CD2012 Principles of Interactive Graphics

Lecture -10

A complete 3D scene

Abir Hussain (JPB6.33, a.hussain@livjm.ac.uk)

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Previous lecture• Smooth and flat shading

• Light source in OpenGL: – glLightfv(source, parameter, pointer_to_array);

• Specifications of light materials in OpenGL:– glMaterialfv(face, type,pointer_to_array);

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Today’s Lecture and Lab

Interaction in 3dand controlling

features

Buffering

Saving andrestoring the

graphics context

Enabling/disabling features

Putting it alltogether: a

complete 3D scene

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Introduction• In lecture 6, we talk about cues to the depth of a

scene– Perspective– Hidden line and surface removal– 3D shapes and surfaces– Viewpoints– Lighting– Shading– Texture and Materials

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Introduction

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Enabling/disabling features

• We need to enable and disable features in OpenGL as required

• For example– Lighting– Textures– Depth testing– Double buffering

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Enabling/disabling features

• For each pixel on the screen, the depth buffer keeps track of the distance between the viewpoint and the object occupying that pixel.

• The depth buffer is generally used for hidden surface elimination. – For example, if a new candidate colour for that pixel

appears, it’s drawn only if the corresponding object is closer than the previous object.

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Enabling/disabling features

• We use double buffering for smooth animation• Depth testing and Double buffering can be

turned off for a static 2D scene.

– Double buffering is needed for animation.– Depth testing is needed for 3D scenes.

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Examples

• Strange effects can see through objects 1.(Might be useful in an engineering illustration!).

• This effect is achieved by disabling depth testing (In Init()).

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Examples

• We might selectively switch off lights for part of the scene Here the second teapot is unlit.

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Examples

• If we don’t disable texture mapping we may get unexpected results.

• Here the teapot is textured and the cubes are black

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Double buffering

• Setting double buffering with• glutInitDisplayMode (GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);

• This function specifies how the display should be initialised. – The built in constant GLUT_DOUBLE and GLUT_RGB

which are Ored together– Indicating that a double display buffer should be

allocated – The colour are specified using desired amounts of red,

green, and blue.

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Double buffering

• This enables the programmer to store old and new copies of the screen in video memory and swap between them for smooth motion

– Without this you would see the scene being redrawn– Change GLUT_DOUBLE to GLUT_SINGLE to see

the effect

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Saving and restoring the graphics context

• So far we have used glLoadIdentity() to restore the graphics context to its original state

• We can also temporally store the current graphics context and then restore it, e.g. drawing 12 cubes in a circle

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Saving and restoring the graphics context

• The steps– glRotated(30,0.0,1.0,0.0);– glTranslated(3.0,0.0,0.0);– coloured_cube(red, 0.5);– glTranslated(-3.0,0.0,0.0);

• draw a cube relative to a centre point where the last object (yellow teapot) was drawn

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Drawing in local context

• We use glPushMatrix/ glPopMatrix to save and restore the context.

• The glPushMatrix pushes all the matrices in the current stack down one level.

• The current stack is determined by glMatrixMode.

• The topmost matrix is copied, so its contents are duplicated in both the top and second from the top matrix.

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Drawing in local context

• If too many matrices are pushed, an error is generated.

• The glPopMatrix pops the top matrix off the stack, destroying the contents of the popped matrix. Current

matrix copy

Add Discard

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Drawing in local context glPushMatrix();glDisable(GL_TEXTURE_2D);glTranslated(-2.0, -0.25, -2.0);teapot(yellow, 0.5);for (i=0; i < 11; i++) {glRotated(30,0.0,1.0,0.0);glTranslated(3.0,0.0,0.0);coloured_cube(red, 0.5);glTranslated(-3.0,0.0,0.0);

}glEnable(GL_TEXTURE_2D);glPopMatrix(); //return to initial location

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Interactions in 3D and controlling features

• In addition to setting features at application start-up we can also change them at run-time.

• The example code show– Changing the colour of an object (pink/blue teapot)– Changing the value of a light’s intensity. – The intensity is set as different values of gray light– The example also shows traversing into and out of a

scene.– Faster speeds are achieved by taking bigger steps for

each key press

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Examples

1. case 't':// Change the colour of teapot

if ( toggle_teapot )toggle_teapot = 0;

elsetoggle_teapot = 1;

glutPostRedisplay();break;

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Examples

2. case 'a': // Move forward slowly

glTranslated(0.0,0.0, 0.05);glutPostRedisplay();

break;

3. case 'A': // Move forward more quickly

glTranslated(0.0,0.0, 0.1);glutPostRedisplay();

break;

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Examples

4. case '-': // Decrease light brightness

new_light_colour = new_light_colour - 0.1;

light_colour[0] = new_light_colour;light_colour[1] = new_light_colour;light_colour[2] = new_light_colour;

glLightfv(GL_LIGHT0, GL_DIFFUSE, light_colour);glutPostRedisplay();

break;

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Next week lecture

Reminder

• Demonstrate your 2nd coursework in the Lab (12th of December)

• Hand in your documentation and code (12th of November)

Next week lecture

• Revision lecture (past exam questions)

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Summary

• A complete 3D program

• Enabling/Disabling features

• Double buffering

• Interactions in 3D and controlling features