CD_CGM - Computer Graphics Metafile Driver (cdcgm.h)

This driver allows generating a Computer Graphics Metafile version 1, which is an ANSI standard for the persistent storage of graphics primitives. The file usually has an extension .CGM.

Use

The file file is created by means of a call to the function cdCreateCanvas(CD_CGM, Data), which opens the file and writes its header. Then, other functions in the CD library can be called as usual. The Data parameter string has the following format:

"filename [width_mmxheight_mm] [resolution] [-t] [-pprec] -d[description]" or in C style "%s %gx%g %g %s"

Only the parameter filename is required. The filename must be inside double quotes (") if it has spaces. width_mm and height_mm are provided in millimeters (note the lowercase "x" between them), and their default value in pixels is INT_MAX for both dimensions. The resolution is the number of pixels per millimeter; its default value is "3.78 pixels/mm" (96 DPI). Width, height and resolution are real values. width_mm, height_mm and resolution are used only by cdGetCanvasSize and in pixel-millimeter conversion. Parameter -t modifies the codification. Parameter -p specifies the precision which can be "16" bits integer (default), "32" bits (integer), "F" (float) or "D" (double). Parameter -d specifies a description, must be the last parameter  (since 5.6).

Any amount of such canvases may exist simultaneously. It is important to note that a call to function cdKillCanvas is required to close the file properly.

Coding - The CGM format supports binary and text encoding. If you are not sure what to do, use binary encoding, which is the default. Should you prefer text coding, add a "-t" string to the Data parameter.

Precision of Coordinates - The primitives can use coordinates in real numbers. But to be effective the parameters "-pF" or "-pD" must be used when creating the canvas (since 5.6).

Behavior of Functions

Control 

• Clear: does nothing.
• Flush: creates a new picture. The CGM format supports multiple picture in the file.
• Play: works with files created with text or binary coding. There are several callbacks for this driver. If one of the callbacks returns a value different from zero, cdPlay's processing is interrupted. The driver implements the callback CD_SIZECB and other callbacks associated to CGM:
  • CD_COUNTERCB - int(*cdcgmcountercb)(cdCanvas* canvas, double percent) - Executed for each header of CGM commands; returns the percentage (0-100%) of headers read.
  • CD_SCLMDECB - int(*cdcgmsclmdecb)(cdCanvas* canvas, short scl_mde, short *drw_mode, double *factor) - Executed for the command CGM SCALE MODE. Returns the current CGM scale mode and allows the callback to modify the scale mode used internally (ABSTRACT=0, METRIC=1). You can choose the METRIC or ABSTRACT scale mode, but if the original scale mode is METRIC, then you must provide the conversion factor in mm per pixel.
  • CD_VDCEXTCB - int(*cdcgmvdcextcb)(cdCanvas* canvas, short type, double *first_x, double *first_y, double *second_x, double *second_y) - Executed for the CGM command CGM VDC EXTENT, returns the VDC SPACE. Currently type is always 1.
  • CD_BEGPICTCB - int(*cdcgmbegpictcb)(cdCanvas* canvas, char *pict) - Executed for the command BEGIN PICTURE, returns the string that describes the image.
  • CD_BEGPICTBCB - int(*cdcgmbegpictbcb)(cdCanvas* canvas) - Executed for the command BEGIN PICTURE BODY.
  • CD_CGMBEGMTFCB - int (*cdcgmbegmtfcb)(cdCanvas* canvas, int *xmin, int *ymin, int *xmax, int *ymax) - Executed for the command BEGIN METAFILE, provides the drawing limits of the image in the file. These are the same parameters passed to cdCanvasPlay.

Coordinate System and Clipping

• UpdateYAxis: does nothing. The axis orientation is the same as the CD library.
• Complex Regions: not supported.
• Transformation Matrix: not supported.

Primitives

• Begin: if parameter CD_CLIP is specified, does nothing. CD_BEZIER and CD_PATH are simulated with lines.
• Pixel: does not exist in CGM, is simulated using a mark with size 1.
• Floating point primitives are supported. But to be effective the parameters "-pF" or "-pD" must be used when creating the canvas.

Attributes

• WriteMode: does nothing, returns CD_REPLACE.
• FontDim: is simulated.
• FillMode: does nothing.
• LineCap: does nothing.
• LineJoin: does nothing.
• TextSize: is simulated.
• Font: see the table bellow for the generated font names. No other fonts are supported.

Colors

• GetColorPlanes: returns 24.
• Palette: does nothing.

Client Images 

• GetImageRGB: does nothing.
• PutImageRGBA: alpha is ignored.

Server Images

• All functions do nothing.