CODEWITHSUNDEEP
c++openglcanvastexturesrender
James C
James C

Reputation: 919

Drawing a primitive ( GL_QUADS ) on top of a 2D texture - no quad rendered, texture colour changed

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 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 ?

Upvotes: 0

Views: 2551

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54602

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);

Upvotes: 1

Related Questions