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Introduction to OpenGL (Part 4). Ref: OpenGL Programming Guide (The Red Book). Part 1 Introduction Geometry Viewing Light & Material Display List. Part 2 Alpha Channel Polygon Offset Part 3 Image Texture Mapping Part 4 Framebuffers Selection & Feedback. Topics. OpenGL.

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Introduction to OpenGL(Part 4)

Ref: OpenGL Programming Guide (The Red Book)

Fall 2009 revised


Part 1

Introduction

Geometry

Viewing

Light & Material

Display List

Part 2

Alpha Channel

Polygon Offset

Part 3

Image

Texture Mapping

Part 4

Framebuffers

Selection & Feedback

Topics


OpenGL

Framebuffers


color (front/back, left/right, aux)

Front/back: animation

Left/right: stereo vision

Aux: not available on PC (use framebuffer object FBO instead)

depth buffer: depth test

VC: 24 bits available

Value : [0.0,1.0]

stencil buffer: restrict drawing to certain portions of the screen

VC: 8 bits available

accumulation buffer: accumulating composite image

Types of Buffers


Using Framebuffers

  • clearing buffers

    • clearing individual buffer is expensive

    • Use glClear with bitwise-ORed masks to clear multiple buffers

  • selecting color buffers for writing/clearing

    • glDrawBuffer: useful in FBO (framebuffer object)


Masking Buffers

  • Before OpenGL writes data into the enabled color, depth, or stencil buffers, a masking operation is applied to the data, as specified with one of the following commands.

  • A bitwise logical AND is performed with each mask and the corresponding data to be written


Masking Buffers (cont)

  • void glColorMask(GLboolean red, GLboolean green, GLboolean blue, GLboolean alpha);

  • void glDepthMask(GLboolean flag);

  • void glStencilMask(GLuint mask);

    • If a 1 appears in mask, the corresponding bit in the stencil buffer is written; where a 0 appears, the bit is not written.

  • The default values of all the GLboolean masks are GL_TRUE, and the default values for the two GLuint masks are all 1's


Only Green Mask TRUE


Per-fragment Operation Sequence

  • Fragments: the pieces after rasterization, sent to framebuffers if they survive these tests

  • glScissor

    • restrict drawing to part of the window

    • a simpler (and faster) version of stencil test (that does not need stencil buffer)

    • application: gui panel

Dithering and logical op not covered here


While the scissor test is enabled, only pixels that lie within the scissor box can be modified by drawing commands.

Including glClear (shown right)

Note: validity of current raster position is determined by frustum (world coordinate); while scissor test affects the window coordinate

Single-pixel window: glScissor (x,y,1,1);

Scissor Test

See also stencil_close


Alpha test:

accept/reject a fragment based on its alpha value

implement transparency

use this test to filter opaque objects

see-through decal (billboarding): reject the transparent fragments (from ruining the depth buffer)

Stencil Test

require stencil buffer

glStencilFunc, glStencilOp

Fragment Operations (cont)


Stenciling


Basic Applications

Restrict rendering to limited portions of the screen

  • “Stenciling”

  • Decal

  • Shadow and lightmap


Mimicking Stencil

  • Compose stencil template

  • Control template then render

  • Multi-pass rendering

silhouette


Decal Using Stencil Buffers

Stencil to resolve z-fighting


Shadow and Lightmap

Shadow and light blended with the textured polygon underneath


Technical Details

  • glStencilFunc (fun, ref, mask)

    • func Specifies the test function. Eight tokens are valid: GL_NEVER, GL_LESS, GL_LEQUAL, GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL, and GL_ALWAYS.

      • If it's GL_LESS, for example, then the fragment passes if reference value is less than the value in stencil buffer

    • ref Specifies the reference value for the stencil test. ref is clamped to the range [0, 2n - 1], where n is the number of bitplanes in the stencil buffer.

