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Grafica Computazionalelezione33
Informatica e Automazione, "Roma Tre"
May 25, 2010
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Contents
OpenGL SuperBible
Vertex transformation pipeline2D projectionModeling transformationsViewing transformationProjection abuse
Write a generic source environment
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OpenGL SuperBible
Main reference:
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OpenGL SuperBible
Main reference:
OpenGL SuperBible
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OpenGL SuperBible
Main reference:
OpenGL SuperBible
book code
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Hello to Glut and OpenGL World
Hello World
First example with window creation and display callback
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Contents
OpenGL SuperBible
Vertex transformation pipeline2D projectionModeling transformationsViewing transformationProjection abuse
Write a generic source environment
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The vertex transformation pipeline
Remember:
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The vertex transformation pipeline
Remember:
Original vertex data ≡ MC (Modeling coordinates)
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The vertex transformation pipeline
Remember:
Original vertex data ≡ MC (Modeling coordinates) Transformed eye coords ≡ VRC (View Reference Coords)
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The vertex transformation pipeline
Remember:
Original vertex data ≡ MC (Modeling coordinates) Transformed eye coords ≡ VRC (View Reference Coords) Normalized device coords ≡ NPC (Norm. Projection coords)
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The vertex transformation pipeline
Remember:
Original vertex data ≡ MC (Modeling coordinates) Transformed eye coords ≡ VRC (View Reference Coords) Normalized device coords ≡ NPC (Norm. Projection coords) Window coordinates ≡ DC (device coordinates)
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Object Coordinates
Object Coordinates are transformed by the ModelView matrix toproduce Eye Coordinates
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Object Coordinates
Object Coordinates are transformed by the ModelView matrix toproduce Eye Coordinates
Eye Coordinates are transformed by the Projection matrix to
produce Clip Coordinates
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Object Coordinates
Object Coordinates are transformed by the ModelView matrix toproduce Eye Coordinates
Eye Coordinates are transformed by the Projection matrix to
produce Clip Coordinates
Clip Coordinate X, Y, and Z are divided by Clip Coordinate Wto produce Normalized Device Coordinates
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Object Coordinates
Object Coordinates are transformed by the ModelView matrix toproduce Eye Coordinates
Eye Coordinates are transformed by the Projection matrix to
produce Clip Coordinates
Clip Coordinate X, Y, and Z are divided by Clip Coordinate Wto produce Normalized Device Coordinates
Normalized Device Coordinates are scaled and translated bythe viewport parameters to produce Window Coordinates
Ob d
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Object Coordinates
Obj C di
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Object Coordinates
Object coordinates are the raw coordinates you submit to
OpenGL with a call to glVertex*() or glVertexPointer()
Obj C di
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Object Coordinates
Object coordinates are the raw coordinates you submit to
OpenGL with a call to glVertex*() or glVertexPointer()
They represent the coordinates of your object or other
geometry you want to render
W ld C di ?
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World Coordinate system ?
Objects are often modeled in one coordinate system, then scaled,translated, and rotated into the world you’re constructing
W ld C di t t ?
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World Coordinate system ?
Objects are often modeled in one coordinate system, then scaled,translated, and rotated into the world you’re constructing
World Coordinates result from transforming ObjectCoordinates by the modelling transforms stored in theModelView matrix
W ld C di t t ?
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World Coordinate system ?
Objects are often modeled in one coordinate system, then scaled,translated, and rotated into the world you’re constructing
World Coordinates result from transforming ObjectCoordinates by the modelling transforms stored in theModelView matrix
However, OpenGL has no concept of World Coordinates
World Coordinate system ?
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World Coordinate system ?
