# OpenGL Need some basic undertanding of OpenGL state mechanics

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Hi all,

I apologize in advance for the ambiguity of the question and the way I'm about to present it. I am trying to create a head's up display in a simple font displaying debugging information which will track x, z and y transformations. I can get that to work, but the scene rendering loses it's texture and is painted in the same color as the text (black). Also, the desired debugging text is not rendered.

If I comment out the scene rendering stuff, the text will render and does what it should. If I comment out the text drawing routine, the scene renders fine. I would like to do both. Not just for debugging but any game needs HUD's to display player / game status etc.

For kicks, I'm using SDL to manage my window. I have a routine which renders an landscape environment from GL_QUADS, with an applied texture, another which does translations on the MODELVIEW_MATRIX based on user input. I have a separate routine which switches to orthographic projection and prints out respective x, y, z positions plus rotation angles in the upper left hand corner of the screen. When I call the text rendering routine, I push the MODELVIEW matrix, do the orthographic rendering, and then pop it back. I've investigated this to some extent and am basing my assumptions about what should be happening based on several tutorials, namely swift and nehe. The font rendering mostly came from NeHe but I adapted it for SDL.

Here is some sample code:

routine, void setupOpengl(). This sets up the perspective, viewport and applies texture (I have lighting and fog etc. But I'm trying to reduce as much noise as possible to solve this.)

void setupOpengl() {    //sky blue color for background    const float bgColor[4] = {.7, .8, 1.0, 1.0};    glClearColor(bgColor[0], bgColor[1], bgColor[2], bgColor[3]);        //basic screen setup. width and height provided by #defines for the screen size sdl has rendered.    glMatrixMode(GL_PROJECTION);    glViewport(0, 0, width, height);    glEnable(GL_DEPTH_TEST);    gluPerspective(45, (float)width/height, .2, 80);    //adapted from a nehe lesson for sdl.     BuildFont();    glShadeModel(GL_SMOOTH);        glMatrixMode(GL_MODELVIEW);        //setup texture:    glEnable(GL_TEXTURE_2D);    myTexture = loadTexture("textures/grassb512.bmp", false);}

the preceding code was called from main which then calls the a buildTerrain() function which renders the terrain of quads. This is compiled as a list since it is only computed once:

void buildTerrain(){//here I set up a 2d array of x and y coordinates containing randomly generated values which are heights. I then apply those to my GL_QUADS vertices:    glNewList(1, GL_COMPILE);    // Terrain rendering routine goes here    glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);    //This is the texture loaded from setupOpenGL.    glBindTexture(GL_TEXTURE_2D, myTexture);    glBegin(GL_QUADS);        for(int x = 0; x<tWidth-1; x++){        for(int y = 0; y<tHeight-1; y++){            glTexCoord2f(0.0f, 0.0f);            glVertex3f(x+0, terrain[x+0][y+1], y+1);            glTexCoord2f(1.0f, 0.0f);            glVertex3f(x+1, terrain[x+1][y+1], y+1);            glTexCoord2f(1.0f, 1.0f);            glVertex3f(x+1, terrain[x+1][y+0], y+0);            glTexCoord2f(0.0f, 1.0f);            glVertex3f(x+0, terrain[x+0][y+0], y+0);        }    }     glEnd();    glEndList();    }

Yah so my rendering and translation occurs in a routine which loops continuously (until esc key is pressed) which renders the list defined above and my text drawing routine (or at least, is supposed to.)
void mainLoop() {    while(true) {        //assign values based on user input:        processEvents();        //values derived from event handler        y_input += y_pos;        x_input += x_pos;        z_input += z_pos;              cw_input  += x_rot;        ccw_input += y_rot;                    // Graphical commands...        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);        glLoadIdentity();        //routine to draw above values in the upper left hand corner of the screen.        drawText();        //perform our necessary translations        glRotatef(cw_input, 0.0, 1.0, 0.0);                glRotatef(ccw_input, 0.0, -1.0, 0.0);                            glTranslatef(-tWidth/2, -12, -tHeight/2);        glTranslatef(x_input, y_input, z_input);  	glCallList(1); // <--(Render terrain from list) described above.	            //I am not sure at all what this does:        SDL_GL_SwapBuffers();            }}