      • VC: n = 8

    • mask Specifies a mask that is ANDed with both the reference value and the stored stencil value when the test is done


Technical Details (cont)

  • glStencilOp (fail, zfail, zpass)

    • fail Specifies the action to take when the stencil test fails.

    • zfail Specifies stencil action when the stencil test passes, but the depth test fails.

    • zpass Specifies stencil action when both the stencil test and the depth test pass, or when the stencil test passes and either there is no depth buffer or depth testing is not enabled.

    • Six symbolic constants are accepted: GL_KEEP, GL_ZERO, GL_REPLACE, GL_INCR, GL_DECR, and GL_INVERT.

Set value to ref

Bitwise invert


Stenciling

  • Steps to draw 2 coplanar rectangles:

    • Make the stencil for yellow one first (by drawing the green polygon)

    • Draw the yellow one with the stencil

    • Draw the green one


Stenciling (cont)

Stencil buffer

Color buffer


Decaling

[0] Base: (-5,0)(5,6)

[1] R: (-3,1)(2,4)

[2] G: (1,2)(3,5)

[3] B: (0,0)(4,3)

Foreground

blocker

  • Draw everything else (set up the depth buffer)

  • Draw base; set the blocker id to be 8; rest 0

  • Arrange the decal id in increasing order

  • The decals can be drawn in any order

Test the program…


2

2

2

2

2

2

Foreground

blocker

3

3

3

3

3

0

0

0

0

0

0

0

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0

3

0

0

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8

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0

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8

0

0

StencilOp(fail, zfail, zpass)

Color buffer

Stencil buffer


2

2

2

2

2

2

Foreground

blocker

3

3

3

3

3

0

0

0

0

0

0

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0

0

0

3

0

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1

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0

8

8

8

8

8

8

8

0

0

0

8

8

8

8

8

8

8

0

0

StencilOp(fail, zfail, zpass)

Color buffer

Stencil buffer

Drawn in different order!


Generating Silhouette

Stencil Buffer

Debug with GLIntercept


Different line style

Hidden line removal

Haloed lines

Cheap silhouette

See also


Shadow matrix to project model onto a flat shadow receiver

Blending + stencil buffer techniques involved

Stencil Planar Shadow


Reflection

Dinosaur is reflected by the planar floor.

Easy hack, draw dino twice, second time hasglScalef(1,-1,1) to reflect through the floor

Notice right image’s reflection falls off the floor!


Stencil Reflection

Clear stencil to zero.Draw floor polygon with stencil set to one.Only draw reflection where stencil is one.

Reflection and shadow


[Shadow Volume Using Stencil Buffers]

Triangle blockinglight source.

Use stencil to tag whetherpixel is inside or outsideof the shadow volume.

Two passes: light pixels outsidevolume; no lighting forpixels inside the volume.

“Shadow” volumeprojected by trianglefrom the light source.


series of images generated on standard color buffers; accumulated one at a time into the accumulation buffers; result copied back into a color buffer for viewing

Accumulation Buffer


scene antialiasing, motion blur, photographic depth of field

[softshadow, jittering]

analogy: multiple exposures

takes longer, but higher quality

accum.buffer may have higher precision

16 bits for each RGBA channels

scene antialiasing by spatially jittering the image

Accumulation Buffer (cont)


Accumulation Buffer APIs

  • glClearAccum (0,0,0,0);

  • glGetIntegerv (GL_ACCUM_RED_BITS, &value);

    • 16 for each RGBA channels

  • glAccum(GL_ACCUM, 1.f/counts);

  • glAccum(GL_RETURN, 1.f);


Accumulation Buffer

scene antialiasing

motion blur

depth of field


Ref


FSAA with Multi-Sampling(ref)

  • glMultiSample


Motion Blur w/o Accum.Buffer

Details:

scene dynamically render to texture;

modulate with a polygon (1,1,1,a)


Scene Anti-aliasing

  • Can be done in real-time w/o accumulation buffer

    • By outlining the silhouette edges

  • See Tom Hall

FPS: 10.8

FPS: 9.6


OpenGL

Selection and Feedback


Revisit unproject.c

A line segment between (x1, y1, zNear) and (x2, y2, zFar)

Check which object got hit

Complexity problem?!

