My code is producing an error when run asynchronously but it works synchronously

Question

I am making a 3d video game with OpenGL with Nim. Originally, I had my code to draw some cubes in my display function, but it uses the {.cdecl.} pragma, meaning I cannot access external variables like the camera position. I switched to an empty display function, with an asynchronous while loop instead. However, when I compiled and ran the program, it produced a GLError, with the message "Error: unhandled exception: OpenGl error: invalid operation [GLError]". Does anyone know why this is happening?

My code:

import opengl, opengl/[glut, glu]
import os, strutils, threadpool
# import camera, controller, dataprotocol, inventory

proc display() {.cdecl.} =
    discard

proc reshape(width: GLsizei, height: GLsizei) {.cdecl.} =
  if height == 0:
    return

  glViewport(0, 0, width, height)

  glMatrixMode(GL_PROJECTION)
  glLoadIdentity()
  
  gluPerspective(45.0, width / height, 0.1, 100.0)

proc drawAll(xpos: float, ypos: float, zpos: float): void =
  glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT)
  glMatrixMode(GL_MODELVIEW)
  glLoadIdentity()
  glTranslatef(1.55+xpos, -1.75+ypos, -7.0+zpos)

  glBegin(GL_TRIANGLES)

  # Top face
  glColor3f(0.0, 1.0, 0.0)  # Green
  glVertex3f(1.0, 1.0, -1.0)
  glVertex3f(-1.0, 1.0, -1.0)
  glVertex3f(-1.0, 1.0,  1.0)
  glVertex3f( 1.0, 1.0,  1.0)
  glVertex3f( 1.0, 1.0, -1.0)
  glVertex3f(-1.0, 1.0,  1.0)

  # Bottom face
  glColor3f(139.0/256.0, 69.0/256.0, 19.0/256.0)  # Brown
  glVertex3f( 1.0, -1.0,  1.0)
  glVertex3f(-1.0, -1.0,  1.0)
  glVertex3f(-1.0, -1.0, -1.0)
  glVertex3f( 1.0, -1.0, -1.0)
  glVertex3f( 1.0, -1.0,  1.0)
  glVertex3f(-1.0, -1.0, -1.0)

  # Front face
  glColor3f(139.0/256.0, 69.0/256.0, 19.0/256.0)  # Brown
  glVertex3f( 1.0,  1.0, 1.0)
  glVertex3f(-1.0,  1.0, 1.0)
  glVertex3f(-1.0, -1.0, 1.0)
  glVertex3f( 1.0, -1.0, 1.0)
  glVertex3f( 1.0,  1.0, 1.0)
  glVertex3f(-1.0, -1.0, 1.0)

  # Back face
  glColor3f(139.0/256.0, 69.0/256.0, 19.0/256.0)  # Brown
  glVertex3f( 1.0, -1.0, -1.0)
  glVertex3f(-1.0, -1.0, -1.0)
  glVertex3f(-1.0,  1.0, -1.0)
  glVertex3f( 1.0,  1.0, -1.0)
  glVertex3f( 1.0, -1.0, -1.0)
  glVertex3f(-1.0,  1.0, -1.0)

  # Left face
  glColor3f(139.0/256.0, 69.0/256.0, 19.0/256.0)  # Brown
  glVertex3f(-1.0,  1.0,  1.0)
  glVertex3f(-1.0,  1.0, -1.0)
  glVertex3f(-1.0, -1.0, -1.0)
  glVertex3f(-1.0, -1.0,  1.0)
  glVertex3f(-1.0,  1.0,  1.0)
  glVertex3f(-1.0, -1.0, -1.0)

  # Right face
  glColor3f(139.0/256.0, 69.0/256.0, 19.0/256.0) # Brown
  glVertex3f(1.0,  1.0, -1.0)
  glVertex3f(1.0,  1.0,  1.0)
  glVertex3f(1.0, -1.0,  1.0)
  glVertex3f(1.0, -1.0, -1.0)
  glVertex3f(1.0,  1.0, -1.0)
  glVertex3f(1.0, -1.0,  1.0)

  glEnd()

  glutSwapBuffers()

# var blocks = string(open("blocks.bmc").readAll).split(";")
# Commented out because not used yet

# glutInit()
glutInitDisplayMode(GLUT_DOUBLE)
glutInitWindowSize(640, 480)
glutInitWindowPosition(50, 50)
discard glutCreateWindow("BlueMC")

if paramCount() == 1 and paramStr(1) == "-fullscreen":
  glutFullScreen()

glutSetCursor(GLUT_CURSOR_CROSSHAIR)

glutDisplayFunc(display)
glutReshapeFunc(reshape)

loadExtensions()

glClearColor(GLFloat(172.0/255.0), GLFloat(246.0/255.0), GLFloat(246.0/255.0), GLFloat(1.0))
glClearDepth(1.0)
glEnable(GL_DEPTH_TEST)
glDepthFunc(GL_LEQUAL)
glShadeModel(GL_SMOOTH)
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST)

proc eventloop(): void {.gcsafe.} =
  while true:
    drawAll(0.0, 0.0, 0.0)

spawn eventloop()
glutMainLoop()

score 2 · Accepted Answer · answered Mar 29 '21 at 15:39

Nim's spawn statement will execute your procedure in a different thread. In OpenGL, the current context is a per-thread property, and an OpenGL conetxt can only be made current to a single thread at each time. Your construct can't work with OpenGL. However, the root cause of your issue is that you use the horribly outdated GLUT framework which forces you to do your render loop with some inverted control flow. If you would use a more modern framework like GLFW, SDL or lots of others, you could implement your event and render loops without any of these hassles.

My code is producing an error when run asynchronously but it works synchronously

1 Answers1