Objects are often modeled in one coordinate system, then scaled,translated, and rotated into the world you’re constructing
World Coordinates result from transforming ObjectCoordinates by the modelling transforms stored in theModelView matrix
However, OpenGL has no concept of World Coordinates
World Coordinates are purely an application construct
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
The ModelView matrix contains both modelling and viewingtransformations that place the viewer at the origin with theview direction aligned with the negative Z axis
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
The ModelView matrix contains both modelling and viewingtransformations that place the viewer at the origin with theview direction aligned with the negative Z axis
Clip Coordinates result from transforming Eye Coordinates bythe Projection matrix
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
The ModelView matrix contains both modelling and viewingtransformations that place the viewer at the origin with theview direction aligned with the negative Z axis
Clip Coordinates result from transforming Eye Coordinates bythe Projection matrix
Clip Coordinate space ranges from -Wc to Wc in all threeaxes, where Wc is the Clip Coordinate W value
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
The ModelView matrix contains both modelling and viewingtransformations that place the viewer at the origin with theview direction aligned with the negative Z axis
Clip Coordinates result from transforming Eye Coordinates bythe Projection matrix
Clip Coordinate space ranges from -Wc to Wc in all threeaxes, where Wc is the Clip Coordinate W value
OpenGL clips all coordinates outside this range
Eye Coordinates
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Eye Coordinates
Eye Coordinates result from transforming Object Coordinates bythe ModelView matrix
The ModelView matrix contains both modelling and viewingtransformations that place the viewer at the origin with theview direction aligned with the negative Z axis
Clip Coordinates result from transforming Eye Coordinates bythe Projection matrix
Clip Coordinate space ranges from -Wc to Wc in all threeaxes, where Wc is the Clip Coordinate W value
OpenGL clips all coordinates outside this range
Perspective division performed on the Clip Coordinatesproduces Normalized Device Coordinates, ranging from -1 to 1in all three axes
Window Coordinates
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Window Coordinates
Window Coordinates result from scaling and translating NormalizedDevice Coordinates by the viewport
Window Coordinates
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Window Coordinates
Window Coordinates result from scaling and translating NormalizedDevice Coordinates by the viewport
The parameters to glViewport() and glDepthRange() controlthis transformation
Window Coordinates
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C
Window Coordinates result from scaling and translating NormalizedDevice Coordinates by the viewport
The parameters to glViewport() and glDepthRange() controlthis transformation
With the viewport, you can map the Normalized DeviceCoordinate cube to any location in your window and depthbuffer
HOWTO set up a 2D projection
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p p j
To set up a 2D projection, you need to change the Projectionmatrix.
HOWTO set up a 2D projection
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p p j
To set up a 2D projection, you need to change the Projectionmatrix.
Normally, it’s convenient to set up the projection so one worldcoordinate unit is equal to one screen pixel, as follows:
HOWTO set up a 2D projection
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p p j
To set up a 2D projection, you need to change the Projectionmatrix.
Normally, it’s convenient to set up the projection so one worldcoordinate unit is equal to one screen pixel, as follows:
HOWTO set up a 2D projection
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To set up a 2D projection, you need to change the Projectionmatrix.
Normally, it’s convenient to set up the projection so one worldcoordinate unit is equal to one screen pixel, as follows:
gluOrtho2D() sets up a Z range of -1 to 1, so you need touse one of the glVertex2*() functions to ensure yourgeometry isn’t clipped by the zNear or zFar clipping planes.
Look carefully !! (from GLRect.py)
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What I did ?!?
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Execute (GLbounce.py) again and look !!
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Howto give each object its own transform?
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Use the ModelView matrix stack.
Howto give each object its own transform?
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Use the ModelView matrix stack.
A typical OpenGL application first sets the matrix mode with acall to glMatrixMode(GL_MODELVIEW) and loads a viewingtransform, perhaps with a call to gluLookAt().
Howto give each object its own transform?
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Use the ModelView matrix stack.
A typical OpenGL application first sets the matrix mode with acall to glMatrixMode(GL_MODELVIEW) and loads a viewingtransform, perhaps with a call to gluLookAt().
Then the code renders each object in the scene with its owntransformation by wrapping the rendering with calls toglPushMatrix() and glPopMatrix()
Howto give each object its own transform?
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For example:
The above code renders a cylinder rotated 90 degrees around theX-axis.
The ModelView matrix is restored to its previous value after theglPopMatrix() call.
Similar call sequences can render subsequent objects in the scene.
Howto give each object its own transform? (Atom.py)
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Of course, we need to massage the ModelView matrix:
Howto give each object its own transform?
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consider the above equivalent with:
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
Absolute versus relative coordinates
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Absolute versus relative coordinates
f d d h b l l l
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Absolute versus relative coordinates
If d d h bj i l i l l i
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Absolute versus relative coordinates
If d t d th bj t i lti l l ti
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
Absolute versus relative coordinates
If d t d th bj t i lti l l ti
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
Absolute versus relative coordinates
If you need to render the same object in multiple locations
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
1. pushing the ModelView matrix stack,
Absolute versus relative coordinates
If you need to render the same object in multiple locations
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
1. pushing the ModelView matrix stack,
2. setting the desired transform, sending the geometry, and
Absolute versus relative coordinates
If you need to render the same object in multiple locations
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
1. pushing the ModelView matrix stack,
2. setting the desired transform, sending the geometry, and
3. popping the stack
Absolute versus relative coordinates
If you need to render the same object in multiple locations
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If you need to render the same object in multiple locations
OpenGL lets you do this two ways:
Use absolute coordinates
Maintain multiple copies of each object, each with its ownunique set of vertices
You don’t need to change the ModelView matrix to render theobject at the desired location
Use relative coordinates
Keep only one copy of the object, and render multiple times by
1. pushing the ModelView matrix stack,
2. setting the desired transform, sending the geometry, and
3. popping the stack
Repeat these steps for each object
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How does the camera work in OpenGL?
