I use the following code trying to render a partial frame of a viewport.
Start and end is a part of the frame, which is correct.
glViewport(start,0,end,SAVE_HEIGHT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(90.0f, 1.0f, NEAR, FAR);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
The code seems out of order; I had it working before. From what I could find of sparse documentation, this seems the correct order however.
The visual output is that I do not see some parts, and the parts that are rendered do not have the correct perspective. As a note, I am rendering to the same FBO each time.
The expected output is that each time the code is called a part of the viewport is rendered to the same fbo.
My question: how can I render to an FBO partially each iteration?
I was looking for glScissor apparently.
Related
I created an OpenGL (GL_TEXTURE_2D) texture, made an OpenCL image2d_t buffer out of it using clCreateFromGLTexture(), I run my OpenCL kernel to draw to the texture, using clEnqueueAcquireGLObjects and clEnqueueReleaseGLObjects before and after, and then I display the result in OpenGL by doing this (I'm trying to simply draw my framebuffer texture to the window with no scaling, is this even the right way to do it?):
glBegin(GL_QUADS);
glTexCoord2f(0.0, 0.0); glVertex2f(0.0f, 0.0f);
glTexCoord2f(1.0, 0.0); glVertex2f(1920.0f, 0.0f);
glTexCoord2f(1.0, 1.0); glVertex2f(1920.0f, 1080.0f);
glTexCoord2f(0.0, 1.0); glVertex2f(0.0f, 1080.0f);
glEnd();
SDL_GL_SwapWindow(window);
It works great except there's one problem, I can't figure out how to fill the texture with zeroes in order to blank it, so I see old pixels where no new ones were written.
Apparently there's no way to blank an OpenCL image object from the host, so I'd have to do it in OpenGL (before calling clEnqueueAcquireGLObjects of course), so I tried this:
glClearColor(0,0,0,0);
glClear(GL_COLOR_BUFFER_BIT);
However this doesn't do anything. It does do something on screen when I comment out the GL_QUADS block above, it's like glClear doesn't do anything on the texture itself but rather the screen directly. It's confusing, I'm not very familiar with OpenGL, how do I blank my texture?
OpenGL glClear clears the currently bound framebuffer. To make this operate on a texture create a framebuffer object, attach the texture to it, bind the framebuffer object for drawing operations and clear it (which will then clear the texture).
I am trying to draw a 2D scene with a texture as background and then ( as the program flows and does computations ) draw different primitives on the "canvas". As a test case I wanted to draw a blue quad on the background image.
I have looked at several resources and SO questions to try get the information I need to accomplish the task ( e.g. this tutorial for first primitive rendering, SOIL "example" for texture loading ).
My understanding was that the texture will be drawn on Z=0, and quad as well. Quad would thus "cover" a portion of texture - be drawn on it, which is what I want. Instead the result of my display function is my initial texture in black/blue colour, and not my texture ( in original colour ) with a blue quad drawn on it. This is the display function code :
void display (void) {
glClearColor (0.0,0.0,0.0,1.0);
glClear (GL_COLOR_BUFFER_BIT);
// background render
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, 1024.0, 512.0, 0.0, 0.0, 1.f); // window size is 1024x512
glEnable( GL_TEXTURE_2D );
glBindTexture( GL_TEXTURE_2D, texture );
glBegin (GL_QUADS);
glTexCoord2d(0.0,0.0); glVertex2d(0.0,0.0);
glTexCoord2d(1.0,0.0); glVertex2d(1024.0,0.0);
glTexCoord2d(1.0,1.0); glVertex2d(1024.0,512.0);
glTexCoord2d(0.0,1.0); glVertex2d(0.0,512.0);
glEnd(); // here I get the texture properly displayed in window
glDisable(GL_TEXTURE_2D);
// foreground render
glLoadIdentity();
gluPerspective (60, (GLfloat)winWidth / (GLfloat)winHeight, 1.0, 100.0);
glMatrixMode(GL_MODELVIEW);
glColor3f(0.0, 0.0, 1.0);
glBegin (GL_QUADS);
glVertex2d(400.0,100.0);
glVertex2d(400.0,500.0);
glVertex2d(700.0,100.0);
glVertex2d(700.0,500.0);
glEnd(); // now instead of a rendered blue quad I get my texture coloured in blue
glutSwapBuffers(); }
I have already tried with many modifications, but since I am just beginning with OpenGL and don't yet understand a lot of it, my attempts failed. For example, I tried with pushing and popping matrices before and after drawing the quad, clearing the depth buffer, changing parameters in gluPerspective etc.
How do I have to modify my code so it will render the quad properly on top of the background texture image of my 2D scene ? Being a beginner, extra explanations of the modifications ( as well as mistakes in the present code ) and principles in general will be greatly appreciated.
EDIT - after answer by Reto Koradi :
I have tried to follow the instructions, and the modified code now looks like :
// foreground render
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glOrtho(0.0f, 1024.0, 512.0, 0.0, 0.0, 1.f);
glColor3f(0.0, 0.0, 1.0);
glBegin (GL_QUADS); // same from here on
Now I can see the blue "quad", but it is not displayed properly, it looks something like this .
Beside that, the whole scene is flashing really quickly.
What do I have to change in my code so that quad will get displayed properly and screen won't be flashing ?
You are setting up a perspective transformation before rendering the blue quad:
glLoadIdentity();
gluPerspective (60, (GLfloat)winWidth / (GLfloat)winHeight, 1.0, 100.0);
The way gluPerspective() is defined, it sets up a transformation that looks from the origin down the negative z-axis, with the near and far values specifying the distance range that will be visible. With this transformation, z-values from -1.0 to -100.0 will be visible. Which does not include your quad at z = 0.0.
