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OpenGL projective textures

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I want to implement shadow mapping, but in order to do this I would like to understand (and implement) projective textures. So far, I've got a lot of docs (from nVidia, Ati, and many others) and a couple of topics here on gd, but I wasn't able to figure them out. I there somewhere a very good and easy tutorial with opengl code that explains how to project a texture onto a mesh? What I understood so far is that I have to build some matrices that would tell opengl texture coordinates routines how to calculate the texture coordinates. Not very much, that is. I've found a small class that should be able to enable a texture projection, but I dont' know how to use it (when I try, I get a black screen. Making it work could be a starting point for me to understand the theory behind:

// Date: 25/07/2004
// By: Lukas Heise

// ###############################################################################

// Usage Instructions
// ==================
// ProjectiveTexture myProjTexture;
// myProjTexture.SetupTexture(myTexture);
// myProjTexture.SetupMatrix(position,rotation,fov);
//
// in the main loop...
//
// RenderScene(RENDER_NORMALY);
// myProjTexture.BeginRender();
// RenderScene(DONT_BIND_TEXTURES);
// myProjTexture.EndRender();

// ###############################################################################


#ifndef __PROJTEXTURE_H
#define __PROJTEXTURE_H

#include <gl/gl.h>

class ProjectiveTexture
{
private:
	UINT textureID;
	float matrix[16];
	double clipPlane[4];

public:
	ProjectiveTexture()
	{
	}

	virtual ~ProjectiveTexture()
	{
		glDeleteTextures(1,&textureID);
	}

	void SetupTexture(UINT id)
	{
		textureID = id;

		glBindTexture(GL_TEXTURE_2D,id);
		glTexEnvi(GL_TEXTURE_2D,GL_TEXTURE_ENV_MODE,GL_ADD);				// add the colors from this texture to the scene
		glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S, GL_CLAMP);
		glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T, GL_CLAMP);
		glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
		glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
	}

	void SetupMatrix(float* Position, float* Rotation, float FOV)
	{
		glMatrixMode(GL_MODELVIEW);

		glPushMatrix();

		static float biasMatrix[16] = { 0.5f, 0.0f, 0.0f, 0.0f,
										0.0f, 0.5f, 0.0f, 0.0f,
										0.0f, 0.0f, 0.5f, 0.0f,
										0.5f, 0.5f, 0.5f, 1.0f };

		// construct the projection matrix (to make things easy let's use the math that OpenGL provides us with)
		glLoadMatrixf(biasMatrix);
		gluPerspective(FOV,1.0f,1.0f,2.0f);
		glRotatef(Rotation[0],1,0,0);
		glRotatef(Rotation[1],0,1,0);
		glRotatef(Rotation[2],0,0,1);
		glTranslatef(Position[0],Position[1],Position[2]);
		glGetFloatv(GL_MODELVIEW_MATRIX,matrix);

		// to eliminate reverse projection we need to construct a clipping plane
		clipPlane[0] = (double)matrix[3];
		clipPlane[1] = (double)matrix[7];
		clipPlane[2] = (double)matrix[11];
		clipPlane[3] = (double)matrix[15];

		glPopMatrix();
	}

	void BeginRender()
	{
		static float planeS[4] = { 1.0f, 0.0f, 0.0f, 0.0f };
		static float planeT[4] = { 0.0f, 1.0f, 0.0f, 0.0f };
		static float planeR[4] = { 0.0f, 0.0f, 1.0f, 0.0f };
		static float planeQ[4] = { 0.0f, 0.0f, 0.0f, 1.0f };

		glPushAttrib(GL_ENABLE_BIT);

		glBindTexture(GL_TEXTURE_2D,textureID);

		glTexGeni(GL_S,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
		glTexGeni(GL_T,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
		glTexGeni(GL_R,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
		glTexGeni(GL_Q,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);

		glTexGenfv(GL_S,GL_EYE_PLANE,planeS);
		glTexGenfv(GL_T,GL_EYE_PLANE,planeT);
		glTexGenfv(GL_R,GL_EYE_PLANE,planeR);
		glTexGenfv(GL_Q,GL_EYE_PLANE,planeQ);

		glEnable(GL_TEXTURE_GEN_S);
		glEnable(GL_TEXTURE_GEN_T);
		glEnable(GL_TEXTURE_GEN_R);
		glEnable(GL_TEXTURE_GEN_Q);

		glDepthMask(GL_FALSE);
		glEnable(GL_BLEND);
		glEnable(GL_ALPHA_TEST);
		glEnable(GL_CLIP_PLANE0);
	