So the orthographic stuff to draw the text, called from above:

void drawText() {        //here we are rendering something in a different matrix:      	glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);      	glMatrixMode   (GL_PROJECTION);        //this is the sticky point for me. I assume I'm pushing necessary matrix data for the ortho stuff, drawing it and reverting back to the former state (all of the stuff above, when I'm finished.       	glPushMatrix();        glLoadIdentity ();        gluOrtho2D(0, width, 0, height); //select the 2d viewport.        glScalef(1, -1, 1); //flip everthing upside down.        glTranslatef(0, height, 0);        //print my font in black      	glColor3f(0.0f,0.0f,0.0f);	     	    //glRasterPos2f(0.0, -height - 5); // prints it at the bottom.            //using glRasterPos2f from Nehe tutorial 	    glRasterPos2f(0.0, -height - (height - 15)); 	    glPrint("x_input - %4.2f", x_input); 	    //and so on. This is one line out of many. 	    //the swift tutorial claims to put this line here: 	    glMatrixMode(GL_PROJECTION);        //now I want to go back to rendering all of the stuff I was doing earlier:        glPopMatrix();        glMatrixMode(GL_MODELVIEW);	    }

So, what am I missing here. It seems that I have a great lack of understanding of the state mechanics of openGL. Do I need to also preserve state for the textures matrix? (it prints my quads as black when they're drawn.) How is it that I can either get the list to compile or the drawText() routine to work but not both? What do I need to do to preserve state for each of these different routines? Is it costly? Is it an SDL issue? I have a pretty good idea that it is basically arising from mistaken assumptions I'm making about how openGL operates - how it preserves state and / or what state conditions it preserves. I need one of those "aha" moments when it will all become clear to me.

Thanks for your time with such a dysfunctional issue.

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Hi,

at a quick glance you're calling glClear twice each frame, once for the terrain and once for the text. This means that the terrain never gets drawn if you draw the text after it. It only needs to be called once each frame.

Hope that helps.

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I certainly wish it did friend but commenting out the glClear() call in the drawText() routine did not change the behavior I described. This is why I'm at such a loss with how states are handled. My assumption is: I compile a list object that draws something in the modelview. before I draw that, I gather information about that and switch to orhtographic and draw the "text" data. I then switch back and perform necessary translations on the modelview data and draw that. There must be something else I'm missing.

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Can you confirm that when you comment out glPrint the terrain is displayed with a texture?
If not it might be a good idea to comment things out in your text drawing function until the terrain is drawn with a texture so that you can narrow it down to figure out exactly what state is causing the terrain texture not to be drawn.

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Wait, sorry about that. I commented out the glClear in the terrain rendering routine as well. That helped but the quads textures are still in black. If you see any glaring missteps in my code, let me know, but I will see if I can get those textures to render.

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@stevenmarky: Apparantly, the texture is rendered, but the color I specify for the text overrides everything and also colors the quads that are textured. I tested this by changing the rgb for the text in the drawText() routine from 0 ,0,0 to 0,5,0 and saw my textures but with a massively green hue to them.
Thanks to JackTheRapper for pointing out a major crux of my issue though. If I or anybody else can figure out why the drawText() routine is coloring my landscape and not just the text, then we can close this thing, call it solved and go home. Thanks for the insight. Much appreciated!!!

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Now that I think about it, when one calls glColor3f() it is outside of any matrix is it not? I am basically stating a global color for rendering the next thing on the stack - which is my modelview - the rendered terrain. So. . . what would I need to tell openGl "hey! apply this color to the rendered font in this context!"?

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The glColor functions are independent of the matrix stack and any scope you are in, I think your solution is to set the color back to white after drawing your text.

e.g. glColor3f(1.0f, 1.0f, 1.0f);

This would work because the color of the texel being drawn is multiplied by the color of the vertex.

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Whew, that is correct. I remember reading that after I posted this response. Thanks for everybody's help and advice. I think we can mark this "Solved" (For now, I just removed the color call altogether and the text is rendered with the ground texture. Which will do for now.)

Thanks!

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Video:

