Introduction to OpenGL(Part 3)

Ref: OpenGL Programming Guide (The Red Book)

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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

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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

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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

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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

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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

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drawf.c

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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

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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)

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Ex: XBM Edit & Display

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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

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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

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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

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Pixel Format

GREY SCALE

GREY SCALE with alpha

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Data Type

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Example

255: 0xFF

See also image.c for CopyPixels

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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

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PixelZoom (xfactor,yfactor)

Enlarge/shrink imagesUse negative factors for reflected images

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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

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

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DevIL

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int pngLoadRaw (filename,rawinfo)

Must be freed manually

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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);

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Imaging Pipeline

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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

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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

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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)

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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)

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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

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Example (depthbuffershow)

Get depth buffer content glReadPixels()

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

Display it as a luminance image glDrawPixels()

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Example (rasterpos3)

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

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Example (sprite)

Process sprite image Add alpha layer

Load png imageSprite animation“feel of depth”

Back-to-front rendering

pixelzoom

Sprites on Google …

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(ref)(ref)

OpenGL

Texture Mapping (Introduction)

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Texture Mapping

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

Parallax mapping

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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

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Example

Screen space view

OpenGL code

Texture Map Texture space view

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Example

Screen space view

OpenGL code

Texture space viewTexture Map

BACK

End of Part 3

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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};

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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};

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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

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(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…

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