usc the same table or different tables I n this genera­ tion of graphics chips, the kature could not be j usti ­

fied in the

TGA2

chip. The Dagger chip, which was

EXPANSION OF 16-BIT 5/6/5 RGB PIXELS TO 8/8/8 RGB

EXPANSION OF 16-BIT 5/5/5 RGB PIXELS TO 8/8/8 RGB

EXPANSION OF 8-BIT 3/3/2 RGB PIXELS TO 8/8/8 RGB

Figure 3

Expanding RGB

Pixels

to

8/8/8

RG B Fonmt

80

intended tc>r lower-cost systems, incl udes neither chrominancc nor luminance adjust LUTs .

T h e other usc for the adjust

LUT

in the TGA2 chip

is for color index conversio n . This operation can be performed when the input pixel tc·m11at is 8 bits wide. In this case, the 8 - bit i np u t pixel is used as an index i nto the t:�blc. The resu lting \'a luc is used as the

Ychanncl value in the rest of the pipeline, and the U

and V channels are ignored . Later in the pipel ine, the

color conversion stage is skipped , and the r�channcl value is used directly as the resulting 8-bi r pixel va lue. Color index conversion is an operation that is particularl y desirable when using the Wind ows

NT

operating system . Typically, 8- bir screen pixels are converted to d isplaved col ors by lllt<lllS of a color LUT in the back-end video logic. Under the X ·wi ndow System graphical windowing environment , the map­ ping between an index and its color can be changed only by the application. Under the Windows NT oper­ ating system, however, the mappi ngs may cha nge dynamically. Therefore, an application that has stored an i mage as 8-bit index ,·alues will ntcd

to

remap those index values

bd(xe

copving it to the screen . This con­ version can be done in software, hut it is flster and simpler ro usc the adjust LUT in the TGA2 chip to per­ form the rcnupping.

Scaling and Filtering

I n the next stage in the rendering pipeline, the chip pertorms scaling and tlltering. The Dagger and TGA2 chips support one-d imensional ( 1 - D) scaling and filter­ ing in hardware . Limiting the chips to 1 -D filteri ng sig­ nificantly simplifies the chip logic, since no line bufkrs are needtd. Somewhat higher-quality images can be achieved using two-dimensional

( 2 - D )

fi ltering, but the difference is not significant. This d i fterence is fu r­ ther reduced by the AccuVid eo dithering algorithm that is implemented by the Dagger and TGA2 chips. Two-dimensiona l smoothing tilters can be su pporttd with added software processing, if required.

Bresenham-style Scaling Image scaling

in

the Dagger and

TGA2

chips uses pixel replication but is not lim­ ited to integer mu ltiples. Instead, images can be scaled hom anv i ntegral source width to

any

integral desti­ nation width. Scal ing is implemented through an adaptation of the Bresenham line-drawing algorithm . A compl ete description o f this Bresen ham-sryle sca ling algorithm appears in "Bresenham -sryle Scaling"; the tol lowing paragraphs provide an outline of the algo­ rithm, which is the same scaling algorithm used in the J 300 fam i ly of adapters 3'

The B rescnham scaling algorithm works l i ke the B resenham line-drawing algorithm. Suppose we are drawing a line ti-om ( 0, 0) to (

10,

6

)

_{, so that dx} = 1 0

and

c�v

= 6 . This is a n X- major line; that is, the line is

longer i n the X di mension than in the V dimension.

Digit.1l Tcchniol )ou rn<li Vol. 7 No. 4 1 995

The Bresenbam algorithm draws this \'ector b\' in itial ­

izing an error term and then i ncrementing it c/.\ rimes,

in this example, 10 rimes. Each rime the algorithm i ncrements the term, a pixel is d rawn . The sign of t he error term determines whether to tind the next

pixel position bv stepping to the right ( incrementing

the X positio n ) or b\' steppi ng d iagonall\' ( increment­

ing both

X

and }"). The error term is incremented in

such a way

t

hat

as

the X position is incremented 10

rimes, the Y position is incremented 6 _{rimes, rhus}

d rawing the desired vector.

