the framebuffer

The FramebufferThe Framebuffer

2004. 02. 17.Hyun-Chul Cho

HyunChul Cho - KUCG - 2

BufferBuffer

Goal of a graphics programDraw pictures on the screen

ScreenRectangular array of pixels

Pixel(2,1)

2.0 3.0

1.0

2.0


BuffersBuffers

BuffersStorage for all the pixels

FramebufferColor buffersDepth bufferStencil bufferAccumulation buffer


Color BuffersColor Buffers

Contain either color-index or RGB color and alpha values

Buffers in OpenGLLeft and right color buffer

For stereoscopic viewing

Front and back color bufferFor double-buffered system

Auxiliary color buffers


Other BuffersOther Buffers

Depth buffer (z-buffer)Store a depth value for each pixel

Distance to the eyeLarger value are overwritten by smaller value

Stencil bufferRestrict drawing to certain portions

Accumulation bufferStore RGBA color dataAccumulate the color data


Clearing BuffersClearing Buffers

void glClearColor (GLclampf r, GLclampf g, GLclampf b, GLclampf alpha);

void glClearIndex (GLfloat index);void glClearDepth (GLclampd depth);void glClearStencil (GLint s);void glClearAccum (GLclampf r, GLclampf g, GLclampf b,

GLclampf alpha);

void glClear (GLbitfield mask);

Hardware!

glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT | GL_ACCUM_BUFFER_BIT );


Selecting Color BuffersSelecting Color Buffers

select the buffers to be written

select the buffer as the source

void glDrawBuffer (GLenum mode);

GL_FRONT GL_FRONT_LEFT GL_AUXiGL_BACK GL_FONT_RIGHT GL_FRONT_AND_BACKGL_LEFT GL_BACK_LFET GL_NONEGL_RIGHT GL_BACK_RIGHT

void glReadBuffer (GLenum mode);

GL_FRONT GL_FRONT_LEFT GL_AUXiGL_BACK GL_FONT_RIGHTGL_LEFT GL_BACK_LFETGL_RIGHT GL_BACK_RIGHT


Masking BuffersMasking Buffers

Mask – bit is writtenRed~alpha – component is writtenFlag – buffer is enabled for writing

void glIndexMask (GLuint mask );

void glColorMask (GLboolean red, GLboolean green, GLboolean blue,

GLboolean alpha);

void glDepthMask (GLboolean flag);

void glStencilMask (GLuint mask);


Testing And OperationTesting And Operation

Scissor test

Alpha test

Stencil test

Depth test

Blending

Dithering

Logical operations


Scissor TestScissor Test

Define a rectangular portion of window

Default – match the size of the window

void glScissor (GLint x, GLint y, GLsizei width, GLsizei height );

glEnable( GL_SCISSOR_TEST );


Alpha TestAlpha Test

Compare the alpha value with refDefault

func - GL_ALWAYSref - 0

void glAlphaFunc (GLenum func, GLclampf ref );

glEnable( GL_ALPAH_TEST );

GL_NEVER GL_ALWAYS

GL_LESS GL_LEQUAL

GL_GREATER GL_GEQUAL

GL_NOTEQUAL GL_EQUAL


Stencil TestStencil Test

void glStencilFunc (GLenum func, GLint ref, GLuint mask );

glEnable( GL_STENCIL_TEST );

GL_NEVER GL_ALWAYS

GL_LESS GL_LEQUAL



void glStencilOp (GLenum fail, GLenum zfail, GLenum zpass );

GL_KEEP GL_ZERO

GL_REPLACE GL_INVERT

GL_INCR GL_DECR


Stencil TestStencil Test

glStencilFunc(GL_ALWAYS, 0x1, 0x1);glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);glBegin(GL_QUADS);

glVertex2f( -5.0, 0.0);glVertex2f( 0.0, 5.0);glVertex2f( 5.0, 0.0);glVertex2f( 0.0, -5.0);

glEnd();

glStencilFunc(GL_EQUAL, 0x1, 0x1);glStencilOp( GL_KEEP, GL_KEEP, GL_KEEP);

DrawMesh();


Depth TestDepth Test

Compare z-value with a value in the depth bufferDefault – GL_LESS

void glDepthFunc (GLenum func);

glEnable( GL_DEPTH_TEST );

GL_NEVER GL_ALWAYS

GL_LESS GL_LEQUAL




Blending And DitheringBlending And Dithering

Blending

DitheringImprove the color resolutionEx. in color index mode

the color index 4.460% of pixel painted with index 440% of pixel painted with index 5


Logical OperationsLogical Operations

void glLogicOp (GLenum opcode);

glEnable( GL_INDEX_LOGIC_OP ); or

glEnable( GL_COLOR_LOGIC_OP );

Parameter Operation Parameter Operation

GL_CLEAR 0 GL_AND s & d

GL_COPY s GL_OR s | d

GL_NOOP d GL_NAND ~( s & d )

GL_SET 1 GL_NOR ~( s | d )

GL_COPY_INVERTED ~s GL_XOR s ^ d

GL_INVERT ~d GL_EQUIV ~( s ^ d )

GL_AND_REVERSE s & ~d GL_AND_INVERTED ~s & d

GL_OR_REVERSE s | ~d GL_OR_INVERTED ~s | d


Accumulation BufferAccumulation Buffer

Higher precision (more bits per color)

Scene AntialiasingMotion BlurDepth of FieldMulti Shadows

void glAccum (GLenum op, GLfloat value );

GL_ACCUM GL_LOAD

GL_RETURN

GL_ADD GL_MULT

Now That You KnowNow That You Know

2004. 02. 17.Hyun-Chul Cho


Error HandlingError Handling

Strongly recommendedCall glGetError() at least once in each display() routine

GLenum glGetError (void);

const GLubyte* gluErrorString (GLenum errorCode);

GLenum errCode;

const GLubyte *errString;

if( (errCode=glGetError())!=GL_NO_ERROR ) {

errString = gluErrorString( errCode );

fprintf( stderr, “OpenGL Error: %s\n”, errString);

}


VersionVersion

const GLubyte* glGetString (GLenum name);

GL_VENDOR GL_RENDERER

GL_VERSION GL_EXTENSIONS

const GLubyte* gluGetString (GLenum name);

GLU_VERSION GLU_EXTENSIONS


Translucency & Fade EffectTranslucency & Fade Effect

If not blending hardware

TranslucencyUse stipple patterns for translucency

Fade EffectDefine a series of polygon stipple patterns


SelectionSelection

Redraw scene in the back buffer with object identifierSimply read back the pixel under the cursor


ShadowShadow

Directional Light Point

zyx

zyxzz

yyzxy

xxxzy

zzyyxx

cdbdad

ddbdadbdad

ddcdcdadad

ddcdbdcdbd

ddscdsbdsa

000

0)()()( DS

),,( zyx dddD

),,( zyx sssS


Hidden-Line RemovalHidden-Line Removal

Draw the outlinesFill the background color

Use polygon offsetUse Stencil Buffer


Depth-Buffered ImagesDepth-Buffered Images

For complex static backgroundDraw an image with depth buffer value

Draw image’s depth-buffer valueSet writemask to 0Draw Stencil buffer

Draw the imageMasked by the stencil buffer


Dirichlet Domains Dirichlet Domains (Voronoi Polygon(Voronoi Polygon))

Draw a cone with its apex

Precompute a cone’s depth in an imageUse the image as depth buffer value

the framebuffer

Documents

test gl

notequalg cho kucg

0x1glstencilop gl

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glenum zpass gl

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