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Inside Macintosh: Imaging With QuickDraw /
Appendix B - Using Picture Comments for Printing

Using Line-Drawing Picture Comments

Line-drawing picture comments, listed in Table B-1 on page B-8, provide your application with the ability to draw dashed lines (as described in the next section) and to display fractional line widths (as described in "Using Fractional Line Widths" on page B-36).

Drawing Dashed Lines

Your application may use dashed lines frequently, particularly if it is a spreadsheet or accounting application. You can use the DashedLine picture comment to draw dashed lines on capable printers without drawing each individual dash. You use the DashedStop picture comment to tell the printer driver when you are finished sending dashed line information.

When you use the DashedLine comment, the printer driver draws the indicated lines or rectangles. You should pass a handle to a TDashedLineRec record in the dataHandle parameter of the PicComment procedure. You use a TDashedLineRec record to specify how the dashed line should look. Here is how you should declare these as Pascal data structures:

   TDashedLineHdl = ^TDashedLinePtr;
   TDashedLinePtr = ^TDashedLineRec;
   TDashedLineRec =
      offset:     SignedByte; {offset}
      centered:   SignedByte; {reserved; set to 0}
      intervals:  ARRAY[0..0] OF SignedByte;
                              {points for drawing and not }
                              { drawing dashes}
Use the offset field to specify an offset as with the PostScript setdash operator.

The centered field is reserved and should be set to 0. Your application must center the dashed lines.

In the intervals field, specify an array of dash intervals describing the number of points drawn for a dash and the number of points not drawn between them.

You must provide both a QuickDraw and a picture comment version of the dashed line. The code in Listing B-9 uses the PostScriptBegin and PostScriptEnd picture comments to hide QuickDraw code from PostScript, and it uses pattern mode 23 to render PostScript drawing invisible in QuickDraw.

Listing B-9 Using the DashedLine picture comment

   magicPen = 23; 
   cx = 280;      {center along x-axis}
   cy = 280;      {center along y-axis}
   r0 = 200;      {radius}
   dashHdl:    TDashedLineHdl;
   i:          Integer;
   a, rad:     Extended;
   {First the PostScript picture comment version. Pattern mode } 
   { 23 makes the line drawing invisible to QuickDraw.}
   dashHdl := TDashedLineHdl(NewHandle(SizeOf(TDashedLineRec)));
   IF dashHdl <> NIL THEN
   WITH dashHdl^^ DO
      offset := 4;       {just for fun}
      centered := 0;     {currently ignored--set to 0}
      intervals[0] := 2; {number of interval specs}
      intervals[1] := 4; {this means 4 points on ...}
      intervals[2] := 6; {... and 6 points off}
      PicComment(DashedLine, SizeOf(TDashedLineRec), 
   rad := 3.14159 / 180;   {conversion degrees -> radians}
   FOR i := 0 TO 9 DO
   BEGIN    {draw some dashed lines}
      a := i * 20 * rad;
      MoveTo(cx, cy);
      Line(round(r0 * cos(a)), - round(r0 * sin(a)));
   PicComment(DashedStop, 0, NIL);  {that's enough!}
   PenMode(srcOr);  {no magic any more}
   {Now, the QuickDraw version. The PostScript driver must }
   { ignore it, so enclose it between PostScriptBegin and }
   { PostScriptEnd comments.}
   PicComment(PostScriptBegin, 0, NIL);
   FOR i := 0 TO 9 DO
      MyDashedQDLine(round(r0 * cos(i * 20 * rad)),
                     - round(r0 * sin(i * 20 * rad)), dashHdl);
   PicComment(PostScriptEnd, 0, NIL);

Using Fractional Line Widths

Your application may need lines as thin as possible or thinner than the screen can display, especially if it is a desktop publishing, spreadsheet, or design application. You can draw hairlines (lines that are less than 1/72 of an inch wide) with printer drivers that support the SetLineWidth picture comment. Your application passes the printer driver a scaling factor (such as 1/4) that the driver applies to the pen size when rendering the picture.

QuickDraw and the PostScript language define 1 point to be 1/72 of an inch, so there are exactly 72 points per inch on the Macintosh screen. The resolution of a PostScript device such as the 300-dpi LaserWriter printer is about four times that of the screen, so the driver can render lines that are approximately 1/4 of a point thick, which is about 1/288 of an inch.

