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Buffers and Processing Fragments
2011 Autumn
Animação e Visualização Tridimensional
2011/2012
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BuffersBuffers
Color Buffers: (front|back)-(left|right),
Depth Buffer; Stencil Buffer; Accumulation Buffer.
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OpenGL ArchitectureImmediate Mode
DisplayList
PolynomialEvaluator
Per VertexOperations &
PrimitiveAssembly
RasterizationPer Fragment
Operations
TextureMemory
CPU
PixelOperations
FrameBuffer
geometry pipeline
Fragments Operations
Fragment tests Blend operations
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Fragments tests
Scissor test Culls fragments in a 2D box on screen
Alpha test Compares fragment alpha with a constant Culls fragments conditionally
Stencil test Compares value of stencil buffer with reference constant Culls fragments conditionally Can apply different operation to stencil value based mode Stencil-fail/S-pass & Z-fail / S-pass & Z-pass
Operations: Set, increment, decrements, … Depth test (visibility/occlusion test)
Compares Z value with value from Z-buffer Culls fragments conditionally, otherwise updates Z-buffer
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Blend OperationsMerge fragments with frame buffer content
Order of operations: Blending operations (aka. compositing)
Weighted combination of fragment and pixel values Dithering operation
Approximation of color by spatial averaging Different rounding based pixel location Half-Toning“
Logical operations 16 combinations of fragment and pixel values (NOT, AND,
OR, XOR)
Window Coordinates (OpenGL API)
void glScissor (GLint x, GLint y, GLsizei width, GLsizei height)
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Scissor TestScissor Test
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Alpha TestAlpha Test
void glAlphaFunc (GLenum func, GLclampf ref);
Enum: GL_NEVER, GL_ALWAYS, GL_LESS, GL_LEQUAL, GL_EQUAL, GL_GEQUAL, GL_GREATER, GL_NOTEQUAL
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How to Use Blending? Remember to enable
glEnable(GL_BLEND); Set up blend function
glBlendFunc(…,…)
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Blending Equation
We can define source and destination blending factors for each RGBA component
s = [sr, sg, sb, s]
d = [dr, dg, db, d]
Suppose that the source and destination colors are b = [br, bg, bb, b]
c = [cr, cg, cb, c]
Blend asc’ = [br sr+ cr dr, bg sg+ cg dg , bb sb+ cb db , b s+ c d ]
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OpenGL Blending and Compositing
Must enable blending and pick source and destination factors glEnable(GL_BLEND) glBlendFunc(source_factor,
destination_factor)Only certain factors supported
GL_ZERO, GL_ONE GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA See Redbook for complete list
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Blend Function 2/2
Parameter (f(R), f(G), f(B), f(A))
GL_ZERO (0,0,0,0)
GL_ONE (1,1,1,1)
GL_SRC_COLOR (RS/kR,GS/kG,BS/kB,AS/kA)
GL_ONE_MINUS_SRC_COLOR (1,1,1,1)-(RS/kR,GS/kG,BS/kB,AS/kA)
GL_DST_COLOR (Rd/kR,Gd/kG,Bd/kB,Ad/kA)
GL_ONE_MINUS_DST_COLOR (1,1,1,1)-(Rd/kR,Gd/kG,Bd/kB,Ad/kA)
GL_SRC_ALPHA AS/kA,AS/kA,AS/kA,AS/kA
GL_ONE_MINUS_SRC_ALPHA (1,1,1,1) -(AS/kA,AS/kA,AS/kA,AS/kA)
GL_DST_ALPHA (AD/kA,AD/kA,AD/kA,AD/kA)
GL_ONE_MINUS_DST_ALPHA (1,1,1,1) -(AD/kA,AD/kA,AD/kA,AD/kA)
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Example
Suppose that we start with the opaque background color (R0,G0,B0,1)
This color becomes the initial destination color We now want to blend in a translucent polygon with color (R1,G1,B1,1)
Select GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA as the source and destination blending factors
R’1 = 1 R1 +(1- 1) R0, ……
Note this formula is correct if polygon is either opaque or transparent
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Order Dependency
Is this image correct?Probably notPolygons are renderedin the order they passdown the pipelineBlending functionsare order dependent
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Red Book Example
Demo: alpha.c example in chapter 6 of the Red Book
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Blending Example 1/6
#include <math.h>#include "glut.h"
GLfloat alpha = 0.0;GLfloat pos[4] = {0, 10, 10, 0};GLfloat dif_l[4] = {1.0, 1.0, 1.0, 1.0};GLfloat dif_t[4] = {1.0, 0.5, 0.8, 1.0};GLfloat dif_m[4] = {0.5, 0.8, 0.8, cos(alpha)};
void display();void reshape(GLsizei, GLsizei);void idle();
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Blending Example 2/6
void main(int argc, char** argv){glutInit(&argc, argv);glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);glutInitWindowSize (500, 500);glutCreateWindow("Blending");glEnable(GL_DEPTH_TEST);glEnable(GL_LIGHTING);glutDisplayFunc(display);glutReshapeFunc(reshape);glutIdleFunc(idle);glutMainLoop();return;
