Introduction to OpenGL (Part 3)

Introduction to OpenGL(Part 3)

Ref: OpenGL Programming Guide (The Red Book)

Fall 2009 revised 2

Topics

Part 1 Introduction Geometry Viewing Light & Material

Part 2 Display List Alpha Channel Polygon Offset

Part 3 Image Texture Mapping

Part 4 FrameBuffers Selection & Feedback

OpenGL

Pixels, Bitmaps, and Images

Fall 2009 revised 4

Bitmaps

A bitmap is a rectangular array of 0s and 1s that serves as a drawing mask for a rectangular portion of the windowSpecification:

Specified, in hex codes, in one dimensional array, row by row, starting from lower-left corner

Width need not be multiples of 8

Fall 2009 revised 5

BitmapsglBitmap (w, h, xbo, ybo, xbi, ybi, bitmapPtr) xbo, ybo: origin xbi, ybi: increment CRP after display

glBitmap(10, 12, 0, 0, 11, 0, bitmapPtr)

Fonts: series of

bitmaps as display list

Fall 2009 revised 6

Bitmap (cont)

Note: You can't rotate bitmap fonts because the bitmap is always drawn aligned to the x and y framebuffer axes.Use glColor* to set GL_CURRENT_RASTER_COLOR

Fall 2009 revised 7

Current Raster Position (CRP)

OpenGL maintains raster position, a 3D-position in world coordinateModified by:

glRasterPos() specify the object coordinates (as in glVertex); transformed by

MODELVIEW & PROJECTION, passed to the clipping stage. If the position is not culled, raster position is updated; otherwise, it is

not valid. glBitmap()

glGetFloatv(GL_CURRENT_RASTER_POSITION, fptr) to obtain the CRPglGetBooleanv (GL_CURRENT_RASTER_POSITION_VALID, &boolvar) to test validity of CRP

Fall 2009 revised 8

drawf.c

Fall 2009 revised 9

Bitmap Editor

This format is called xwindow bitmap, with xbm extensionBitmaps can be created by the GIMP (GNU Image Manipulation Program)Or, seek format converters

Fall 2009 revised 11

Another XBM Editor (Here)

A simple Tk Program; require Tcl/Tk installed

Get tcltk for Windows from ActiveTcl

Note that xbm and the opengl xbitmap format is slightly different (details)


Ex: XBM Edit & Display


APIs for Images (Pixel Rectangles)

glReadPixels() Reading pixel data

from framebuffer to processor memory.

glDrawPixels() Writing pixel data

from processor memory to framebuffer

glCopyPixels() Copying pixel data

within the framebuffer


Function Arguments

glReadPixels (x, y, w, h, F, T, ptr) x,y: window coordinate F: pixel format T: is data type ptr: pointer to image memory

glDrawPixels (w, h, F, T, ptr) Draw to current raster position

glCopyPixels (x, y, w, h, buffer) Buffer: GL_COLOR | GL_DEPTH | GL_STENCIL Equivalent to: Read then Draw


More on glCopyPixels

Note that there's no need for a format or data parameter for glCopyPixels(), since the data is never copied into processor memory. The read source buffer and the destination buffer of glCopyPixels() are specified by glReadBuffer() and glDrawBuffer() respectively Default:

single-buffer: GL_FRONT Double-buffered: GL_BACK


Pixel Format

GREY SCALE

GREY SCALE with alpha


Data Type


Example

255: 0xFF

See also image.c for CopyPixels


Example image.c

Copy the lower left corner to the CRP (where the mouse is)

For single-buffer version, only GL_FRONT is involvedWhile motion is in action, display is not calledDouble-buffer version: [from the API doc]

glutSwapBuffers promotes the contents of the back buffer to become the contents of the front buffer. The contents of the back buffer then become undefined. Reality is… two have same content


PixelZoom (xfactor,yfactor)

Enlarge/shrink imagesUse negative factors for reflected images


Image Loader (TGA)simple TGA utility in gluit.raronly load and save uncompressed images in greyscale, RGB or RGBA mode.Info in TGA header: image type [unsigned char]

