JPS63167977A - Three-dimensional shape picking method - Google Patents

Abstract

PURPOSE:To specify part of a three-dimensional shape by obtaining a Z coordinate from picked X and Y coordinates and then searching for a segment where a polygon which contains the pick point belongs by a specific method, and thus selecting the picked segment. CONSTITUTION:The three-dimensional coordinate values of the pick point are obtained from a Z coordinate value left in an area of a Z buffer 103 corresponding to X and Y coordinate values at the time of display processing by implicit- surface elimination based upon the X and Y coordinate of the picking on a screen. Then, sign bits obtained by subtracting the X-Z coordinate values of the pick point from the X-Z coordinate values of the vertexes of respective polygons in a primitive set and sign bits obtained by subtracting the coordinate values of the vertexes from the pick point coordinate values are stored in specific bits of flags consisting of 6-bit areas, and the flags as to all vertexes of the individual polygons are ANDed. Then a pane system for a plane containing a polygon whose AND result is zero is found to check whether the pick point is contained or not in detail.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for editing a three-dimensional shape displayed on a graphic display after hidden surfaces have been removed. This invention relates to a three-dimensional shape picking method suitable for selecting three-dimensional shapes.

[Conventional technology]

Conventionally, pick processing of two-dimensional shapes has been carried out using the picked screen as described in Computer Graphics The picked segment is selected depending on whether the upper X and Y coordinates are included in the screen range, which is a vertical rectangle surrounding the segment.

Furthermore, when a pick point is included in multiple segments, the distance between the pick point and a primitive in the segment is compared, and the segment that includes the primitive closest to the pick point is selected as the picked segment.

[Problem that the invention seeks to solve]

The above conventional technology is aimed at picking two-dimensional shapes, and only the X and Y coordinates on the screen are taken into account as pick point information. Therefore, when the conventional technology is used for picking three-dimensional shapes, the screen A problem arises in that it is unclear which segment to select from a plurality of segments that overlap in the depth direction. Furthermore, when the calculation for determining the distance between a pick point and a primitive is applied to a three-dimensional system, it is necessary to determine a plane equation for a polygon, which poses a problem in that the amount of calculation is large.

An object of the present invention is to make it possible to specify a portion of a three-dimensional shape displayed on the screen by picking a point on the terminal screen.

[Means for solving problems]

The above purpose is that when a part of the 3D shape is picked,
It is assumed that one point on the most proximal segment in the screen depth direction has been picked, and after obtaining the z coordinate of the pick point from within the Z buffer based on the picked X and Y coordinates on the screen, this pick point is included. This is possible by searching the segment buffer for the segment to which the polygon belongs using a method applying the Cohen-Sutherland clipping technique and selecting it as the picked segment.

[Effect]

The Z-coordinate of each point on the segment currently displayed on the screen is stored in the Z-buffer.

Therefore, by using the Z coordinate in the Z buffer that corresponds to the picked X, Y coordinate,
Y and Z coordinate values can be obtained.

In addition, in a 3D area clipping method that applies Cohen-Sutherland's clipping technique to 3D,
Minimum of 3D area,! By replacing both &major coordinates with the coordinates of the pick point, all vertices of the polygon are above, below, and to the right of the pick point.

Those existing on the left side or the rear side can be removed as polygons that clearly do not include pick points.

Therefore, it is possible to select a picked segment with a relatively small amount of calculation.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an example in which the present invention is applied to a three-dimensional graphic CAD system. FIG. 2 is a PAD showing the processing contents performed in the pick processing section 101 in FIG. In FIG. 1, reference numeral 101 denotes a pick processing unit that performs processing associated with pick operations. A pick device 102 serves as an input device for a pick operation. Reference numeral 103 denotes a Z buffer used in display processing that performs hidden surface removal, and stores the two coordinate values of the display object that is most present in the procedure for each point on the currently displayed screen. 104
is a segment buffer in which shape data of the displayed three-dimensional shape is stored in units of segments.