}
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Blending Example 3/6
void reshape(GLsizei w, GLsizei h){glViewport(0, 0, 500, 500);glMatrixMode(GL_PROJECTION);glLoadIdentity();gluPerspective(45, 1 / 1, 0.1, 1000);glMatrixMode(GL_MODELVIEW);glLoadIdentity();gluLookAt(-10, 10, 30, 0, 0, 0, 0, 1, 0);
}
void idle(){alpha += 0.05;dif_m[3] = (cos(alpha) + 1) / 2;glutPostRedisplay();
}
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Blending Example 4/6
void display(){glClearColor(0, 0, 0, 0);glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);glEnable(GL_LIGHT0);glLightfv(GL_LIGHT0, GL_POSITION, pos);glLightfv(GL_LIGHT0, GL_DIFFUSE, dif_l);glMaterialfv(GL_FRONT_AND_BACK, GL_DIFFUSE, dif_t);glutSolidTeapot(5);
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Blending Example 5/6
glEnable(GL_BLEND);glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);glMaterialfv(GL_FRONT_AND_BACK, GL_DIFFUSE, dif_m);glBegin(GL_POLYGON);
glNormal3f(0, 0, 1);glVertex3f(-5, -5, 10);glVertex3f(5, -5, 10);glVertex3f(5, 5, 10);glVertex3f(-5, 5, 10);
glEnd();glFlush();glutSwapBuffers();
}
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Blending Example 6/6
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Billboards
•Replace a mesh with a pre-computed picture of the mesh
•Fast to render: a single textured polygon versus many
•Sometimes called“impostors”
•Always “aligned” with the viewing direction
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What to use it for? Scenery
Trees, grass, spectators Mesh simplification
Replace far-away objects with billboards Non-polygonal objects
Fire, smoke, clouds, particles
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Billboard basics A billboard is a
textured rectangle Texture is static Draw billboard
where mesh would have been drawn
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Billboard basics Need to remove
texture background Two ways:
Masking Alpha blending
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Billboards by using Masking
Billboard fragments alpha values are 0 or 1 Draw billboard fragments with alpha equal to 1
OpenGL: Alpha test
void glAlphaFunc (GLenum func, GLclampf ref);Enumerados: GL_NEVER, GL_ALWAYS, GL_LESS,
GL_LEQUAL, GL_EQUAL, GL_GEQUAL, GL_GREATER, GL_NOTEQUAL
glAlphaFunc (GL_GREATER, 0.0f)glEnable(GL_ALPHA_TEST)
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Billboards with Blending Billboard fragments alpha values between [0,
1] Blending
void glBlendFunc (GLenum sfactor, GLenum dfactor);
Enumerados: GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ONE_MINUS_DST_COLOR GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA...
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
glEnable(GL_BLEND)
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Opaque and Translucent Polygons
Suppose that we have a group of polygons some of which are opaque and some translucent
How do we use hidden-surface removal? Opaque polygons block all polygons behind them and
affect the depth buffer Translucent polygons should not affect depth buffer
Render with glDepthMask(GL_FALSE) which makes depth buffer read-only
Sort polygons first to remove order dependency
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3D Blending with the Depth Buffer
Steps:
Enable Depth Buffering Draw all the opaque objects (how?) Make the depth-buffer read-only (why?) Draw the translucent objects with
blending. Figure out the correct order of rasterization
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How to Use Fog? Remember to enable
glEnable(GL_FOG); Set up fog function
glFogf(…,…)
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Fog Function 1/2
glFog{f,i}[v]{GLenum pname, param}Pname param
GL_FOG_MODE GL_LINEAR, GL_EXP, GL_EXP2
GL_FOG_DENSITY default : 1.0
GL_FOG_START default : 0.0
GL_FOG_END default : 1.0
GL_FOG_INDEX default : 0.0
GL_FOG_COLOR default : (0,0,0,0)
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Fog Function 2/2
GL_LINEAR
GL_EXP
GL_EXP2
COLOR:
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How to use stencil buffer?
1. glutInitDisplayMode(GLUT_STENCIL);
2. glEnable(GL_STENCIL_TEST);
3. glClearStencil(0);4. glClear(GL_STENCIL_BUFFER_BIT);
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Stencil Testing 1/2
void glStencilFunc( GLenum func, GLint ref, GLuint mask ); sets the function and reference value
for stencil testing. func: test function, see next page. ref: reference value mask:A mask that is ANDed with both
the reference value and the stored stencil value when the test is done.
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Stencil Testing 2/2
param Meaning
GL_NEVER Always fails.
