OpenGL OpenGL Isamechanismtocreateimagesinaframebuffer - PDF document

11/4/09 OpenGL
 OpenGL
 • OpenGL
 – Is
a
mechanism
to
create
images
in
a
frame
buffer
 – Is
an
API
to
access
that
mechanism
 – Is
well
specified
 • OpenGL
 – Is
not
a
window
system
 – Is
not
a
user
interface
 – Is
not
a
display
mechanism
 – Does
not
even
own
the
framebuffer
 • It
is
owned
by
the
window
system
so
it
can
be
shared
 • But
OpenGL
defines
its
aAributes
carefully
 White‐Square
Code
 // Draw a white square against a black background #include <windows.h> #include <stdio.h> #define GLUT_DISABLE_ATEXIT_HACK // yuck! #include <GL/glut.h> void draw() { glClear(GL_COLOR_BUFFER_BIT); glLoadIdentity(); glOrtho(0, 4, 0, 4, -1, 1); glBegin(GL_POLYGON); glVertex2i(1, 1); OpenGL glVertex2i(3, 1); glVertex2i(3, 3); glVertex2i(1, 3); glEnd(); glFlush(); } int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA); GLUT glutCreateWindow("whitesquare"); glutDisplayFunc(draw); glutMainLoop(); } 1

11/4/09 OpenGL
PorGon
of
White‐Square
Code
 glClear(GL_COLOR_BUFFER_BIT); // black background glLoadIdentity(); glOrtho(0, 4, 0, 4, -1, 1); // int cast to double glBegin(GL_POLYGON); // draw white square glVertex2i(1, 1); glVertex2i(3, 1); glVertex2i(3, 3); glVertex2i(1, 3); glEnd(); glFlush(); // force completion Red‐Book
Example
 glClearColor(0.0, 0.0, 0.0, 0.0); glClear(GL_COLOR_BUFFER_BIT); glColor3f(1.0, 1.0, 1.0); glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0); glBegin(GL_POLYGON); glVertex3f(0.25, 0.25, 0.0); glVertex3f(0.75, 0.25, 0.0); glVertex3f(0.75, 0.75, 0.0); glVertex3f(0.25, 0.75, 0.0); glEnd(); glFlush(); // force completion State
tables
 COLOR_CLEAR_VALUE 0,0,0,0 OpenGL 2.0 Spec, Table 6.21. Framebuffer Control 2

11/4/09 State
tables
 OpenGL
2.0,
Table
6.5.
Current
Values
and
Associated
Data 
 Snippet
From
the
OpenGL
2.0
Spec
 • VerGces
are
specified
by
giving
their
coordinates
in
two,
three,
 or
four
dimensions.
This
is
done
using
one
of
several
versions
 of
the
 Vertex 
command:
 • 

void
 Vertex{234}{sifd} (
T
 coords
 );
 • 

void
 Vertex{234}{sifd}v (
T
 coords
 );
 • A
call
to
any
 Vertex 
command
specifies
four
coordinates:
x,
y,
 z,
and
w.
The
x
coordinate
is
the
first
coordinate,
y
is
second,
z
 is
third,
and
w
is
fourth.
A
call
to
 Vertex2 
sets
the
x
and
y
 coordinates;
the
z
coordinate
is
implicitly
set
to
zero
and
the
 w
coordinate
to
one.
 OpenGL
Philosophy
 • PlaXorm
and
window
system
independent
 • Rendering
only
 • Aims
to
be
real‐Gme
 • Takes
advantage
of
graphics
hardware
where
 it
exists
 • State
system
 • Client‐server
system

 • Immediate
mode
 3

11/4/09 The
OpenGL
Pipeline
 OpenGL
Shaded‐Quad
Code
 glClearColor(1, 1, 1, 1); // white glClear(GL_COLOR_BUFFER_BIT); glLoadIdentity(); glOrtho(0, 100, 0, 100, -1, 1); glBegin(GL_TRIANGLE_STRIP); glColor3f(0, 0.5, 0); // dark green glVertex2i(11, 31); glVertex2i(37, 71); glColor3f(0.5, 0, 0); // dark red glVertex2i(91, 38); glVertex2i(65, 71); glEnd(); glFlush(); OpenGL
Pipeline
 Application • Emphasis
is
on
data
types
 Vertex assembly • Diagram
ignores
 Vertex operations – Pixel
pipeline
 Primitive assembly – Texture
memory
 – Display
lists
 Primitive operations – …
 Rasterization Fragment operations • Display
is
not
part
of
OpenGL
 Framebuffer Display 4

11/4/09 Vertex
Assembly
(data
types)
 Application struct { Vertex assembly float x,y,z,w; float r,g,b,a; Vertex operations } vertex; Primitive assembly Primitive operations Rasterization Fragment operations Framebuffer Display Vertex
Assembly
(OpenGL)
 Application • Vertex
assembly
 Vertex assembly – Force
input
to
canonical
format
 • Convert
to
internal
representaGon
 Vertex operations – E.g.,
x,
y
to
float
 • IniGalize
unspecified
values

