GPIGuide - Graphics Functions

Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation

GPI Guide and Reference

How to Use the GPI Guide and Reference Graphics Functions Data Types Graphics Orders Graphics Orders Data Types Errors Area and Polygon Primitives Bit Maps Creating and Drawing Retained Graphics Character String Primitives Clipping and Boundary Determination Color and Mix Attributes Correlation Coordinate Spaces and Transformations Editing Retained Graphics and Graphics Segments Fonts Graphics Attributes Line and Arc Primitives Marker Primitives Matrix Multiplication Metafiles Print Job Submission and Manipulation Presentation Spaces and Device Contexts Paths Regions Notices Glossary

GPI coordinate values that are in world or model space are passed in variables of data type LONG. For a presentation space of format GPIF_LONG (see GpiCreatePS), the signed value must be contained within the low-order 28 bits.

For a presentation space with a format of GPIF_SHORT, the signed value must be contained within the low-order 16 bits. Coordinates that exceed this limit are truncated without error, when stored in a segment. As a consequence, a large positive number may appear as a negative number.

In both instances, after transformation to media space (that is, device space, possibly including a translation for the window origin), coordinate values must be in the range -32 768 through +32 767.

The PMERR_COORDINATE_OVERFLOW error condition occurs if a coordinate is too large to be handled.

Region coordinates must be within the range -32 767 through +32 765.

Matrix Parameter Values

These GPI functions define transforms:

GpiSetSegmentTransformMatrix

GpiSetModelTransformMatrix

GpiCallSegmentMatrix

GpiSetViewingTransformMatrix

GpiSetDefaultViewMatrix

GpiCreatePS

GpiSetPageViewport.

Note

The last two functions define the device transform; the page viewport may be defaulted.

Concatenation of transform matrixes can occur as the transform is specified, for example, if TRANSFORM_ADD is specified. Concatenation also occurs during drawing, between the various transforms in the viewing pipeline.

During the process of concatenation, it is possible for the matrix parameter overflow error, PMERR_INV_MATRIX_ELEMENT, to occur. This error is raised if either of the following conditions occurs for any intermediate value during the concatenation arithmetic (see, for example, GpiSetSegmentTransformMatrix for an explanation of matrix element numbers):

• Any of the matrix elements 1, 2, 4, or 5 is greater than 32 767 or less than -32 768 (ñ1 for a GPIF_SHORT format presentation space), or

• Either of elements 7 or 8 is greater than 134 217 727 (2 -1) or less than -134 217 728 (-2) (greater than 32 767 or less than -32 768 for a GPIF_SHORT format presentation space).

Rounding Errors

In general for graphics coordinates, when non-unity transforms (apart from simple translation) are involved, rounding errors occur. For example, adding the coordinates of one point to a delta value, to produce the coordinates of a second point (all in world coordinates) does not always map to the same device pel as if the computation had been done in device coordinates. Such errors can be avoided if calculations are done in device coordinates, or if there are no scaling (or rotational, or shear) elements in the transforms. Alternatively, the problems can be reduced, though not eliminated, by defining very fine world coordinates.

Drawing Process Check Errors

Some GPI functions involve processing buffers of graphics orders or retained graphics segments (the data for which consists of graphics orders). These functions can give rise to Drawing Process Check (DPC) errors if an order is found that either is not valid in its context or that contains invalid data. If this happens, processing of the function stops and the error is recorded. Note that orders up to the one found to be in error are processed by the function, and output occurs if drawing is being performed.

Each function that can return these errors has Drawing Process Check errors in its error condition list. The full list of DPC errors is:

PMERR_INV_IN_AREA PMERR_INV_IN_PATH

PMERR_INV_IN_ELEMENT

PMERR_ALREADY_IN_ELEMENT

PMERR_STOP_DRAW_OCCURRED (warning)

PMERR_PATH_INCOMPLETE

PMERR_AREA_INCOMPLETE

PMERR_IMAGE_INCOMPLETE

PMERR_INV_ORDER_LENGTH

PMERR_NOT_IN_IMAGE

PMERR_NOT_IN_AREA

PMERR_NOT_IN_ELEMENT

PMERR_NOT_IN_PATH

PMERR_INSUFFICIENT_MEMORY

PMERR_SEG_CALL_STACK_EMPTY

PMERR_SEG_CALL_STACK_FULL

PMERR_TRUNCATED_ORDER

PMERR_CALLED_SEG_NOT_FOUND

PMERR_DYNAMIC_SEG_SEQ_ERROR

PMERR_PROLOG_ERROR

PMERR_INV_IN_VECTOR_SYMBOL

Functions

The GPI Functions are listed here

GPI Function Context

A number of the GPI error conditions indicate that a function has been used in the wrong context. This appendix lists every function and shows, for each one, whether it can be used:

• In a micro presentation space
• While there is an open segment bracket
• While there is an open area bracket
• While there is an open element bracket
• While there is an open path bracket.

A yes (Yes) means that a function can be used; a no (No) means that it cannot. There are some additional qualifiers in the form of superscript numbers. These generally indicate some further restriction on the context in which a function can be called, and are explained at the end of this appendix.

Where GPI Functions Can Be Called

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