When you specify the SetLineWidth picture comment in the kind parameter of the PicComment procedure, you also specify a TLineWidthHdl handle (a handle to a data structure of type TLineWidth) in the dataHandle parameter. The TLineWidth data structure is defined by the Point data type. Here is how you should declare these as Pascal data types in your application:

TLineWidthHdl  = ^TLineWidthPtr;
TLineWidthPtr  = ^TLineWidth;
TLineWidth     = Point; {v = numerator, h = denominator}
Use the vertical coordinate of the point as the numerator and the horizontal coordinate as the denominator of the scaling factor: the driver multiplies the horizontal and vertical components of the pen by the scaling factor to obtain the new pen width. For example, if you have a pen size of (1,2) and your SetLineWidth picture comment uses 2 for the horizontal and 7 for the vertical, the pen size will then be (7/2) 1 pixel wide and
(7/2) 2 pixels high.

In Figure B-9, the original pen size is 1 point. The first scaling factor is 5.0 or (5,1), which gives the pen a width of 5 points. The second scaling factor, applied to the new pen width, is 0.2 or (1,5), which gives the pen a width of 1 point again.

Figure B-9 Changing the pen width using the SetLineWidth picture comment

The SetLineWidth picture comment is implemented by all PostScript LaserWriter printer drivers and by some QuickDraw printer drivers. However, not all QuickDraw printer drivers support SetLineWidth, and there is no backup solution for cases where it is not supported. Among QuickDraw printer drivers that do support SetLineWidth, some drivers emulate PostScript printer drivers, while others--such as the QuickDraw LaserWriter SC driver--implement SetLineWidth differently.

The difference between the implementations of the SetLineWidth comment by the PostScript LaserWriter driver and the QuickDraw LaserWriter SC driver is apparent as soon as SetLineWidth is used a second time. The PostScript driver keeps an internal line-scaling factor, which is initialized to 1.0 when a job is started. Each number passed through SetLineWidth is multiplied by the current internal scaling factor to get the effective scaling factor for the pen size. The LaserWriter SC driver, on the other hand, replaces its current scaling factor for the pen size by the new value passed through SetLineWidth.

To support both implementations, you must always use an additional SetLineWidth picture comment to reset the PostScript driver line width to 1.0 before scaling to a new value width, as illustrated by the following lines of code:

PicComment(SetLineWidth, SizeOf(TLineWidth), Handle(1/oldLineWidth));
PicComment(SetLineWidth, SizeOf(TLineWidth), Handle(newLineWidth));
For example, suppose your application set the line width to 0.25, and now it needs a line width of 0.5. The following two SetLineWidth comments have the desired effect on all PostScript and QuickDraw drivers that implement the SetLineWidth comment.
Current line width,
PS driver
Current line width,
QD driver
Value passed along with SetLineWidthNew line width,
PS driver
New line width,
QD driver

The sample code in Listing B-10 gives the expected results on PostScript LaserWriter and QuickDraw printer drivers that implement the SetLineWidth comment.

Listing B-10 Using the SetLineWidth picture comment

PROCEDURE MySetNewLineWidth(oldWidth,newWidth: TLineWidth);
   tempWidthH: TLineWidthHdl;
   tempWidthH := TLineWidthHdl(NewHandle(SizeOf(TLineWidth)));
   tempWidthH^^.v := oldWidth.h;
   tempWidthH^^.h := oldWidth.v;
   PicComment(SetLineWidth, SizeOf(TLineWidth), Handle(tempWidthH));
   tempWidthH^^ := newWidth;
   PicComment(SetLineWidth, SizeOf(TLineWidth), Handle(tempWidthH));
PROCEDURE MyLineWidthDemo;
   y0 = 50;    {top left of demo}
   x0 = 50;
   d0 = 440;   {length of horizontal lines}
   e0 = 5;     {distance between lines}
   kN = 5;     {number of lines}
   oldWidth,newWidth:   TLineWidth;
   i,j,y:               Integer;
   y := y0;
   SetPt(oldWidth,1,1);             {initial line width = 1.0}
   FOR i := 1 TO 5 DO
      {want to set it to i/4 = 0.25, 0.50, 0.75 ...}
      MoveTo(x0, y);
      Line(d0, 0);
      y := y + e0;
      oldWidth := newWidth;

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© Apple Computer, Inc.
7 JUL 1996