		glAlphaFunc(GL_GREATER,0.0f);			// we decide not to render the transparent areas of our texture (alpha value is less or equal 0.05)
		glBlendFunc(GL_DST_COLOR,GL_SRC_COLOR);	// this blending effect looks good but do experiment with others!
		glClipPlane(GL_CLIP_PLANE0,clipPlane);

		glMatrixMode(GL_TEXTURE);
		glPushMatrix();
		glLoadMatrixf(matrix);					// load our projection matrix
	}

	void EndRender()
	{
		glPopMatrix();
		glMatrixMode(GL_MODELVIEW);

		glDisable(GL_CLIP_PLANE0);
		glDisable(GL_ALPHA_TEST);
		glDisable(GL_BLEND);
		glDepthMask(GL_TRUE);

		glDisable(GL_TEXTURE_GEN_S);
		glDisable(GL_TEXTURE_GEN_T);
		glDisable(GL_TEXTURE_GEN_R);
		glDisable(GL_TEXTURE_GEN_Q);

		glPopAttrib();
	}
};

#endif


I use it in the following way:
    ProjectiveTexture myProjTexture;
    myProjTexture.SetupTexture(1);
    float pos[] = {1.0,1.0,-1.0,1.0};
    float r[] = {0.0,0.0,1.0,1.0};
    myProjTexture.SetupMatrix(pos,r,45.0);
.
.
.
        //Render the frame
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        Render();
        myProjTexture.BeginRender();
        Render();
        myProjTexture.EndRender();
        SDL_GL_SwapBuffers();

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first try everything with identity texture matrix and disable the clip plane. what is the result now?

and: your pushattrib is senseless if you disable everything hardcoded

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When BeginRender() finishes you have the texture matrix as the current matrix. If you execute any matrix manipulation routine (glMultMatrixf(), glTranslatef() etc.) in your Render() function then it will be destroyed.

Try setting MODELVIEW as the current matrix mode before returning from BeginRender(). When the EndRender() function begins set the TEXTURE matrix as the current matrix mode in case to pop the correct matrix.

Hope that helps.

HellRaiZer

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Thank you for answering. OK, I took a bit more time to think about the theory, and I think I understand now most of it. That is what I know:
-I create a new 'camera', just like the primary one, with gluLookAt and so on. After this step the MODELVIEW matrix is set.
-If I transform with it a vertex, it is projected on the viewplane (does ogl use this term?), in the range [-1, 1].
-Now I have to transform again the vertex to strech the coords in the range [0,1], suitable for textures.
-If I imagine to 'overlap' this projected scene with my texture (i.e, overlapping the screen with a printed version of the texture), I can get the uv coords simply by observing where, through the page, I see the vertex.

Theorethically, it seems that the steps are:
-setting a new camera where I want the projector to be.
-tranforming the vertices.
-taking the screen-space coords as uv coords. this step can be done by gltexgen.

Now I have a questions:
-In the papers I downloaded I've found that I need the inverse of the modelview matrix to correctly calculate the texture coords. Somewhere the inverse is explicitly assigned, somewhere else it seems a gltexgen job. Either ways, why do I need the inverse?

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Ok, finally I think to understand the theory. AFAIK I'm following exactly what described in the nVidia paper, then I suppose it's an opengl error.
The steps are:
-Build a bias matrix (B) that transform coords from eye plane into [0,1].
-Build a new modelview matrix (let's call it Vp) for the projector.
-Build a new Projection matrix for the projector (Pp).
-Multiply them all.

Now I should multiply the result with the camera view inverse. Following the paper, opengl does this if when I set the texture matrix(or the texture planes?) the modelview contains the camera view matrix. I think that this code, mathematically, is correct. Could some of you check it and tell me where are the errors, please?

void LightMap()
{
glMatrixMode(GL_MODELVIEW);

glPushMatrix();
static float biasMatrix[16] = { 0.5f, 0.0f, 0.0f, 0.0f,
0.0f, 0.5f, 0.0f, 0.0f,
0.0f, 0.0f, 0.5f, 0.0f,
0.5f, 0.5f, 0.5f, 1.0f };

// construct the projection matrix (to make things easy let's use the math that OpenGL provides us with)
glLoadMatrixf(biasMatrix);
gluPerspective(45.0,1.0f,1.0f,2.0f);
glRotatef(Rotation[0],1,0,0);
glRotatef(Rotation[1],0,1,0);
glRotatef(Rotation[2],0,0,1);
glTranslatef(Position[0],Position[1],Position[2]);
glGetFloatv(GL_MODELVIEW_MATRIX,matrix);

// to eliminate reverse projection we need to construct a clipping plane
clipPlane[0] = (double)matrix[3];
clipPlane[1] = (double)matrix[7];
clipPlane[2] = (double)matrix[11];
clipPlane[3] = (double)matrix[15];

glPopMatrix();

