How can i draw a model on front screen in opengl - opengl

I want to draw a logo(3D awards) at the corner of the window(fixed position when change camera)

here is my code for drawing fullscreen rectangle (in old opengl)
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glBegin(GL_QUADS);
glTexCoord2f( 0,0 );
glVertex3d( -1.0,-1.0, 0 );
glTexCoord2f( 1,0 );
glVertex3d( 1.0,-1.0, 0 );
glTexCoord2f( 1,1 );
glVertex3d( 1.0, 1.0, 0 );
glTexCoord2f( 0,1 );
glVertex3d( -1.0, 1.0, 0 );
glEnd();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
just disable depth buffer abd set the texture to be able to draw your logo in front of eferything
Of course you can change the position and the size of it

Could do it like so:
Draw your scene
Disable the depth test (or clear the depth buffer if you need self-depth testing to draw the model correctly)
Set a new matrix on the stack which ignores camera position.
Draw the logo.

Related

OpenGL 2D co-ordinate system

How does the 2D openGL co-ordinate system work?
glOrtho( 0.0, SCREEN_WIDTH, SCREEN_HEIGHT, 0.0, 1.0, -1.0);
creates a 2d perspective SCREEN_WIDTH wide and SCREEN_HEIGHT high
glTranslatef( 0.f, SCREEN_HEIGHT, 0.f );
moves the origin(i think) to the bottom of the screen on the left.
If I wanted to create a square here what would the co-ords be?
To draw a square I would use:
glBegin( GL_QUADS );
glVertex2f( ?.f, ?.f );
glVertex2f( ?.f, ?.f );
glVertex2f( ?.f, ?.f );
glVertex2f( ?.f, ?.f );
glEnd();
what coordinates do I need to enter to draw a square at the bottom left corner(any size)

Large Object disappears when behind another smaller object Opengl?

I am trying to draw a large polygon behind the cube to appear like the floor. How ever when ever the background floor is behind the cube it disappears completely. Here is the display function including the background I want to add and one of the cubes sides.
void display(){
// Clear screen and Z-buffer
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
// Reset transformations
glLoadIdentity();
//background floor
glBegin(GL_POLYGON);
glColor3f( 0.5, 0.5, 0.5);
glVertex3f( 1, -0.9, 1 ); // x-y-z right bottom
glVertex3f( 0.6, 0.5, 1 ); //right top
glVertex3f( -0.6, 0.5, 1 ); //left top
glVertex3f( -1, -0.9, 1 ); //left bottom
glEnd();
// Rotate when user changes rotate_x and rotate_y
glRotatef( rotate_x, 1.0, 0.0, 0.0 );
glRotatef( rotate_y, 0.0, 1.0, 0.0 );
// FRONT side of cube
glBegin(GL_POLYGON);
glColor3f( 1.0, 0.5, 0.0 );
glVertex3f( 0.05, -0.05, -0.05 );
glVertex3f( 0.05, 0.05, -0.05 );
glVertex3f( -0.05, 0.05, -0.05 );
glVertex3f( -0.05, -0.05, -0.05 );
//I have omitted the other 5 sides
glEnd();
glFlush();
glutSwapBuffers();
}
I suspect your poligon is facing outwards.
Try disabling backface culling with glDisable(GL_CULL_FACE). If this works swap the order of the vertices (should be CCW or trigonometric, on the side that the polygon should be visible from).
Don't leave the face culling disabled, unless you don't care about performance.

qt + open Gl= paintgl function for drawing a triangle

i have coded what i have read in previous post about a well known subject but i continue to get a green window without any triangle inside.
Here is my paint function :
void mGLWidget::paintGL()
{
glClearColor( Qt::green );
QSize viewport_size = size();
glViewport(0, 0, viewport_size.width(), viewport_size.height());
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-1, 1, -1, 1, 5, 7); // near and far match your triangle Z distance
glMatrixMode(GL_MODELVIEW);
glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE );
glColor4f( 1.0, 1.0, 1.0, 1.0 );
glTranslatef(-1.5f,0.0f,-15.0f);
glBegin(GL_TRIANGLES); // Drawing Using Triangles
glVertex3f( 0.0f, 1.0f, 0.0f); // Top
glVertex3f(-1.0f,-1.0f, 0.0f); // Bottom Left
glVertex3f( 1.0f,-1.0f, 0.0f); // Bottom Right
glEnd();
// Finished Drawing The Triangle
// swapBuffers();
}
I only get a green window without any triangle. My QT is qt4.8 and opengl 4.0.
What am I doing wrong ?-\
your near and far value for your glfrustrum is quite a small range, make it 1, 100 instead of 5, 7. might not be the actual solution but it will help if its not
Your near and far value is small and you are translating the triangle very far by -15f unit that means your triangle is not draw or act as a point. So you only see the window color.