1 - colour map image 2 - RGB(A) uncompressed 3 - greyscale uncompressed 9 - greyscale RLE (compressed) 10 - RGB(A) RLE (compressed)

pixel depth [unsigned char] 8 – greyscale | 24 – RGB | 32 - RGBA


Image Loader (PNG)Offline; Local copy at webhd2:game-lib


DevIL


int pngLoadRaw (filename,rawinfo)

Must be freed manually


Remark

Most graphics formats have image origin at upper/left cornerWhile OpenGL has the image origin at lower/left cornerHence, if no correction is done, every image is drawn upside down.Correction in PNG loader: pngSetStandardOrientation (1);


Imaging Pipeline


Imaging Pipeline(cont.)

glPixelStore() Controlling Pixel-Storage Modes glPixelStorei(GL_UNPACK_ALIGNMENT

, 1); Packing and unpacking refer to the way

that pixel data is written to and read from processor memory.

tells OpenGL not to skip bytes at the end of a row


PixelStorei(GL_UNPACK_ALIGNMENT, x)

Specifies the alignment requirements for the start of each pixel row in memory. The allowable values are: 1 (byte-alignment), 2 (rows aligned to even-numbered bytes), 4 (word-alignment [default]), and 8 (rows start on double-word boundaries).

Byte:8-bitWord:16-bitDouble-word:32-bit

Byte:8-bitWord:16-bitDouble-word:32-bit


Details

Byte aligned (1)

In client memory, the start of each row of pixels is …

w=3,h=2

Assuming the RGB image is of size 3x2:(three bytes per pixel)

1st row 2nd row

Double word aligned (8)

Aligned to even bytes (2); Word aligned (4)


Settings

For RGBA images, it doesn’t matter (each pixel has 4 bytes: RGBARGBA…)

For RGB and luminance images and images with odd width, it should be set to byte-aligned (data are densely packed)

glPixelStorei(GL_UNPACK_ALIGNMENT, 1)

glPixelStorei(GL_UNPACK_ALIGNMENT, 4)


Improving Pixel Pipeline Performance

A series of fragment operations is applied to pixels as they are drawn into the framebuffer. For optimum performance, disable all fragment operations (depth test, …)While performing pixel operations, disable other costly states, such as texturing and lightingIt is usually faster to draw a large pixel rectangle than to draw several small ones, since the cost of transferring the pixel data can be amortized (分攤 ) over many pixels


Example (depthbuffershow)

Get depth buffer content glReadPixels()

Reverse and scale! Near (white) far (black)

Display it as a luminance image glDrawPixels()


Example (rasterpos3)

Illustrate that CRP is a point in R3Image displayed is always parallel to projection planeFake perspective by zooming with distance to camera


Example (sprite)

Process sprite image Add alpha layer

Load png imageSprite animation“feel of depth”

Back-to-front rendering

pixelzoom

OpenGL

Texture Mapping (Introduction)


Steps in Texture Mapping

Specify (create) the texture Set texture parameters: Indicate how the texture is to be applied to

each pixel

Enable texture mappingDraw the scene, supplying both texture and geometric coordinates

Works only in RGB mode


Specify the TextureglTexImage1D()glTexImage2D()glTexImage3D()


Two Dimensional Texture

glTexImage2D (GLenum target, GLint level, GLint components, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, const GLvoid *pixels) target ： GL_TEXTURE_2D level ： LOD number (0: base image) components ： number of color components

(1|2|3|4) Format ： pixel data format Border: 0 or 1 Width & height

2m + 2(border bit) w and h can be different

There are new extensions that removes this restriction


glTexImage*D supplement

In GL version 1.1 or greater, pixels may be a null pointer. In this case texture memory is allocated to accommodate a texture of width width and height height. You can then download subtextures to initialize this texture memory. The image is undefined if the user tries to apply an uninitialized portion of the texture image to a primitive.