GL_LESS Passes if ( ref & mask) < ( stencil & mask).
GL_LEQUAL Passes if ( ref & mask) ≤ ( stencil & mask).
GL_GEQUAL Passes if ( ref & mask) ≥ ( stencil & mask).
GL_NOTEQUAL Passes if ( ref & mask) ( stencil & mask).
GL_ALWAYS Always passes.
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Modify stencil buffer 1/2
void glStencilOp( GLenum fail, GLenum zfail, GLenum zpass ); sets the stencil test actions. fail: The action to take when the
stencil test fails zfail: Stencil action when the stencil
test passes, but the depth test fails. zpass: both the stencil test and the
depth test pass
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Stencil buffer + Z buffer
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Modify stencil buffer 2/2
param Meaning
GL_KEEP keep the current value
GL_ZERO set the value in stencil buffer to zero
GL_REPLACE set the value in stencil buffer to ref in glStencilFunc()
GL_INCR increase the current value in stencil buffer
GL_DECR decrease the current value in stencil buffer
GL_INVERT bitwise inverse the current value in stencil buffer
Red Book example Demo: stencil.c example in chapter
10 of the Red Book
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Another Stencil buffer example 1/3
#include<gl/glut.h>
void GL_display(){glEnable(GL_DEPTH_TEST);glEnable(GL_STENCIL_TEST);glClearStencil(0);glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);glStencilFunc(GL_ALWAYS, 1, 1);glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);glColor3f(0.0, 1.0, 1.0);glutSolidCube(16.0);glClear(GL_DEPTH_BUFFER_BIT);glStencilOp(GL_KEEP, GL_KEEP, GL_INVERT);glStencilFunc(GL_EQUAL, 1, 1);glColor3f(1.0, 1.0, 1.0);glutSolidTeapot(8);glFlush();
}
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Stencil buffer example 2/3
void GL_reshape(GLsizei w, GLsizei h){
glViewport(0, 0, w, h);glMatrixMode(GL_PROJECTION);glLoadIdentity();gluPerspective(60.0, (GLfloat)w/(GLfloat)h, 0.5, 100.0);glMatrixMode(GL_MODELVIEW);glLoadIdentity();gluLookAt(0.0, 0.0, 50.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
}
int main(int argc, char** argv){
glutInit(&argc, argv);glutInitWindowSize(400, 400);glutInitWindowPosition(0, 0);glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA | GLUT_DEPTH | GLUT_STENCIL);glutCreateWindow("Stencil Buffer");glutDisplayFunc(GL_display);glutReshapeFunc(GL_reshape);glutMainLoop();
}
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Stencil buffer example 3/3
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Other example… 1/2
void GL_display(){glEnable(GL_DEPTH_TEST);glEnable(GL_STENCIL_TEST);glClearStencil(0);glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);glStencilFunc(GL_ALWAYS, 1, 1);glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);glColor3f(0.0, 1.0, 1.0);glutSolidCube(16.0);glClear(GL_DEPTH_BUFFER_BIT);glStencilOp(GL_KEEP, GL_KEEP, GL_INVERT);glStencilFunc(GL_EQUAL, 1, 1);glColor3f(1.0, 1.0, 1.0);glutSolidTeapot(8);glStencilFunc(GL_NOTEQUAL, 1, 1);glColor3f(1.0, 1.0, 0.0);glPushMatrix();glTranslatef(10, 0, 0);glutSolidSphere(12, 10, 6);glPopMatrix();glFlush();
}
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Other example… 2/2
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Cooler example…
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How to use accumulation buffer? glutInitDisplayMode(GLUT_ACCUM)
;
glClear(GL_ACCUM_BUFFER_BIT);
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Accumulation function void glAccum( GLenum op,
GLfloat value ); operates on the accumulation buffer. op: The accumulation buffer
operation. See next page. value: also see next page.
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Accumulation functionop Meaning
GL_ACCUM read each pixel from the color buffer, multiplies the R, G, B, and alpha values by value, then add
the result to accumulation buffer.
GL_LOAD the same as GL_ACCUM, but replace the value in accumulation rather than add.
GL_RETURN read values from the accumulation buffer, multiplies them by value, and places the result in the
color buffers.
GL_ADD / GL_MULT simply add / multiply the values of each pixel in the accumulation buffer by value, and return to
the accumulation buffer. (GL_MULT will clamp value from –1.0 to 1.0 while GL_ADD will not.)
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Supersampling Multiple samples per pixel: each
pixel is divided into several sub-pixels
Multiple samples per pixel are taken in various ways and these are blended together to give the resulting pixel
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Supersampling example Viewport resolution 2 x 2
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Filter 3 x 3
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Virtual Image
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Weighted filter 3 x 3
Rendering multiple images Example: 3 x 3 filter Avoid to use 9 x original image resolution
memory Solution: rendering 9 times the entire scene
with the original resolution Each rendering is obtained by jittering a
sub-pixel amount in screen space Each final pixel is the average of the images
that intersect it.