//////////////////////////////////
static float planeS[4] = { 1.0f, 0.0f, 0.0f, 0.0f };
static float planeT[4] = { 0.0f, 1.0f, 0.0f, 0.0f };
static float planeR[4] = { 0.0f, 0.0f, 1.0f, 0.0f };
static float planeQ[4] = { 0.0f, 0.0f, 0.0f, 1.0f };

glBindTexture(GL_TEXTURE_2D, lightmap);

glMatrixMode(GL_MODELVIEW);


glTexGeni(GL_S,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
glTexGeni(GL_T,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
glTexGeni(GL_R,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);
glTexGeni(GL_Q,GL_TEXTURE_GEN_MODE,GL_EYE_LINEAR);

glTexGenfv(GL_S,GL_EYE_PLANE,planeS);
glTexGenfv(GL_T,GL_EYE_PLANE,planeT);
glTexGenfv(GL_R,GL_EYE_PLANE,planeR);
glTexGenfv(GL_Q,GL_EYE_PLANE,planeQ);

glEnable(GL_TEXTURE_GEN_S);
glEnable(GL_TEXTURE_GEN_T);
glEnable(GL_TEXTURE_GEN_R);
glEnable(GL_TEXTURE_GEN_Q);

glAlphaFunc(GL_GREATER,0.0f); // we decide not to render the transparent areas of our texture (alpha value is less or equal 0.05)
glBlendFunc(GL_DST_COLOR,GL_SRC_COLOR); // this blending effect looks good but do experiment with others!*/
glClipPlane(GL_CLIP_PLANE0,(double*)clipPlane);

glMatrixMode(GL_TEXTURE);
glPushMatrix();
glLoadMatrixf(matrix); // load our projection matrix
glMatrixMode(GL_MODELVIEW);
////////////////////////////////////
}
void Render()
{
glClearColor(1.0, 1.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0, 0.0, -5.0);
glRotatef(r, 0.0, 1.0, 0.0);

r += 0.2;

LightMap(); //Call the lightmapping routine

glEnable(GL_TEXTURE_2D);
glBegin(GL_QUADS);
glColor3f(0.8, 0.5, 0.5);
glNormal3f(0.0,0.0,1.0); glVertex3f(-1.0, 1.0, 0.0);
glNormal3f(0.0,0.0,1.0); glVertex3f(1.0, 1.0, 0.0);
glNormal3f(0.0,0.0,1.0); glVertex3f(1.0, -1.0, 0.0);
glNormal3f(0.0,0.0,1.0); glVertex3f(-1.0, -1.0, .0);
glEnd();
glBegin(GL_QUADS);
glColor3f(0.8, 0.5, 0.5);
glNormal3f(0.0,1.0,0.0); glVertex3f(-1.0, -1.0, 0.0);
glNormal3f(0.0,1.0,0.0); glVertex3f(1.0, -1.0, 0.0);
glNormal3f(0.0,1.0,0.0); glVertex3f(1.0, -1.0, 2.0);
glNormal3f(0.0,1.0,0.0); glVertex3f(-1.0, -1.0, 2.0);
glEnd();

SDL_GL_SwapBuffers();