Setting Texture Environment

glTexEnv{if}{v}(GLenum target, GLenum pname, TYPEparam); Setting how textures are to be interpreted: Target ： GL_TEXTURE_ENV Pname: GL_TEXTURE_ENV_MODE Param: modes (DECAL|REPLACE|

MODULATE|BLEND)


Texture Environment Modes

GL_REPLACEGL_MODULATE (default)GL_DECALGL_BLEND

New environment modes:GL_ADD: Cv = Cf + Ct

GL_COMBINE (ARB, see here)


GL_MODULATE

Color of polygon affects the display of texture

Tree: (r,g,b,a) acutout = 0Polygon: (1,0,0)


GL_BLEND

Use with:glTexEnvfv (GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, colorPtr)

Cc: texture environment color


GL_REPLACE

Appearance solely determined by texture

FOR TEXTURE CUT-OUTS



GL_DECAL

Cp: replace

RGB: DECAL=REPLACE



Texture + Lighting

To show fragment color, use GL_MODULATEApply specular color AFTER texture mapping:

glLightModeli (GL_LIGHT_MODEL_COLOR_CONTROL, GL_SEPARATE_SPECULAR_COLOR);

See FAQ 21.040

GL_SINGLE_COLOR

GL_SEPARATE_SPECULAR_COLOR


Texture Coordinates

Texture coordinate: Associate texture location (in texture

space) with vertices in the polygonglTexCoord2i (s, t); glVertex2i (x, y);

Order cannot be reversed![Think of TexCoord as state

assignment]


Texture Coordinates of Quadrics

Most quadric primitives have default setting for texture coordinatesTo turn on the default setting: gluQuadricTexture

(qobj, GL_TRUE)

Fall 2009 revised 51rasterization

Polygon (in screen space) and texture

coordinates

(1,1)

(1,0)

(0,.75) (1,1)

(1,0)

(0,.75)

Texture map (4x4)

nearestlinear

Interpolate (s,t) and lookup (r,g,b,a) in the texture map

Filters

Texture Access and Lookup


Magnification & Minification

Nature of problem: Mismatch between texels and pixels

1

Pixels

TexelsPixels

Magnification

Minification


Options

MagnificationGL_NEARESTGL_LINEAR

MinificationGL_NEAREST, GL_LINEAR, GL_NEAREST_MIPMAP_NEARESTGL_LINEAR_MIPMAP_NEARESTGL_NEAREST_MIPMAP_LINEARGL_LINEAR_MIPMAP_LINEAR

Chooses the mipmap that most closely matches the size of the pixel being textured

Chooses the two mipmaps that most closely match the size of the pixel being textured

Nearest: in Manhattan distance to the center of the pixel


Magnification OptionsGL_LINEAR Interpolate the

texels More time

consuming but more accurate

GL_NEAREST Snap to the

nearest texel


Problem with Minification

without mipmap with mipmap


MipmappingThe most popular method of anti-aliasing for textures‘Mip’ stands for “multum in parvo” = “many things in a

small place”The texture is downsampled to a quarter of the original

area.

2m2n

m, n∈+

2m-12n-1

d = the Level Of Detail (LOD)

Level 0

Level 1

Level 2

Level 3

Level 0 Texture

If a pixel covers2222 texels,go to level 2

and get a texel.2121,level 1

Mipmapping was invented by Lance Williams in 1983 and is described in his paper Pyramidal parametrics.


Minification OptionsglTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR, GL_NEAREST, GL_LINEAR_MIPMAP_LINEAR,

GL_LINEAR_MIPMAP_NEAREST, GL_NEAREST_MIPMAP_LINEAR, GL_NEAREST_MIPMAP_NEAREST);

Remarks: Mipmap selection is done per pixel

Using not-yet-ready mipmap disables the texture mapping function.

Remarks: Mipmap selection is done per pixel

Using not-yet-ready mipmap disables the texture mapping function.


Mipmap Generation

gluBuild2DMipmaps

Storage overhead

3

4

1

11

414

141

2


Texture Wrap

When the texture coordinates are outside [0,1]glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,param); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,param); param: GL_CLAMP|GL_REPEAT


Parameters for Wrapping

GL_CLAMP use the border

texel Diagonal: corner

texel

GL_REPEAT repeat the texels

in [0,1]


Texture Object

A texture object stores texture data and makes it readily availableYou can now control many textures and go back to textures that have been previously loaded into your texture resourcesUsing texture objects is usually the fastest way to apply textures, resulting in big performance gains

it is almost always much faster to bind (reuse) an existing texture object than it is to reload a texture image using glTexImage*D()


Texture Object API

glGenTextures Allocate n textures (integer identifiers)

glBindTexture Bind the current texture to specified identifier

glDeleteTextures Free the allocated texture identifiers Reverse of glGenTextures


checker.c


texbind.c

Once created, a named texture may be re-bound to the target of the matching dimensionality as often as needed. It is usually much faster to use glBindTexture to bind an existing named texture to one of the texture targets than it is to reload the texture image using glTexImage1D, glTexImage2D, or glTexImage3D.


