OpenGL四相不渲染

Question

我有一個GL_QUADS從一本書例子，我把顏色黑一個，我仍然沒有得到它，我得到的是我清晰的白色屏幕 什麼地方出錯了？

#include "SDL.h" 
#include "SDL_opengl.h" 
#include <iostream> 
int main(int argc, char* args[]) 
{ 
    // Memory usage specifics 
    SDL_Init(SDL_INIT_EVERYTHING); 
    SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8); 
    SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8); 
    SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8); 
    SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8); 
    SDL_GL_SetAttribute(SDL_GL_BUFFER_SIZE, 32); 
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 16); 
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); 

    // name on the window 
    SDL_WM_SetCaption("our first game", NULL); 

    //set screen 
    SDL_SetVideoMode(600,400,32, SDL_OPENGL); 

    // when screen is clear 
    glClearColor(1,1,1,1); //red, green, blue, alpha 

    //portion to display 
    glViewport(0,0,600,400); 

    //shade 
    glShadeModel(GL_SMOOTH); 

    // 3D or 2D 
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); //saves 

    //if only 2D? 
    glDisable(GL_DEPTH_TEST); 
    //this handles the main loop 
    bool isRunning = true; 
    //this is for the events 
    SDL_Event event; 

    //main rectangle 
    float myX = 300; 
    float myY = 350; 
    float width = 50; 
    float height = 30; 

    //main loop 
    while (isRunning) 
    { 
     //EVENTS 
     while (SDL_PollEvent(&event)) 
     { 
      //logic that should happen for certain events 
      //quiting the window 
      if (event.type == SDL_QUIT) 
      { 
       isRunning = false; 
      } 
      // if ESC pressed 
      if (event.type == SDL_KEYUP && event.key.keysym.sym == SDLK_ESCAPE) 
      { 
       isRunning = false; 
      } 
     } 
     //LOGIC 

     //RENDERING TO THE SCREEN 
     glClear(GL_COLOR_BUFFER_BIT); 
     //start rendering phase 
     glPushMatrix(); 
     //set the matix 
     glOrtho(0,600,400,0,-1,1); 
     //black color 
     glColor4ub(0,0,0,255); 

     //drawing 
     //GL_QUADS, GL_POINTS, GL_LINES, GL_LINE_STRIP, 
     //GL_LINE_LOOP, GL_QUADS, GL_TRIANGLES, GL_POLIGON 

     glBegin(GL_QUADS); 
      glVertex2f(myX,myY); 
      glVertex2f(myX+width,myY); 
      glVertex2f(myX+width,myY+height); 
      glVertex2f(myX,myY+height); 
     glEnd(); 



     //render 
     SDL_GL_SwapBuffers(); 
    } 
    //house keeping 
    SDL_Quit(); 
    return 0; 
} 

Answer 1

這並不奇怪，你有一個堆棧溢出等待發生，由於這一行：

//start rendering phase 
glPushMatrix(); 

每一幀，你推「當前」矩陣壓入堆棧的頂部，從不彈出任何東西。最終你會溢出並且glOrtho(0,600,400,0,-1,1);將會有未定義的結果。

我可以問你爲什麼把矩陣推到棧上？如果您在每一幀覆蓋了投影矩陣，那麼您的代碼就可以很好地工作。

我也爲這行代碼不解：

// 3D or 2D 
glMatrixMode(GL_PROJECTION); 
glLoadIdentity(); //saves 

glLoadIdentity (...)不保存任何東西，它寫一個單位矩陣，以目前的矩陣：

1 0 0 0 
0 1 0 0 
0 0 1 0 
0 0 0 1 

由於glOrtho (...)乘以當前矩陣，我實際上會替換第一個提到的代碼。這樣你將總是產生相同的結果，而不是乘以矩陣的前一個值。但是，更重要的是，假設您從不在代碼中的任何其他位置修改矩陣，您不必在每次繪製時都設置投影矩陣。 OpenGL通過緩衝區交換，清除等來保持其所有矩陣的狀態......狀態不會因爲您開始新的框架而消失。