Reflection and depth in OpenGL

I'm trying to create a mirror in OpenGL. Most references I've found recommend using the stencil buffer to define the boundaries of the mirror itself and using a combination of translation and scaling matrices for the actual reflection. I managed to get this to work when the scene is viewed so that no objects are in front of the mirror. However, when an object is in front of the mirror the overlapping part is not shown, making it look as if it's behind the mirror. There's a good chance that I'm misunderstanding how the stencil buffer works, as this is the first time I've tried using it, but I could also be making some mistake regarding depth or something else. I'd appreciate any suggestions. Here is the code.
#include <gl/glut.h>
// perspective
void view(void) {
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-2.0, 2.0, -2.0, 2.0, -2.0, 2.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotatef(-85.0, 1.0, 0.0, 0.0);
glScalef(0.25, 0.25, 0.25);
}
void init(void) {
glEnable(GL_DEPTH_TEST);
glClearStencil(0);
glEnable(GL_STENCIL_TEST);
}
void mirror(GLboolean inside, GLfloat vertSet[4][3]) {
GLint i;
if(inside)
glBegin(GL_QUADS); // draw inside of mirror
else
glBegin(GL_LINE_LOOP); // draw frame of mirror
for(i = 0; i < 4; i++)
glVertex3fv(vertSet[i]);
glEnd();
}
void scene(void) {
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glRotatef(90.0, 1.0, 0.0, 0.0);
glutSolidTeapot(1.0);
glPopMatrix();
}
void display(void) {
GLfloat vertSet1[4][3] = {{-3.0, 3.0, 0.0}, {2.0, 3.0, 0.0},
{2.0, 3.0, 2.0}, {-3.0, 3.0, 2.0}};
view();
// store mirror shape in the stencil buffer.
glClear(GL_STENCIL_BUFFER_BIT);
glStencilFunc(GL_ALWAYS, 1, 1);
glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);
mirror(true, vertSet1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// draw mirror frame.
glColor3f(0.0, 0.0, 0.0);
mirror(false, vertSet1);
// draw scene outside mirror
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
glStencilFunc(GL_NOTEQUAL, 1, 1);
scene();
// draw reflection of scene in mirror
glStencilFunc(GL_EQUAL, 1, 1);
glTranslatef(0.0, 3.0, 0.0);
glScalef(1.0, -1.0, 1.0);
glTranslatef(0.0, -3.0, 0.0);
scene();
glFlush();
glutSwapBuffers();
glutPostRedisplay();
}
int main (int argc, char **argv) {
glutInit(&argc, argv);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0, 0);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_STENCIL | GLUT_DEPTH);
glutCreateWindow("");
glClearColor(1.0, 1.0, 1.0, 0.0);
init();
glutDisplayFunc(display);
glutMainLoop();
}
The idea of the stencil is, that you draw it when you'd normally render the mirror's glass within the scene. You want the drawing of the stencil to be depth tested. When you finished rendering your scene, you add a clip plane in the plane of the mirror, then you clear the depth buffer and redraw the scene. Since you clear the depth buffer you need the stencil to be depth tested first to make the mirror not overdraw already drawn objects in the world.
Note that drawing a mirror is essentially the same as drawing a portal.
This is my code for doing reflection in old opengl, hope this helps
Please note that most of the code comes from NeHe tutorial: http://nehe.gamedev.net/tutorial/clipping__reflections_using_the_stencil_buffer/17004/
double eqr[] = { 0.0f, -1.0f, 0.0f, 0.0f };
glColorMask(0, 0, 0, 0);
glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_ALWAYS, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glDisable(GL_DEPTH_TEST);
DrawFloor(); // draw flor to the stencil buffer
glColorMask(1, 1, 1, 1);
glStencilFunc(GL_EQUAL, 1, 1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
glEnable(GL_CLIP_PLANE0);
glClipPlane(GL_CLIP_PLANE0, eqr);
glPushMatrix();
glScalef(1.0f, -1.0f, 1.0f);
//glLightfv(GL_LIGHT0, GL_POSITION, light_pos);
glFrontFace(GL_CW);
glTranslatef(0.0f, 0.3f, 0.0);
RenderScene();
glFrontFace(GL_CCW);
glPopMatrix();
glDisable(GL_CLIP_PLANE0);
glDisable(GL_STENCIL_TEST);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(0.6f, 0.7f, 1.0f, 0.5f);
DrawFloor(); // draw floor second time
glDisable(GL_BLEND);
// draw normal scene
glPushMatrix();
glColor3f(1.0f, 1.0f, 1.0f);
glTranslatef(0.0f, 0.3f, 0.0);
RenderScene();
glPopMatrix();
For better result you might use RenderToTexture or use cube maps

How can I apply a fragment shader to every pixel?

I have some transparent decals rendered into a separate framebuffer. Now I'm trying to blend this buffer with the back buffer (active target).
I'm assuming that I have to load the color buffer of the fbo as a texture in a fragment shader and output the texture color to the active target. How can I tell the graphics card to do this operation for every pixel?
Draw a quad or two triangles that cover the whole screen.
my code for the fullscreen rectangle/quad
this is old method - from OpenGL 1.1, but works :) For new OpenGL 3.* you have to use VBO/Arrays
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glBegin(GL_QUADS);
glTexCoord2f( 0,0 );
glVertex3d( -1.0,-1.0, 0 );
glTexCoord2f( 1,0 );
glVertex3d( 1.0,-1.0, 0 );
glTexCoord2f( 1,1 );
glVertex3d( 1.0, 1.0, 0 );
glTexCoord2f( 0,1 );
glVertex3d( -1.0, 1.0, 0 );
glEnd();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
of course you have to setup textures before