Can do better?

How to do Picking?

Simple tracking on XZ plane


Selection (Basic Idea)

  • draw scene into framebuffer

  • enter selection mode and redraw the scene

    • the content of framebuffer unchanged until you exit the selection mode

  • when exit, returns a list of primitives that intersected the viewing volume

    • as an array of names (integer) and hit records

  • Picking: a variation of selection

    • triggered by mouse; mouse defines v.volume


Selection (Steps)

  • glSelectBuffer (maxselects, selectbuffer)

  • glRenderMode (GL_SELECT)

  • glInitNames, glPushName (0)

    • Something needs to be pushed on the empty stack (otherwise, it cannot be “loaded”)

  • define viewing volume

    • Need to save current projection matrix for further rendering …

  • issue drawing commands

    • Load a name; then draw something; if the thing drawn is in v.volume, it generates a hit (with the name)

  • glRenderMode (GL_RENDER)

    • Return the number of hits

  • process select buffer for hits


Name Stack

  • name: GLuint

  • glInitNames

    • causes the name stack to be initialized to its default empty state

  • glPushName/glPopName

    • as the name implies

  • glLoadName

    • replace the top of the stack

    • need to push something right after InitNames for first load

  • commands ignored unless in selection mode


Primitive intersecting v.vol causes a hit

stack pointer points to the beginning of selection array and updates when hit occurs

culled polygons generate no hit

composite objects (w/ single name) generate one hit

hit records aren’t updated until glRenderMode is called

each hit record:

number of names on the name stack

min and max window-coord z values

[0,1]

contents of name stack w/ bottommost element first

Hit Record


void selectObjects(void)

{

GLuint selectBuf[512];

GLint hits;

glSelectBuffer (512, selectBuf);

(void) glRenderMode (GL_SELECT);

glInitNames(); glPushName(0);

glPushMatrix ();

glMatrixMode (GL_PROJECTION);

glLoadIdentity ();

glOrtho (0.0, 5.0, 0.0, 5.0, 0.0, 10.0);

glMatrixMode (GL_MODELVIEW);

glLoadIdentity ();

glLoadName(1);

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0);

glLoadName(2);

drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0);

glLoadName(3);

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0);

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0);

glPopMatrix ();

hits = glRenderMode (GL_RENDER);

processHits (hits, selectBuf);

}

void display(void)

{

glClearColor (0.0, 0.0, 0.0, 0.0);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

drawScene (); selectObjects ();

glFlush();

}

void drawScene (void)

{

glMatrixMode (GL_PROJECTION);

glLoadIdentity ();

gluPerspective (40.0, 4.0/3.0, 1.0, 100.0);

glMatrixMode (GL_MODELVIEW);

glLoadIdentity ();

gluLookAt (7.5, 7.5, 12.5, 2.5, 2.5, -5.0, 0.0, 1.0, 0.0);

glColor3f (0.0, 1.0, 0.0);/* green */

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0);

glColor3f (1.0, 0.0, 0.0);/* red */

drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0);

glColor3f (1.0, 1.0, 0.0);/* yellow */

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0);

drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0);

drawViewVolume (0.0, 5.0, 0.0, 5.0, 0.0, 10.0);

}

void processHits (GLint hits, GLuint buffer[])

{

unsigned int i, j;

GLuint names, *ptr;

printf ("hits = %d\n", hits);

ptr = (GLuint *) buffer;

for (i = 0; i < hits; i++) {/* for each hit */

names = *ptr;

printf (" number of names for hit = %d\n", names); ptr++;

printf(" z1 is %g;", (float) *ptr/0x7fffffff); ptr++;

printf(" z2 is %g\n", (float) *ptr/0x7fffffff); ptr++;

printf (" the name is ");

for (j = 0; j < names; j++) {/* for each name */

printf ("%d ", *ptr); ptr++;

}

printf ("\n");

}

}


Process Hits

  • When returning to RENDER mode,

  • All hit records got recorded in a GLuint array as follows

    [# of names, zmin, zmax, names…]

  • Names for an object need not be a single integer

    • [object type, object number, object part,…]


About the depth value (refs: 1,2)

  • The depth is taken from the Z buffer (where it lies in the range [0,1]).

