了texture, texturen bound to texture unit 0 , sampler set to use texture uint 0.
回到simple_texture2D sample，可以看到，shader code使用了内建函数texture,来fetch from texture map，

vec4 texture(sampler2D sampler, vec2 coord[,float bias])
sampler: sampler bound to a texture unit, 来指定要fetch的texture
coord: a 2D texture coordinate, 用来fetch from texture map
bias: 可选参数，提供texture fetch所用的mipmap bias(mip贴图 偏置),

texture()返回一个vec4，代表color fetched fromt texture map.
texture data mapped到color channels的方式，依赖于texture base format。
下表展示了texture format map到vec4 color的方式，texture swizzles决定了，每个分量的值，如何map到shader的分量

base format Texel Data 纹素
GL_RGB R G B 1.0
GL_RGBA R G B A
GL_LUMINANCE L L L 1.0
GL_LUMINANCE_ALPHA L L L A
GL_ALPHA 0.0 .0.0 0.0 A

这里有个texture swizzles，9.1.9纹理调配讲了：texture swizzles control how color components in input R,RG,RGB,RGBA texture map to components when fetched from in the shader.
感觉和allocation->flag.swizzled不一样，这个对应的是tiled，

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