[23-1 컴퓨터 그래픽스 정리] shader programing part2

본 글은 국민대학교 김준호 교수님의 "컴퓨터 그래픽스" 강의자료를 정리 목적으로 만들었습니다.
글 자료의 모든 권한은 김준호 교수님께 있습니다.

Shader Programming at a Glance

GLSL Programming at a glance

코드 불러오는 순서

void init_shader_program()
{
// shaderfile불러오기
GLuint vertex_shader = create_shader_from_file('~~~',GL_VERTEX_SHADER);
GLuint fragment_shader = create_shader_from_file('~~~', GL_FRAGMENT_SHADER);


//프로그램 만들기
program = glCreateProgram();
//프로그램에 shader 붙이기
glAttachShader(program, vertex_shader);
glAttachShader(program, fragment_shader);

//gpu에서 uniform 변수의 위치 =>값 할당은 나중에해줌
loc_u_PVM = glGetUniformLocation(program, "u_PVM");
//gpu에서 attribute 변수 의 위치 =>값 할당은 나중에해줌
loc_a_position = glGetAttribLocation(program, "a_position");
loc_a_color = glGetAttribLocation(program, "a_color");
}

void render_object()
{
    //use a program
    glUseProgram(program);

    //Load uniforms
    mat_PVM = mat_proj * mat_view * mat_model;
    glUniformMatrix4fv(loc_u_PVM, 1, GL_FALSE, mat_PVM);

    //Load attributes as per-vertex data
    glBindBuffer(...);
    glEnableVertexAttribArray(...);
    glVertexAttribPointer(...);

    //Draw with per-vertex data
    glDrawArrays(...);
}

More about Shader Programming

shader들이 input을 바꿔서는 안된다. Input값은 Read only

 

I/O Storage Qualifiers in GLSL 1.2

Three types of I/O storage qualifiers in shader

Uniforms

Declare global variables whose values are the same across the entire shaders

 

Attributes

• Declare variables that are passed to a vertex shader from OpenGL on a per-vertex basis

 

Varyings

• Variables that provide the interface between the vertex shader, the fragment shader, and the fixed functionality between them

 

Built-in variables and generic variables are availabe

Built-in type : OpenGL pre-defined constants and uniform state

Generic type : User-defined variables

 

Uniform

Declare values, which do not change during a rendering

Available in both of vertex shader and fragment shader

Read-only

• Initialized either directly by an appliction via API commands or indirectly by OpenGL

 

Attribute

Declare values, which vary for per-vertex

Available in vertex shader only

Read-only

• Passed through the OpenGL vertex API or as part of a vertex array

 

Varyings

Used for passing data from vertex shader to fragment shader

Read/writable in vertex shader

read-only in fragment shader

shader code example

//vertex shader
uniform mat4 u_PVM;

attribute vec3 a_position;
attribute vec43 a_color;

varying vec3 v_color;

void main()
{   // Uniform 변수 PVM과 attribute 변수 a_position을 이용해 vertex마다 위치계산
    gl_Position = u_PVM * vec4(a_position, 1.0f); //gl_position : 예약어
    v_color = a_color; // 여기서 color를 어떻게 가중치를 부여하냐에따라 색이 다르게 나올 수 있음.
}

//fragment shader
varying vec3 v_color; //vertexshader에서 계산한 color를 받아옴
void main()
{
    gl_FragColor = vec4(v_color,1.0f);
}