  • It gets multiplied by 2^32 -1 (0xffff ffff)and is rounded to the nearest integer.

  • Note that the depths you get are not linearly proportional to the distance to the viewpoint due to the nonlinear nature of the z buffer (See Opengl-2.ppt, p.44)


Picking

restrict drawing to a small region of the viewport (near the cursor)

mouse click to initiate the selection mode

Issues:

interpreting mouse (x,y)!

hierarchical pick: name stack manipulation

3D pick (picking and depth values)


creates a projection matrix that can be used to restrict drawing to a small region of the viewport

gluPickMatrix

Often called from mouse callback; need to convert glut to OpenGL mouse coordinates


void drawSquares(GLenum mode) {

GLuint i, j;

for (i = 0; i < 3; i++) {

if (mode == GL_SELECT)

glLoadName (i);

for (j = 0; j < 3; j ++) {

if (mode == GL_SELECT)

glPushName (j);

glColor3f ((GLfloat) i/3.0, (GLfloat) j/3.0,

(GLfloat) board[i][j]/3.0);

glRecti (i, j, i+1, j+1);

if (mode == GL_SELECT)

glPopName ();

}

}

}

void processHits (GLint hits, GLuint buffer[]) {

unsigned int i, j;

GLuint ii, jj, names, *ptr;

printf ("hits = %d\n", hits);

ptr = (GLuint *) buffer;

for (i = 0; i < hits; i++) {/* for each hit */

names = *ptr;

printf (" number of names for this hit = %d\n", names); ptr++;

printf(" z1 is %g;", (float) *ptr/0x7fffffff); ptr++;

printf(" z2 is %g\n", (float) *ptr/0x7fffffff); ptr++;

printf (" names are ");

for (j = 0; j < names; j++) { /* for each name */

printf ("%d ", *ptr);

if (j == 0) /* set row and column */

ii = *ptr;

else if (j == 1)

jj = *ptr;

ptr++;

}

printf ("\n");

board[ii][jj] = (board[ii][jj] + 1) % 3;

}

}

void pickSquares(int button, int state, int x, int y)

{

GLuint selectBuf[512];

GLint hits;

GLint viewport[4];

if (button != GLUT_LEFT_BUTTON || state != GLUT_DOWN)

return;

glGetIntegerv (GL_VIEWPORT, viewport);

glSelectBuffer (512, selectBuf);

(void) glRenderMode (GL_SELECT);

glInitNames(); glPushName(0);

glMatrixMode (GL_PROJECTION);

glPushMatrix ();

glLoadIdentity ();

gluPickMatrix ((GLdouble) x, (GLdouble) (viewport[3] - y),

5.0, 5.0, viewport);

gluOrtho2D (0.0, 3.0, 0.0, 3.0);

drawSquares (GL_SELECT);

glMatrixMode (GL_PROJECTION);

glPopMatrix ();

glFlush ();

hits = glRenderMode (GL_RENDER);

processHits (hits, selectBuf);

glutPostRedisplay();

}


Example: picking spheres and cubes


Hints on Using Selection

  • 2D application

    • handle your own picking

  • 3D:

    • organize your program and data structure so that it is easy to draw list of object in either selection or rendering mode

    • Use name creatively


key difference: what information is sent back

transformed primitives is sent back to an array of floating point values (window coordinate)

tokens of primitive type and other data for that primitive

steps

glFeedbackBuffer

glRenderMode (GL_FEEDBACK)

draw primitives

glRenderMode (GL_RENDER)

parse the feedback array

[Feedback]


void drawGeometry (GLenum mode)

