# OpenGL Cannot get my screen to clear

## Recommended Posts

Hi all,

I cannot get my openGL program to clear the screen when I call glClear. All I get during the running of my program is just get an empty white screen. And that's all I'm actually doing at the moment, just trying to clear the screen.

I am terrified that I am about to break the fourth rule of the openGL forum faq, so I will try to keep the code I'm posting to an absolute minimum. However, this project was in directx, and I'm attempting to port it over to opengl. Currently, I cannot get the screen to clear, so I suspect that there's something very simple I'm doing wrong. I'm checking the glGetError function pretty consistently, but it's always returning GL_NO_ERROR. I suspect that the problem may lie in one of two areas, either my openGL instantiation process, or something I'm not doing in my draw function. I'm very new to openGL, so it's probably something very very basic in my draw function. Feel free to ask for more code, I've got tons and tons in this project, but very little actual drawing code.

My draw function:
[source lang = "cpp"]int DrawGLScene(GLvoid)								// Here's Where We Do All The Drawing{  char buf[200];  if(0 != glGetError())  {    sprintf(buf, "OpenGL error code:%d\n",glGetError());    debugText->Add(buf);  }  glClearColor(1.0f, 0.156f, 0.392f, 0.0f);  glPushAttrib(GL_ALL_ATTRIB_BITS);  glPushClientAttrib(GL_CLIENT_ALL_ATTRIB_BITS);  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);			// Clear The Screen And The Depth Buffer  glViewport(0, 0, game_info.vert, game_info.horiz);					// Reset The Current Viewport  glMatrixMode(GL_PROJECTION);						// Select The Projection Matrix  glLoadIdentity();							// Reset The Projection Matrix  // Calculate The Aspect Ratio Of The Window  gluPerspective(45.0f,(GLfloat)game_info.vert/(GLfloat)game_info.horiz,0.1f,100.0f);    glMatrixMode(GL_MODELVIEW);						// Select The Modelview Matrix  glLoadIdentity();							// Reset The Modelview Matrix  if(0 != glGetError())  {    sprintf(buf, "OpenGL error code:%d\n",glGetError());    debugText->Add(buf);  }  return TRUE;								// Everything Went OK}

- Goishin

##### Share on other sites
Hmm not sure. Two things I maybe see, but I'm not confident this is your problem:

1) If you're using doublebuffering, are you swapping the buffer after you finish your draw call?
2) You're calling PushAttrib every loop but not popping it, do you do this somewhere else? I would expect this to crash your application pretty quickly if you don't.

##### Share on other sites
Ah, I am such an idiot!

I wasn't calling SwapBuffers. So I could draw and draw and draw all I want, but if I'm never actually presenting what I'm drawing, I just get a white screen.

Thanks folks, but I wrapped this one up on my own.

- Goishin

##### Share on other sites
Hey Karwosts,

Wow! That was faaaast! But yes, you were exactly right. I wasn't swapping my buffers. And, no, I'm currently not popping my buffers. But thanks very much for the tip! I'll go add that now :)

- Goishin

## Create an account

Register a new account

• ### Forum Statistics

• Total Topics
627726
• Total Posts
2978813
• ### Similar Content

• Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
So, here's what the plan is so far as far as loading goes:
Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!

• I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks

• A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

-What I'm using:
C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.
-Questions
Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?

• Both functions are available since 3.0, and I'm currently using glMapBuffer(), which works fine.
But, I was wondering if anyone has experienced advantage in using glMapBufferRange(), which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
• By xhcao
Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness.

• 10
• 9
• 21
• 14
• 12