 Primitive assembly – E.g.,
z
=
0,
w=1
 • Insert
current
modal
state
 Primitive operations – E.g.,
color
to
0,0.5,0,1
 – Or
create
using
evaluators
 Rasterization • Error
detecGon
 Fragment operations – INVALID_ENUM
 – INVALID_VALUE
 Framebuffer – INVALID_OPERATION
 • Especially
between
Begin
and
End
 Display Vertex
Assembly
(OpenGL)
 Application glColor3f(0, 0.5, 0); glVertex2i(11, 31); Vertex assembly glVertex2i(37, 71); glColor3f(0.5, 0, 0); // no effect Vertex operations Primitive assembly Primitive operations struct { float x,y,z,w; // 11, 31, 0, 1 float r,g,b,a; // 0, 0.5, 0, 1 Rasterization } vertex; Fragment operations struct { float x,y,z,w; // 37, 71, 0, 1 Framebuffer float r,g,b,a; // 0, 0.5, 0, 1 } vertex; Display 5

11/4/09 Vertex
OperaGons
 Application • OpenGL
 Vertex assembly – Transform
coordinates
 Vertex operations • 4x4
matrix
arithmeGc
 – Compute
(vertex)
lighGng
 Primitive assembly – Compute
texture
coordinates
 – …
 Primitive operations • In
our
case:
 Rasterization – Scale
(arbitrary
100x100)
coordinates

 to
fit
window
 Fragment operations – No
lighGng,
no
texture
coordinates
 Framebuffer Display PrimiGve
Assembly
(data
types)
 Application struct { Vertex assembly float x,y,z,w; float r,g,b,a; Vertex operations } vertex; Primitive assembly struct { vertex v0,v1,v2; Primitive operations } triangle; or Rasterization struct { vertex v0,v1; Fragment operations } line; or Framebuffer struct { Display vertex v0; } point; PrimiGve
Assembly
 Application OpenGL
 • Vertex assembly – Group
verGces
into
primiGves:
 • points,
 Vertex operations • lines,
or
 • triangles
 Primitive assembly – Decompose
polygons
to
triangles
 – Duplicate
verGces
in
strips
or
fans
 Primitive operations In
our
case:
 • Rasterization – Create
two
triangles
from
a
strip:
 1 3 Fragment operations glBegin(GL_TRIANGLE_STRIP); glColor(green); glVertex2i(…); // 0 Framebuffer glVertex2i(…); // 1 glColor(red); glVertex2i(…); // 2 glVertex2i(…); // 3 Display 2 glEnd(); 0 6

11/4/09 PrimiGve
OperaGons
 Application • OpenGL
 Vertex assembly – Clip
to
the
window
boundaries
 Vertex operations • Actually
to
the
frustum
surfaces
 – Perform
back‐face
/
front‐face
ops
 Primitive assembly • Culling
 Primitive operations • Color
assignment
for
2‐side
lighGng
 • In
our
case
 Rasterization – Nothing
happens
 Fragment operations Framebuffer Display RasterizaGon
(data
types)
 Application struct { Vertex assembly float x,y,z,w; float r,g,b,a; Vertex operations } vertex; Primitive assembly struct { vertex v0,v1,v2 Primitive operations } triangle; Rasterization struct { short int x,y; Fragment operations float depth; float r,g,b,a; Framebuffer } fragment; Display RasterizaGon 
 Application OpenGL
 – Determine
which
pixels
are
included
in
 Vertex assembly the
primiGve
 Vertex operations • Generate
a
fragment
for
each
such
pixel
 – Assign
aAributes
(e.g.,
color)
to
each
 Primitive assembly fragment
 Primitive operations In
our
case:
 Rasterization Fragment operations Framebuffer Display 7

11/4/09 Fragment
OperaGons
 Application OpenGL
 – Texture
mapping
 Vertex assembly – Fragment
lighGng
(OpenGL
2.0)
 Vertex operations – Fog
 Primitive assembly – Scissor
test
 – Alpha
test
 Primitive operations In
our
case,
nothing
happens:
 Rasterization Fragment operations Framebuffer Display Framebuffer
(2‐D
array
of
pixels)
 Application struct { float x,y,z,w; Vertex assembly float r,g,b,a; } vertex; Vertex operations struct { vertex v0,v1,v2 Primitive assembly } triangle; Primitive operations struct { short int x,y; Rasterization float depth; float r,g,b,a; Fragment operations } fragment; Framebuffer struct { int depth; Display byte r,g,b,a; } pixel; Fragment
  
Framebuffer
Ops
 Application OpenGL
 Vertex assembly – Color
blending
 – Depth
tesGng
(aka
z‐buffering)
 Vertex operations – Conversion
to
pixels
 Primitive assembly In
our
case,
conversion
to
pixels:
 Primitive operations Rasterization Key
idea:
images
are
 Fragment operations built
in
the
 framebuffer,
not
just
 Framebuffer placed
there! 
 Display 8