Texture Objects

Texture properties saved in texture objects: Minification and magnification filters, wrapping

modes, border color and texture priority

When a texture object is bound again, one may edit the contents of the bound texture object. Any commands that change the above properties will change the currently bound texture as well as the current texture state.Texture environment, texgen, etc. are NOT stored in texture objects


Texture Loader (PNG, glpng.rar)

Features Build mipmap Load and bind 2D textures Load image data (image loader) Handle transparent image (cut-out texture)

Important API id = pngBind(filename, mipmap, trans, info,

wrapst, minfilter, magfilter) pngSetStencil(red, green, blue) pngSetStandardOrientation(1) pngLoad(filename, mipmap, trans, info)

TEXTURE ID IS RETURNED TO YOU BY PNG

LOADER


Texture Transforms

Texture coordinates are multiplied by a 4 by 4 matrix before any texture mapping occurs.Texture animation: using this, you can make the texture slide over the surface, rotate around it, stretch and shrink, …All matrix operations apply: Push/Pop/Mult/…


Texture Suite

cloud

explodesmoke

waterfire


Automatic TEXture Coordinate GENeration

void glTexGen{ifd}{v}(GLenum coord, GLenum pname, TYPEparam);

Coord: GL_S, GL_T, GL_R, GL_Q Pname: GL_TEXTURE_GEN_MODE Type: GL_OBJECT_LINEAR, GL_EYE_LINEAR, or

GL_SPHERE_MAP

Used when the tex coords are too cumbersome to specify, or will be changed with time.Texture environment, texgen, etc. are NOT stored in texture objects.


1D Texture According to Height

Continuous color gradation(0,1/32,2/32, …, 31/32)

Discrete white lines(0,0,0,1,0,0,0,1,…)


TexGen: Object/Eye_Linearfor a vertex with object coordinates (xo,yo,zo,wo), generated coordinate = p1xo + p2yo + p3zo + p4wo

Meaning: when p1, p2, p3 are normalized, this coord corresponds to signed distanceSimilarly, EYE_LINEAR applies to eye coordinates

(0,0,1,0)(xo,yo,zo,wo)

Initially, all texture generation functions are set to GL_EYE_LINEAR and are disabled. Both s plane equations are (1, 0, 0, 0), both t plane equations are (0, 1, 0, 0), and all r and q plane equations are (0, 0, 0, 0).


Eye_linear (ref)

Computing this distance in eye coordinates is a little tricky, since the plane and vertex must first be transformed to eye coordinates. if M is the modelview matrix at the time glTexGenfv( GLX, GL EYE PLANE, plane ) is called, then the transformed vertex V’ is V’ = MV, let’s call V’ = (xe, ye, ze, we). The plane must also be transformed to get a new plane A’x + B’y + C’z + D’w = 0 We get (A’,B’,C’,D’) = (A,B,C,D)M−1 and M is the modelview matrix when glTexGen is invoked. The texture coordinate is then computed as A’xe + B’ye + C’ze + D’we.


Eye_linear Summary

e

e

e

e

o

o

o

o

e

e

e

e

w

z

y

x

dcbatex

Mdcbadcba

M

w

z

y

x

M

w

z

y

x

coord

matrix modelview:,

1


Multi-Texture

What this is about … Extension Wrangler (GLEW)


Usages of Multitexture


can cascade up to 32 TE’sNote: default blending mode is GL_MODULATE

ARB_multitexture (ref)

(First) ARB-approved extension (1998)Support up to 32 texture units*Texture matrix and texgen affects the active texture environment

* glGetIntegerv (GL_MAX_TEXTURE_UNITS, &units);

My platform has only 4! OpenGL 3.1 requires minimum of 16 texture units


Details

OpenGL's multitexture support requires that every texture unit be fully functional and maintain state that is independent of any other texture units. Each texture unit has its own texture coordinate generation state, texture matrix state, texture enable state, and texture environment state. However, each texture unit within an OpenGL context shares the same set of texture objects.


Multi Texture


TexGen with MultitextureWhen the GL_ARB_multitexture extension is supported, glTexGen set the texture generation parameters for the currently active texture unit, selected with glActiveTexture.