For Bresenham scali ng, dx rcpresems the width of the source image, and

c/1•

represents the \\'idth of the destination image on the screen . vVhen reducing the size of the sou rce image, dx is greater th:m d)' and the error terms and increments are set u p in rhe same way as the X-major Brescnham line drawing, :ts described in the previous paragraph. One source pixel

is

processed each rime the error term is incrcmcmed . When Bresenham 's line algorithm i ndicates a step in the X dimension on I)·, the source pixel is skipped . vVhen the algorithm i ndicates :1 step in both the .\· and the V dimensions, the source pixel is written to the destina­ tion . As a result,

cx:tcrly dx so urce

pixels arc processed , and exactly d1• ofrhem JIT drawn to the screen .

Enlarging an im:�gc works in a similar bshion . For cx:1mplc, consider �1 source i mage rbar is nJITO\\'er than

the destination image, that is, dx is less th;m d ) ' This is

cqui,·alent to d rawing a )

:major Brcscnh:tm line in which the error term is incremented d ) ' tin1es :�nd rhe X dimension is incrcmcmcd

dx

ti mes. The scaling algo­ rith m draws a sou rce pixel to the desti n;Hion at each step. If the l ine-drawing algorithm i ncrements only in rhc Ydimension, it repeats the cu rrent pixel. If the line­

dra\\'ing algorithm increments

in

both t he X : llld the Y dimensions, it steps to and d isplavs rhc next source pixe l. Consequent!\', the dx source pixe ls arc

rcplicJted

to yield c/J! destination pixels, thus enlarging the image.

The Bresenham line-drawing algorithm has t\\'O

nice properties that arc shared by the Brescnham scal­ ing algorithm. First, it requ i res no divisions to com­

pur e the error increments. Second,

it

produces lines

that are as smooth as possible, gi,·en the pixel grid .

ThJt is, tor an X-major line, each of the ch pixels has

a l' posi tion that is the

closest

pixel to the i ntersection

of i rs

X position wirh the real

\'ector.

Simi larlv, the B rescnham scaling Jlgorithm selects pixels ti1Jt h:l\'e the most even spacing possible, given the pixel gri d . Just a s l ines c a n h e d rawn ti-om lett to right o r fi·om

right to left, images can

be

dra\\'11 in either direction .

An image dram1 in one d irection is tht mi rror image of the image drJ\\'n in rhc other direction . J'vlirror i magi ng is sometimes used in telecon krcncing, so that

users can look at themselves rhe way

they

norm:dl\' see

themselves. Similarly, images can be ru rncd upside

down by simply drawing to the d isplay fi·om

bottom

Scaling in the

V dimension is pertcmned simi larly to

X-d imension sc:1.li ng. On the TGA2 chip, scaling

is performed i n software instead of in hardware: the

software increments an error term to decide whether

to skip lines ( tor reducing) or repeat lines ( for enlarg­

ing). This is accepc1.blc because the CPU has plenty of

spare cycles to pertc>rm the scaling compu tations while

the al gorithm draws the preceding line. The Dagger

chip su pports Y-d imension scaling in hardware to

red uce the number of commands that are needed

to scale an im;1ge .

Smooth ing and Sharpening Fi lters

Like the J300, the

Dagger and TGA2 chips provide both smoothing and

s harpening tilrers. Table

1

shows the available filters.

All are three-tap fi lters that are inexpensive to imple­

ment in hardware . The smoothing tilters are used to

improve the quality of scaled i m:�ges. The sharpening

ti lters provide edge e n hancement. The two filters

marked with asterisks ( * ) arc ava i l a ble only on the

TGA2 chip. The others are avaibblc on both the

In document dtj v07 04 1995 pdf (Page 81-83)