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Take a look at my code:
Sprite Class
(You mostly need to see the Constructor, the Render Method and the Move Method)
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Window Class:
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Brain Class
#include "Brain.h" #include "Sprite.h" #include "Window.h" struct Brain::Implementation { //Just A Flag. bool started; //Window Pointer. Window *window; //Sprite Pointer. Sprite *sprite; }; Brain::Brain(Window *window, Sprite *sprite) { //Create Pointer To Implementation. m_Impl = new Implementation(); //Initialize Implementation. m_Impl->started = true; m_Impl->window = window; m_Impl->sprite = sprite; } Brain::~Brain() { //Delete Pointer To Implementation. delete m_Impl; } void Brain::Start() { } void Brain::Update() { } Window * Brain::GetWindow() { return m_Impl->window; } Sprite * Brain::GetSprite() { return m_Impl->sprite; } bool Brain::GetStart() { return m_Impl->started; } void Brain::SetStart(bool value) { m_Impl->started = value; } Script Class (Its a Brain Subclass!!!)
#include "Script.h" Script::Script(Window *window, Sprite *sprite) : Brain(window, sprite) { } Script::~Script() { } void Script::Start() { std::cout << "Game Started!" << std::endl; } void Script::Update() { Input *input = this->GetWindow()->GetInput(); Sprite *sp = this->GetSprite(); //Move this sprite. this->GetSprite()->Move(200 * this->GetWindow()->GetDeltaTime(), input->GetKeyDown("left"), input->GetKeyDown("right"), input->GetKeyDown("up"), input->GetKeyDown("down")); std::cout << sp->GetTag().c_str() << ".x = " << sp->GetPos()->x << ", " << sp->GetTag().c_str() << ".y = " << sp->GetPos()->y << std::endl; }
Main:
#include "SpaceShooterEngine.h" #include "Script.h" int main() { Window w("title", 600,600); Scene *scene = new Scene(); Sprite *player = new Sprite("Resources/Images/player.png", "Player", 100,100); Sprite *other = new Sprite("Resources/Images/cherno.png", "Other", 400, 100); Sprite *other2 = new Sprite("Resources/Images/cherno.png", "Other", 300, 400); Brain *brain = new Script(&w, player); player->AddBrain(brain); scene->AddSprite(player); scene->AddSprite(other); scene->AddSprite(other2); w.LoadScene(scene); w.MainLoop(); return 0; }

I literally can't find what is wrong. If you need more code, ask me to post it. I will also attach all the source files.
Brain.cpp
Error.cpp
IndexBuffer.cpp
Input.cpp
Renderer.cpp
Scene.cpp
Sprite.cpp
Texture.cpp
VertexArray.cpp
VertexBuffer.cpp
VertexBufferLayout.cpp
Window.cpp
Brain.h
Error.h
IndexBuffer.h
Input.h
Renderer.h
Scene.h
SpaceShooterEngine.h
Sprite.h
Texture.h
VertexArray.h
VertexBuffer.h
VertexBufferLayout.h
Window.h

• Hello fellow programmers,
For a couple of days now i've decided to build my own planet renderer just to see how floating point precision issues
can be tackled. As you probably imagine, i've quickly faced FPP issues when trying to render absurdly large planets.

I have used the classical quadtree LOD approach;
I've generated my grids with 33 vertices, (x: -1 to 1, y: -1 to 1, z = 0).
Each grid is managed by a TerrainNode class that, depending on the side it represents (top, bottom, left right, front, back),
creates a special rotation-translation matrix that moves and rotates the grid away from the origin so that when i finally
normalize all the vertices on my vertex shader i can get a perfect sphere.
T = glm::translate(glm::dmat4(1.0), glm::dvec3(0.0, 0.0, 1.0)); R = glm::rotate(glm::dmat4(1.0), glm::radians(180.0), glm::dvec3(1.0, 0.0, 0.0)); sides[0] = new TerrainNode(1.0, radius, T * R, glm::dvec2(0.0, 0.0), new TerrainTile(1.0, SIDE_FRONT)); T = glm::translate(glm::dmat4(1.0), glm::dvec3(0.0, 0.0, -1.0)); R = glm::rotate(glm::dmat4(1.0), glm::radians(0.0), glm::dvec3(1.0, 0.0, 0.0)); sides[1] = new TerrainNode(1.0, radius, R * T, glm::dvec2(0.0, 0.0), new TerrainTile(1.0, SIDE_BACK)); // So on and so forth for the rest of the sides As you can see, for the front side grid, i rotate it 180 degrees to make it face the camera and push it towards the eye;
the back side is handled almost the same way only that i don't need to rotate it but simply push it away from the eye.
The same technique is applied for the rest of the faces (obviously, with the proper rotations / translations).
The matrix that result from the multiplication of R and T (in that particular order) is send to my vertex shader as r_Grid'.
// spherify vec3 V = normalize((r_Grid * vec4(r_Vertex, 1.0)).xyz); gl_Position = r_ModelViewProjection * vec4(V, 1.0); The r_ModelViewProjection' matrix is generated on the CPU in this manner.
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Each frame i'm rendering each node by sending the generated matrices this way.
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Everything works well, the node's split and merge as you'd expect, however whenever i get close to the surface
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I've read that if i could render each grid relative to the camera i could get better precision on the surface, effectively
getting rid of those rounding errors.

My question is how can i achieve this relative to camera rendering in my scenario here?
I know that i have to do most of the work on the CPU with double, and that's exactly what i'm doing.
I only use double on the CPU side where i also do most of the matrix multiplications.
As you can see from my vertex shader i only do the usual r_ModelViewProjection * (some vertex coords).