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As far as OpenGL is concerned, there is no camera
How does the camera work in OpenGL?
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As far as OpenGL is concerned, there is no camera
More specifically, the camera is always located at the eyespace coordinate (0., 0., 0.)
How does the camera work in OpenGL?
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As far as OpenGL is concerned, there is no camera
More specifically, the camera is always located at the eyespace coordinate (0., 0., 0.)
To give the appearance of moving the camera, your OpenGLapplication must move the scene with the inverse of thecamera transformation.
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
1. an eye position,
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
1. an eye position,
2. a position to look at,
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
1. an eye position,
2. a position to look at,
3. and an up vector, all in object space coordinates
How can I move my eye, or camera, in my scene?
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OpenGL doesn’t provide an interface to do this using a camera
model
However, the GLU library provides the gluLookAt() function,which takes
1. an eye position,
2. a position to look at,
3. and an up vector, all in object space coordinates
This function computes the inverse camera transformaccording to its parameters and multiplies it onto the currentmatrix stack.
My camera should go to the ModelView or Projectionmatrix?
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The GL_PROJECTION matrix should contain only the projection
transformation calls it needs to transform eye space coordinatesinto clip coordinates.
The GL_MODELVIEW matrix, as its name implies, should containmodeling and viewing transformations, which transform objectspace coordinates into eye space coordinates
My camera should go to the ModelView or Projectionmatrix?
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The GL_PROJECTION matrix should contain only the projection
transformation calls it needs to transform eye space coordinatesinto clip coordinates.
The GL_MODELVIEW matrix, as its name implies, should containmodeling and viewing transformations, which transform objectspace coordinates into eye space coordinates
Remember to place the camera transformations on theGL_MODELVIEW matrix and never on theGL_PROJECTION matrix.
My camera should go to the ModelView or Projectionmatrix?
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The GL_PROJECTION matrix should contain only the projection
transformation calls it needs to transform eye space coordinatesinto clip coordinates.
The GL_MODELVIEW matrix, as its name implies, should containmodeling and viewing transformations, which transform objectspace coordinates into eye space coordinates
Remember to place the camera transformations on theGL_MODELVIEW matrix and never on theGL_PROJECTION matrix.
Think of the projection matrix as describing the attributes of your camera, such as field of view, focal length, fish eye lens,etc
My camera should go to the ModelView or Projectionmatrix?
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The GL_PROJECTION matrix should contain only the projection
transformation calls it needs to transform eye space coordinatesinto clip coordinates.
The GL_MODELVIEW matrix, as its name implies, should containmodeling and viewing transformations, which transform objectspace coordinates into eye space coordinates
Remember to place the camera transformations on theGL_MODELVIEW matrix and never on theGL_PROJECTION matrix.
Think of the projection matrix as describing the attributes of your camera, such as field of view, focal length, fish eye lens,etc
Think of the ModelView matrix as where you stand with thecamera and the direction you point it.
From "GL_PROJECTION abuse (By Steve Baker)"
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One of the most common OpenGL programming errors is misuse of
the GL_PROJECTION matrix stack
From "GL_PROJECTION abuse (By Steve Baker)"
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One of the most common OpenGL programming errors is misuse of
the GL_PROJECTION matrix stack
From "GL_PROJECTION abuse (By Steve Baker)"
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One of the most common OpenGL programming errors is misuse of
the GL_PROJECTION matrix stack
This matrix stack is often perceived as being the correct placeto put the position and orientation of the camera - but this is
not in fact the case.
From "GL_PROJECTION abuse (By Steve Baker)"
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One of the most common OpenGL programming errors is misuse of
the GL_PROJECTION matrix stack
This matrix stack is often perceived as being the correct placeto put the position and orientation of the camera - but this is
not in fact the case.
Unfortunately, when someone does go and put their cameratransform into the GL_PROJECTION matrix (instead of into
GL_MODELVIEW where it belongs)
From "GL_PROJECTION abuse (By Steve Baker)"
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One of the most common OpenGL programming errors is misuse of
the GL_PROJECTION matrix stack
This matrix stack is often perceived as being the correct placeto put the position and orientation of the camera - but this is
not in fact the case.
