为什么glOrtho会触发GL_INVALID_OPERATION？

Question

在以下代码中，GL_INVALID_OPERATION调用期间设置了glOrtho()。我无法弄清楚为什么。你看到这些电话有什么问题吗？

#include <SDL2/SDL.h>
#include <GL/gl.h>

#include <iostream>

int main()
{
  SDL_Init( 0 );
  SDL_InitSubSystem( SDL_INIT_VIDEO );

  SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
  SDL_GL_SetAttribute( SDL_GL_CONTEXT_MAJOR_VERSION, 1 );
  SDL_GL_SetAttribute( SDL_GL_CONTEXT_MINOR_VERSION, 1 );
  SDL_GL_SetAttribute( SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES );

  SDL_GL_CreateContext
    ( SDL_CreateWindow
      ( "Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1280, 768,
        SDL_WINDOW_OPENGL ) );

  glViewport( 0, 0, 1280, 768 );
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

  std::cout << "No error: " << glGetError() << " ?= " << GL_NO_ERROR << '\n';

  glOrtho( 0, 1280, 0, 768, -1, 0 );

  std::cout << "Invalid operation: " << glGetError() << " ?= "
            << GL_INVALID_OPERATION << '\n';

  return 0;
}

根据the documentation of glOrtho，在glBegin()和glEnd()之间进行通话时会设置此错误，此处未调用此错误。在the documentation for GLES 1.1中，甚至没有记录此错误。

你知道发生了什么吗？