}

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      #include "Window.h" #include <GL/glew.h> #include <GLFW/glfw3.h> #include "Error.h" #include "Renderer.h" #include "Scene.h" #include "Input.h" //Global Variables. int screen_width, screen_height; //On Window Resize. void OnWindowResize(GLFWwindow *window, int width, int height); //Implementation Structure. struct Window::Implementation { //GLFW Window. GLFWwindow *GLFW_window; //Renderer. Renderer *renderer; //Delta Time. double delta_time; //Frames Per Second. int fps; //Scene. Scene *scnene; //Input. Input *input; //Deconstructor. ~Implementation(); }; //Window Constructor. Window::Window(std::string title, int width, int height) { //Initializing width and height. screen_width = width; screen_height = height; //Create Pointer To Implementation. m_Impl = new Implementation(); //Try initializing GLFW. if (!glfwInit()) { std::cout << "GLFW could not be initialized!" << std::endl; std::cout << "Press ENTER to exit..." << std::endl; std::cin.get(); exit(-1); } //Setting up OpenGL Version 3.3 Core Profile. glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3); glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3); glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE); /* Create a windowed mode window and its OpenGL context */ m_Impl->GLFW_window = glfwCreateWindow(width, height, title.c_str(), NULL, NULL); if (!m_Impl->GLFW_window) { std::cout << "GLFW could not create a window!" << std::endl; std::cout << "Press ENTER to exit..." << std::endl; std::cin.get(); glfwTerminate(); exit(-1); } /* Make the window's context current */ glfwMakeContextCurrent(m_Impl->GLFW_window); //Initialize GLEW. if(glewInit() != GLEW_OK) { std::cout << "GLEW could not be initialized!" << std::endl; std::cout << "Press ENTER to exit..." << std::endl; std::cin.get(); glfwTerminate(); exit(-1); } //Enabling Blending. GLCall(glEnable(GL_BLEND)); GLCall(glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)); //Setting the ViewPort. GLCall(glViewport(0, 0, width, height)); //**********Initializing Implementation**********// m_Impl->renderer = new Renderer(); m_Impl->delta_time = 0.0; m_Impl->fps = 0; m_Impl->input = new Input(this); //**********Initializing Implementation**********// //Set Frame Buffer Size Callback. glfwSetFramebufferSizeCallback(m_Impl->GLFW_window, OnWindowResize); } //Window Deconstructor. Window::~Window() { delete m_Impl; } //Window Main Loop. void Window::MainLoop() { //Time Variables. double start_time = 0, end_time = 0, old_time = 0, total_time = 0; //Frames Counter. int frames = 0; /* Loop until the user closes the window */ while (!glfwWindowShouldClose(m_Impl->GLFW_window)) { old_time = start_time; //Total time of previous frame. start_time = glfwGetTime(); //Current frame start time. //Calculate the Delta Time. m_Impl->delta_time = start_time - old_time; //Get Frames Per Second. if (total_time >= 1) { m_Impl->fps = frames; total_time = 0; frames = 0; } //Clearing The Screen. m_Impl->renderer->Clear(0, 0, 0); //Render The Scene. if (m_Impl->scnene != NULL) m_Impl->scnene->Render(this); //Updating the Screen. m_Impl->renderer->Update(m_Impl->GLFW_window); //Increasing frames counter. frames++; //End Time. end_time = glfwGetTime(); //Total time after the frame completed. total_time += end_time - start_time; } //Terminate GLFW. glfwTerminate(); } //Load Scene. void Window::LoadScene(Scene * scene) { //Set the scene. m_Impl->scnene = scene; } //Get Delta Time. double Window::GetDeltaTime() { return m_Impl->delta_time; } //Get FPS. int Window::GetFPS() { return m_Impl->fps; } //Get Width. int Window::GetWidth() { return screen_width; } //Get Height. int Window::GetHeight() { return screen_height; } //Get Input. Input * Window::GetInput() { return m_Impl->input; } Renderer * Window::GetRenderer() { return m_Impl->renderer; } GLFWwindow * Window::GetGLFWindow() { return m_Impl->GLFW_window; } //Implementation Deconstructor. Window::Implementation::~Implementation() { delete renderer; delete input; } //OnWindowResize void OnWindowResize(GLFWwindow *window, int width, int height) { screen_width = width; screen_height = height; //Updating the ViewPort. GLCall(glViewport(0, 0, width, height)); }  
      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
      Shader.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
      Shader.h
      SpaceShooterEngine.h
      Sprite.h
      Texture.h
      VertexArray.h
      VertexBuffer.h
      VertexBufferLayout.h
      Window.h
    • By Cristian Decu
      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.
      // No the most efficient way, but it works. glm::dmat4 Camera::getMatrix() { // Create the view matrix // Roll, Yaw and Pitch are all quaternions. glm::dmat4 View = glm::toMat4(Roll) * glm::toMat4(Pitch) * glm::toMat4(Yaw); // The model matrix is generated by translating in the oposite direction of the camera. glm::dmat4 Model = glm::translate(glm::dmat4(1.0), -Position); // Projection = glm::perspective(fovY, aspect, zNear, zFar); // zNear = 0.1, zFar = 1.0995116e12 return Projection * View * Model; } I managed to get rid of z-fighting by using a technique called Logarithmic Depth Buffer described in this article; it works amazingly well, no z-fighting at all, at least not visible.
      Each frame i'm rendering each node by sending the generated matrices this way.
      // set the r_ModelViewProjection uniform // Sneak in the mRadiusMatrix which is a matrix that contains the radius of my planet. Shader::setUniform(0, Camera::getInstance()->getMatrix() * mRadiusMatrix); // set the r_Grid matrix uniform i created earlier. Shader::setUniform(1, r_Grid); grid->render(); My planet's radius is around 6400000.0 units, absurdly large, but that's what i really want to achieve;
      Everything works well, the node's split and merge as you'd expect, however whenever i get close to the surface
      of the planet the rounding errors start to kick in giving me that lovely stairs effect.
      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).
       
      Thank you for your suggestions!
       
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