TexUnit0:texture2D;(s,t) given

TexUnit1:texture1D; texGen

End of Part 3


Results from Xbm Editor

From top to bottomByte swapping

#define sample_width 16#define sample_height 10static char sample_bits[] = {0xff,0x00,0x00,0xff,0x00,0x00,0x07,0xe0,0x00,0x00,0xf0,0xf8,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00};


Converting to OpenGL Bitmap

Bottom to top & byte swapping#define sample_width 16#define sample_height 10static char sample_bits[] = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x0f,0x1f,0x00,0x00,0xe0,0x07,0x00,0x00,0x00,0xff,0xff,0x00};


Solution

Byte swap: resolved by glPixelStorei (GL_UNPACK_LSB_FIRST, 1);

Bottom to top: Recall width of bitmap need not be

multiples of 8, but it is stored in units of unsigned characters

But, the XBM Editor only allows width of multiples of 8 (8,16,24,…)

See the example code on reading it reversely


(almost) Useless API!

There seems to no way to make the bitmap larger (than it should be, pixels)

glPixelZoom won’t work!

Therefore, its use is limitedNevertheless, the color is free to change…

BACK


Texture Mapping

A way to render realistic picture w/o using too many polygons

Parallax mapping


Texture Coordinate System

Texture: a 2D array of color values, each unit is called texel (texture element)Every texture map has (s,t) coordinates [0,0] to [1,1]Each vertex in a polygon specifies its texture coordinates (s,t), then map to the given image to determine the corresponding texture


Example

Screen space view

OpenGL code

Texture Map Texture space view


Example

Screen space view

OpenGL code

Texture space viewTexture Map

BACK





BACK


OpenGL Extension (Wikipedia)

The OpenGL standard allows individual vendors to provide additional functionality through extensions as new technology is created. Extensions may introduce new functions and new constants, and may relax or remove restrictions on existing OpenGL functions. NV: Each vendor has an alphabetic abbreviation that is used in naming their new functions and constants. For example, NVIDIA's abbreviation (NV) is used in defining their proprietary function glCombinerParameterfvNV() and their constant GL_NORMAL_MAP_NV. EXT: It may happen that more than one vendor agrees to implement the same extended functionality. In that case, the abbreviation EXT is used. ARB: It may further happen that the Architecture Review Board "blesses" the extension. It then becomes known as a standard extension, and the abbreviation ARB is used.


Introduction

The second ARB extension was GL_ARB_multitexture, introduced in version 1.2.1. Following the official extension promotion path, multitexturing is no longer an optionally implemented ARB extension, but has been a part of the OpenGL core API since version 1.3.Before using an extension a program must first determine its availability, and then obtain pointers to any new functions the extension defines. The mechanism for doing this is platform-specific and libraries such as GLEW and GLEE exist to simplify the process.Specifications for nearly all extensions can be found at the official extension registry


Introduction

What extension wrangler does: Your application may find some extensions already available

through Microsoft's opengl32.lib. However, depending on your OpenGL device and device driver, a particular vendor-specific extension may or may not be present at link time.

If it's not present in opengl32.lib, you'll need to obtain the address of the extension's entry points at run time from the device's ICD.

After you obtain the entry point addresses of the extension functions you wish to use, simply call through them as normal function pointers:

FAQ: How to know which version of OpenGL you are running? What’s the difference among versions? How to know what extensions are supported?

Ref: All About OpenGL Extensions


Utilities

GLEW: OpenGL extension wrangler

Glview: OpenGL extension viewer

glewinfo.txt


Commonly Used Extensions

GL_ARB_multitextureGL_ARB_texture_cube_mapGL_ARB_texture_env_dot3GL_ARB_shadowGL_ARB_vertex_program, GL_ARB_fragment_programGL_ARB_occlusion _queryGL_ARB_texture_non_power_of_twoGL_EXT_texture3DGL_EXT_framebuffer_object

ARB – Extensions officially approved by the OpenGL Architecture Review Board

EXT – Extensions agreed upon by multiple OpenGL vendors

NV – NVIDIA Corporation

SGI – Silicon Graphics

SGIX – Silicon Graphics (experimental)

BACK

Documents

Introduction to OpenGL (Part 3)