{

glBegin (GL_LINE_STRIP);

glNormal3f (0.0, 0.0, 1.0);

glVertex3f (30.0, 30.0, 0.0);

glVertex3f (50.0, 60.0, 0.0);

glVertex3f (70.0, 40.0, 0.0);

glEnd ();

if (mode == GL_FEEDBACK)

glPassThrough (1.0);

glBegin (GL_POINTS);

glVertex3f (-100.0, -100.0, -100.0);

/* will be clipped */

glEnd ();

if (mode == GL_FEEDBACK)

glPassThrough (2.0);

glBegin (GL_POINTS);

glNormal3f (0.0, 0.0, 1.0);

glVertex3f (50.0, 50.0, 0.0);

glEnd ();

}

/* Write contents of one vertex to stdout */

void print3DcolorVertex (GLint size,

GLint *count, Lfloat *buffer)

{

int i;

printf (" ");

for (i = 0; i < 7; i++) {

printf ("%4.2f ", buffer[size-(*count)]);

*count = *count - 1;

}

printf ("\n");

}

void display(void)

{

GLfloat feedBuffer[1024];

GLint size;

glMatrixMode (GL_PROJECTION);

glLoadIdentity ();

glOrtho (0.0, 100.0, 0, 100.0, 0.0, 1.0);

glClearColor (0.0, 0.0, 0.0, 0.0);

glClear(GL_COLOR_BUFFER_BIT);

drawGeometry (GL_RENDER);

glFeedbackBuffer (1024, GL_3D_COLOR, feedBuffer);

(void) glRenderMode (GL_FEEDBACK);

drawGeometry (GL_FEEDBACK);

size = glRenderMode (GL_RENDER);

printBuffer (size, feedBuffer);

}

/* Write contents of entire buffer. (Parse tokens!)*/

void printBuffer(GLint size, GLfloat *buffer)

{

GLint count;

GLfloat token;

count = size;

while (count) {

token = buffer[size-count]; count--;

if (token == GL_PASS_THROUGH_TOKEN) {

printf ("GL_PASS_THROUGH_TOKEN\n");

printf (" %4.2f\n", buffer[size-count]);

count--;

}

else if (token == GL_POINT_TOKEN) {

printf ("GL_POINT_TOKEN\n");

print3DcolorVertex (size, &count, buffer);

}

else if (token == GL_LINE_TOKEN) {

printf ("GL_LINE_TOKEN\n");

print3DcolorVertex (size, &count, buffer);

print3DcolorVertex (size, &count, buffer);

}

else if (token == GL_LINE_RESET_TOKEN) {

printf ("GL_LINE_RESET_TOKEN\n");

print3DcolorVertex (size, &count, buffer);

print3DcolorVertex (size, &count, buffer);

}

}

}


feedback + picking

more refined picking (not just pick object, but some point on the object)

feedback buffer type:

2D/3D/4D + COLOR + TEXTURE

glPassThrough

tokens that helps parsing the feedback data

feedback array syntax

(table 12-2)

an application:

rendereps.c

use feedback buffer info to write EPS

Feedback (cont)


End of Part 4


GLIntercept

  • An OpenGL debugging facility that intercepts OpenGL calls to create informative logs

  • Comes with some built-in logging scripts

  • Rename the one you choose to gliConfig.ini

  • The one particular useful for stencil buffer debugging is gliConfig_XMLFrame.ini

  • Ctrl-shift-key to create frame log

  • View the resulting XML in IE


Copy OpenGL32.dll and the chosen/renamed gliConfig.ini to the directory where EXE resides

Each ctrl-shift-F

creates a Frame log

Use IE to open the XML


gliConfig_XMLFrame.ini

By default, it only does color buffer logging

If you need to see depth and/or stencil buffers,

modify these two lines.


Example (Stencil Silhouette)

(pre,post,diff)

Red means no difference

Green means same pixels

Click to enlarge


Example (cont)

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