If you want to draw your quad in 2D coordinate space, the easiest solution is to not use gluPerspective() at all. Just use a glOrtho() type transformation like you did for your initial drawing.
If you want perspective, you will need a GL_MODELVIEW transformation as well. You can start with a translation in the negative z-direction, within a range of 1.0 to 100.0. You may have to adjust your coordinates for the different coordinate system as well, or use additional transformations that also translate in xy-direction, and possibly scale.
The code also has the coordinates in the wrong order for drawing the blue quad. You either have to change the draw call to GL_TRIANGLE_STRIP (recommended because it at least gets you one step closer to using features that are not deprecated), or swap the order of the last two vertices:
glBegin (GL_QUADS);
glVertex2d(400.0,100.0);
glVertex2d(400.0,500.0);
glVertex2d(700.0,500.0);
glVertex2d(700.0,100.0);
glEnd(GL_QUADS);
I just started loading some obj files and render it with opengl. When I render these meshes I get this result (see pictures).
I think its some kind of depth problem but i cant figure it out by myself.
Thats the parameters for rendering:
// Dark blue background
glClearColor(0.0f, 0.0f, 0.4f, 0.0f);
// Enable depth test
glEnable( GL_DEPTH_TEST );
// Cull triangles which normal is not towards the camera
glEnable(GL_CULL_FACE);
I used this Tutorial code as template. https://code.google.com/p/opengl-tutorial-org/source/browse/#hg%2Ftutorial08_basic_shading
The problem is simple, you are doing FRONT or BACK culling.
And the object file contains CCW(Counter-Clock-Wise) or CW (Clock-Wise) cordinates, so written from left to right or right to left.
Your openGL code is expecting it in the other way round, so it hides the surfaces which you are looking backward on.
To check this solves your problem, just take out the glEnable(GL_CULL_FACE);
As this exactly seems to be producing the problem.
Additionally you can use glCullFace(ENUM); where ENUM has to be GL_FRONT or GL_BACK.
If you don't in at least one of both cases can't see your mesh (means in both cases: GL_FRONT or GL_BACK your just seeing the partial mesh) , thats a problem with your code of interpreting the .obj. or the .obj uses not strict surface vectors. (A mix of CCW and CW)
I am actually unsure what you mean, however glEnable(GL_CULL_FACE); and then GL_CULL_FACE(GL_BACK); will cull out or remove the back face of the object. This greatly reduces the lag while rendering objects, and only makes a difference if you are inside or "behind" the object.
Also, have you tried glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); before your render code?
I'm trying to use GWEN to draw some GUI elements on top of my opengl scene. It seems to have set up correctly but nothing from gwen is actually being drawn (visibly at least). I'm using a custom renderer which is essentially GWEN's stock opengl renderer but with a different function for loading textures. And OpenGL::Begin() and OpenGL::End() replaced with these:
void coRenderer::Begin()
{
glUseProgram(0);
glDisable(GL_DEPTH_TEST);
glDepthMask(0);
glEnable(GL_BLEND);
glMatrixMode(GL_PROJECTION); // Select The Projection Matrix
glPushMatrix(); // Store The Projection Matrix
glLoadIdentity();
glOrtho(0, screen->w, screen->h, 0, -1, 1 );
glMatrixMode(GL_MODELVIEW);
glActiveTexture(GL_TEXTURE0);
}
void coRenderer::End()
{
Flush();
glMatrixMode(GL_PROJECTION); // Select The Projection Matrix
glPopMatrix(); // Restore The Old Projection Matrix
glDisable(GL_BLEND);
glEnable(GL_DEPTH_TEST);
glDepthMask(1);
glEnable(GL_TEXTURE_2D);
}
the code for gwen's opengl renderer is here:
http://gwen.googlecode.com/svn/trunk/trunk/gwen/Renderers/OpenGL/OpenGL.cpp
BTW I'm using OpenGL 2.1 not 3.0+
Ah GWEN. That frustrating GUI library.
When I started using it, and integrating it into the engine we wrote in school, I had the same issue as you, using the stock OpenGL renderer however. Turned out it was being positioned wrong, calling glLoadIdentity() to reset the identity matrix seemed to resolve it.
The issue you are having, could well end up being the same as what I had, or there could be a problem with your custom OpenGL renderer. I'm not sure if you know much about GWEN, or how it works, but it runs on a single texture, that skins the GUI. Are you loading that in? Perhaps your texture loader isn't loading it correctly.
Try using your Debugger and stepping through your program. Areas of interest would be where you're attempting to load the GUI skin, where you're assigning the screen space that GWEN can use, and when you're actually attempting to render the GUI.
I have been trying to create a hud in my OpenGL application. Having looked around, it seems the way to do it is with an ortho projection, but so far I have not been able to get the program to render correctly. What is happening is instead of rendering on top of my display, I'm getting odd graphical glitches as seen here:
If I comment out the hud code, everything renders perfectly.
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
//Set up projection matrix
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//Using gluPerspective. It's pretty easy and looks nice.
gluPerspective(fov, aspect, zNear, zFar);
//Set up modelview matrix
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//3D rendering
glDepthMask(GL_FALSE);
glDisable(GL_DEPTH_TEST);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0,window_width,0,window_height); //left,right,bottom,top
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(0.0,0.0,1.0);
glBegin(GL_QUADS);
glVertex2f(50,50);
glVertex2f(50,100);
glVertex2f(100,100);
glVertex2f(100,50);
glEnd();
Once you're done rendering the HUD, you need to re-enable the depth writes
glDepthMask(GL_TRUE);
When you clear the buffers when rendering your HUD, all that has been
drawn so far (your 3D scene) will also be cleared. So don't clear the buffer twice.