Unfortunately, when someone does go and put their cameratransform into the GL_PROJECTION matrix (instead of into
GL_MODELVIEW where it belongs)
The consequences are rather subtle:
Lighting
OpenGL has to transform vertex normals into world coordinate space that is to say WITHOUT the effects of
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coordinate space - that is to say WITHOUT the effects of perspective - but WITH the effects of the camera position
Lighting
OpenGL has to transform vertex normals into world coordinate space that is to say WITHOUT the effects of
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coordinate space - that is to say WITHOUT the effects of perspective - but WITH the effects of the camera position
Lighting
OpenGL has to transform vertex normals into world coordinate space that is to say WITHOUT the effects of
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coordinate space - that is to say WITHOUT the effects of perspective - but WITH the effects of the camera position
Hence, only the GL_MODELVIEW matrix is applied to the
normals for lighting
Lighting
OpenGL has to transform vertex normals into world coordinate space - that is to say WITHOUT the effects of
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coordinate space - that is to say WITHOUT the effects of perspective - but WITH the effects of the camera position
Hence, only the GL_MODELVIEW matrix is applied to the
normals for lighting
If you put the camera transform into the GL_PROJECTIONmatrix then your lighting will be wrong
Lighting
OpenGL has to transform vertex normals into world coordinate space - that is to say WITHOUT the effects of
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coordinate space that is to say WITHOUT the effects of perspective - but WITH the effects of the camera position
Hence, only the GL_MODELVIEW matrix is applied to the
normals for lighting
If you put the camera transform into the GL_PROJECTIONmatrix then your lighting will be wrong
However, some people do their own lighting - and in any case,it can be a subtle error that you might not have noticedpreviously.
Fog
OpenGL has to figure out how far each vertex is from the
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camera
Fog
OpenGL has to figure out how far each vertex is from the
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camera
Fog
OpenGL has to figure out how far each vertex is from the
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camera
Once again, perspective effects are not relevent to thiscalculation - so the GL_PROJECTION matrix is not used
Fog
OpenGL has to figure out how far each vertex is from the camera
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camera
Once again, perspective effects are not relevent to thiscalculation - so the GL_PROJECTION matrix is not used
If you put the camera transform into the GL_PROJECTIONmatrix then your fogging will be wrong
Fog
OpenGL has to figure out how far each vertex is from the camera
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camera
Once again, perspective effects are not relevent to thiscalculation - so the GL_PROJECTION matrix is not used
If you put the camera transform into the GL_PROJECTIONmatrix then your fogging will be wrong
Since few people use fog, many people have the error anddon’t know it.
TexGen
Since OpenGL uses the eyepoint to figure out some of the
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p y p gTexGen’ed texture coordinates,
TexGen
Since OpenGL uses the eyepoint to figure out some of the
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TexGen’ed texture coordinates,
TexGen
Since OpenGL uses the eyepoint to figure out some of the
8/9/2019 lezione33
http://slidepdf.com/reader/full/lezione33 93/102
TexGen’ed texture coordinates,
if the camera position is all mixed up with the projectioninformation then you’ll end up with some pretty strangetexture coordinates
TexGen
Since OpenGL uses the eyepoint to figure out some of the
8/9/2019 lezione33
http://slidepdf.com/reader/full/lezione33 94/102
TexGen’ed texture coordinates,
if the camera position is all mixed up with the projectioninformation then you’ll end up with some pretty strangetexture coordinates
(Thanks to Brian Sharp at 3Dfx for pointing this one out)
Z-buffer
I b li (b t h f) th t d
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I believe (but have no proof) that a screwed up
GL_PROJECTION matrix could cause your Z values to be computed strangely
Z-buffer
I believe (but have no proof) that a screwed up
8/9/2019 lezione33
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I believe (but have no proof) that a screwed up
GL_PROJECTION matrix could cause your Z values to be computed strangely
Z-buffer
I believe (but have no proof) that a screwed up
8/9/2019 lezione33
http://slidepdf.com/reader/full/lezione33 97/102
I believe (but have no proof) that a screwed up
GL_PROJECTION matrix could cause your Z values to be computed strangely
This would likely only manifest itself as a lack of Z precisionunder some circumstances - so you might not notice - and itmight not matter.
The Golden Rule ( "GL_PROJECTION abuse")
The rule is:
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The rule is:
The only functions that should be called when the glMatrixModeis set to ‘GL_PROJECTION‘ are:
glLoadIdentity
to initialise the stack.
gluPerspective/glFrustum/glOrtho/gluOrtho2
to set the appropriate projection onto the stack.
You could use glLoadMatrix to set up your own projection matrix(only if you understand the restrictions and consequences)
Menu management GLUT callbacks
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Menu management GLUT callbacks
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download the file (triangle.py)
Menu management GLUT callbacks
8/9/2019 lezione33
http://slidepdf.com/reader/full/lezione33 101/102
download the file (triangle.py)
study it
Menu management GLUT callbacks
8/9/2019 lezione33
http://slidepdf.com/reader/full/lezione33 102/102
download the file (triangle.py)
study it
produce a generic framework where the keybord keys are usedto either ROTATE on two axes about the origin, or to MOVEthe viewer far or close to the origin.