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Antialiasing with accumulation buffer entire scene is offset (jittered) by small, subpixel amounts
in screen space, and accumulated. Enable and clear the accumulator GL ACCUM reads each pixel from the current color buffer
and multiplies the R,G,B,and alpha values by 1/n and adds the result to the accumulator.
Scene gets drawn n times with slight perturbations (jittering), so that each pixel is the average of the images that intersect it.
GL_RETURN copies the values into the color buffer for viewing.
glClearAccum(r; g; b; alpha)glClear(GL ACCUM BUFFER BIT)glAccum(GL ACCUM; 1/n)glAccum(GL RETURN; 1.0)
Antialiasing example Demo: accpersp.c example in Chapter 10 “The
Framebuffer” of the Red Book Final scene results from the accumulation of 8
jittered images Jittering an image implies a slightly changing
in the corresponding frustumdx = jitter_x*(xwsize / viewport[2])
dy = jitter_y*(ywsize / viewport[3])
For each image rendering the viewpoint remains unchanged which implies the use of non-symmetric frustum
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Depth of Field effect Demo: dof.c example in chapter 10 of the
Red Book Scene drawn 8 times corresponding to
jittering 8 times the viewpoint Jitter the volume while holding it stationary
at the focal plane Teapot located in focal plane is “in focus” Teapots not in the focal plane get blurred
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Depth of Field effect
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Non-symmetric frustum
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Other accumulation example 1/3
#include <GL/glut.h>void GL_display(){
// clear the bufferglClearColor(0.0, 0.0, 0.0, 0.0);glClearAccum(0.0, 0.0, 0.0, 0.0); glClear(GL_ACCUM_BUFFER_BIT);for(int i = 0; i < 360; i++){
glClear(GL_COLOR_BUFFER_BIT);glColor3f(1.0, 1 - (float)i / 180, 1 - (float)i / 360);glPushMatrix();glTranslatef(0, -6, -10);glRotatef(i,0, 1, 0);glutWireTeapot(5);glPopMatrix();glAccum(GL_ACCUM, 0.01);
}glAccum(GL_RETURN, 1.0) ;glutSwapBuffers();
}
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Accumulation example 2/3
void GL_reshape(GLsizei w, GLsizei h){
glViewport(0, 0, w, h);glMatrixMode(GL_PROJECTION);glLoadIdentity();gluPerspective(60.0, (GLfloat)w/(GLfloat)h, 1.0, 20.0);glMatrixMode(GL_MODELVIEW);glLoadIdentity();gluLookAt(0.0, 3.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
}
int main(int argc, char** argv){
glutInit(&argc, argv);glutInitWindowSize(500, 500);glutInitWindowPosition(0, 0);glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_ACCUM);glutCreateWindow("Accumulation");glutDisplayFunc(GL_display);glutReshapeFunc(GL_reshape);glutMainLoop();
}
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Accumulation example 3/3
Motion blur
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Accumulation example 2 1/3
#include <GL/glut.h>void GL_display(){
glClearColor(0.0, 0.0, 0.0, 0.0);glClearAccum(0.0, 0.0, 0.0, 0.0); glClear(GL_ACCUM_BUFFER_BIT);for(int i = 0; i < 30; i++){
glClear(GL_COLOR_BUFFER_BIT);glLoadIdentity();gluLookAt(i * 0.01, 0.0, 3.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);glPushMatrix();glTranslatef(0, 4, -10);glColor3f(1, 0, 0);glutWireTeapot(2);glPopMatrix();glAccum(GL_ACCUM, 0.03333);
}glAccum(GL_RETURN, 1.0) ;glPushMatrix();glColor3f(1, 1, 1);glutWireTeapot(1);glPopMatrix();glutSwapBuffers();glutSwapBuffers();
}
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Accumulation example 2 2/3
void GL_reshape(GLsizei w, GLsizei h){
glViewport(0, 0, w, h);glMatrixMode(GL_PROJECTION);glLoadIdentity();gluPerspective(60.0, (GLfloat)w/(GLfloat)h, 1.0, 50.0);glMatrixMode(GL_MODELVIEW);glLoadIdentity();
}
int main(int argc, char** argv){
glutInit(&argc, argv);glutInitWindowSize(500, 500);glutInitWindowPosition(0, 0);glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_ACCUM);glutCreateWindow("Accumulation");glutDisplayFunc(GL_display);glutReshapeFunc(GL_reshape);glutMainLoop();
}
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Accumulation example 2 3/3
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Cooler example
Buffers Wrap-Up Open GL Buffer
n x m elements of k bits
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Buffers Wrap-up OpenGL Frame Buffer
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