參考資訊:
https://wiki.libsdl.org/FrontPage
https://kaprikawn.wordpress.com/2018/01/07/learning-to-learn-opengl-es-2-0-part-01/
http://www.subdude-site.com/WebPages_Local/RefInfo/Computer/Linux/LinuxGuidesOfOthers/linuxProgrammingGuides/pdfs/MobileDeviceProgramming/MOBogl_OpenGL_ES_2-0_ProgrammingGuide_2009_457pgs.pdf
OpenGL ES的Shader包含:Vertex、Fragment兩類,分別用來描述頂點和顏色,使用的程式語言則是Shader Language,Attribute是用來描述Vertex頂點的資料,而Uniform則是傳入給Shader的資料,由於是傳入的資料,因此是Read Only屬性,無法在Shader裡面做任意修改,另外還有一個叫Varying,它是用來在Vertex和Fragment之間傳遞使用
Vertex Shader會將Attribute資料轉換成對應的Varying資料,Varying資料就是要帶給Fragment Shader的資料,gl_Position是一個內建的Varying變數,需要將轉換後的資料傳入gl_Position
Fragment Shader會將Varying資料轉換成對應的顏色,gl_FragColor是一個內建的顏色變數,需要將轉換後的顏色傳入gl_FragColor
main.c
#include <stdio.h> #include <stdlib.h> #include <SDL2/SDL.h> #include <GLES2/gl2.h> const char *vShaderSrc = "attribute vec4 vPosition; \n" "void main(void) \n" "{ \n" " gl_Position = vPosition; \n" "}"; const char *fShaderSrc = "#ifdef GL_ES \n" "precision mediump float; \n" "#endif \n" "uniform vec4 vColor; \n" "void main(void) \n" "{ \n" " gl_FragColor = vColor; \n" "}"; int main(int argc, char **argv) { SDL_Window *window = NULL; SDL_GLContext context = {0}; SDL_Init(SDL_INIT_VIDEO); SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES); SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2); SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0); window = SDL_CreateWindow("main", 0, 0, 320, 240, SDL_WINDOW_OPENGL); context = SDL_GL_CreateContext(window); GLuint vShader = 0; GLuint fShader = 0; GLuint pObject = 0; vShader = glCreateShader(GL_VERTEX_SHADER); glShaderSource(vShader, 1, &vShaderSrc, NULL); glCompileShader(vShader); fShader = glCreateShader(GL_FRAGMENT_SHADER); glShaderSource(fShader, 1, &fShaderSrc, NULL); glCompileShader(fShader); pObject = glCreateProgram(); glAttachShader(pObject, vShader); glAttachShader(pObject, fShader); glLinkProgram(pObject); glUseProgram(pObject); GLfloat v[] = { 0.0f, 0.5f, 0.0f, -0.5f, -0.5f, 0.0f, 0.5f, -0.5f, 0.0f }; GLuint vp = glGetAttribLocation(pObject, "vPosition"); glVertexAttribPointer(vp, 3, GL_FLOAT, GL_FALSE, 0, v); glEnableVertexAttribArray(vp); GLuint vc = glGetUniformLocation(pObject, "vColor"); glUniform4f(vc, 1.0f, 0.0f, 0.0f, 1.0f); glViewport(0, 0, 320, 240); glClear(GL_COLOR_BUFFER_BIT); glDrawArrays(GL_TRIANGLES, 0, 3); SDL_GL_SwapWindow(window); SDL_Delay(3000); SDL_DestroyWindow(window); SDL_GL_DeleteContext(context); SDL_Quit(); return 0; }
vShaderSrc:宣告一個Attribute vPosition,vPosition會直接把Vertex頂點資料傳給gl_Position
fShaderSrc:宣告一個Uniform vColor,vColor會直接把顏色資料傳遞給gl_FragColor
LN59~61:取得vPosition位置並且設定三角形的頂點資料
LN63~64:取得vColor位置並且設定顏色為紅色
編譯、執行
$ gcc main.c -lSDL2 -lGLESv2 -o main $ ./main