105 performs coordinate transformation and clipping on data in the segment buffer.

This is a display processing unit that performs hidden surface removal, hidden line removal, and brightness calculation processing to display a three-dimensional shape on both sides of the terminal. 106
is a central processing unit that executes application programs and controls the operation of the entire CAr system.

Next, the operation of the pick processing section 101 will be explained according to the PAD shown in FIG. First, in the pick processing section, the pick point X, Y position 4! inputted by the user using the pick device! ? The receiver receives I (Xp, Yp) and starts processing. Next, locus 5 (Xp, Yp
), extract the Z coordinate value ZP of the pick point from the area corresponding to
p) is determined. FIG. 3 shows the relationship between pick points and Z buffers. First, a pick segment number indicating the identification number of the picked segment is initialized. If the pick segment number remains at the initial value at the end of the pick process, it is assumed that the pick has failed. For this reason.

As the initial value of the pick segment number, it is necessary to select a number that cannot exist as a segment identification number. Next, the following process is repeated for all segments whose display attributes are turned on in the segment buffer. First, the segment data is read and the vertex coordinates of each polygon belonging to the segment are obtained. Next, using the functions included in the display processing section, coordinate transformation processing and clipping processing are performed on the segment data. Then, the following processing is performed for each polygon displayed as a result of clipping. First, all bits of a check flag consisting of 6 bits are set to 1. Next, for each vertex of the polygon (coordinate values are (XV, Yv, Zv)), Y
p-Yv, Yv-Yp, Xp-Xv,
Calculate the subtraction of Xv - Xp e Zp - Zv , Zv - Zp, calculate the AND of the sign bit of the subtraction result and a predetermined bit of the check flag, and store the result in the check flag. FIG. 4 shows a model diagram of the above processing. Also, in FIG. 5, Xp.

9 shows nine regions divided according to the magnitude relationship between Yp, Zp and Xv, Yv, and Zv, and the sign bit of the subtraction result in each region. As is clear from FIG. 4, for example, if all vertices of a polygon are above the pick point, the relationship Yp < Yv is consistently held for all vertex coordinates, so the first bit of the check flag is set to 1. Become. Also,
If the vertices of the polygon exist, for example, both above and below the pick point, all bits of the check flag will be zero. Therefore, at least one of the check flags
A polygon whose bit is non-zero has all vertices above, below, right, left, front, or behind the pick point, so it is clearly treated as a polygon that does not include the pick point, and will be used from now on. Can be excluded from inspection. This is the minimum of a three-dimensional area in a three-dimensional area clipping method that applies the Cohen-Sutherland clipping technique.

The maximum coordinates are replaced with the pick coordinates (see the above-mentioned reference: Computer Graphics, page 302).

Next, a detailed check is performed on the polygon for which all bits of the check flag are zero to see if it contains a pick point. First, the intersections of the plane Y=Yp and the line segments on each side of the polygon are found. Sort the resulting intersection points in ascending order of X, and the X coordinate is
If there is an odd number of intersection points smaller than , there is a possibility that the pick point is included in the polygon. If the number of intersections is an even number, the pick point is clearly outside the polygon, so no further checks are performed. For polygons that may include pick points, a plane equation that includes the polygons is determined. Then, if the difference between the Z coordinate value obtained by substituting X=Xp and Y=Yp into this equation and ZP is smaller than a predetermined tolerance, the polygon includes a pick point. If the difference between the above Z coordinate value and Zp is larger than the allowable error, it can be seen that the polygon does not include a pick point.

When a polygon including a pick point is found through the above-described inspection, the identification number of the segment to which the polygon belongs is immediately set as the pick segment number, the pink error flag is turned off, and the pick process is terminated. If a polygon including a pick point is not detected even after inspecting all segments, the pick error flag is turned on and the pick process is terminated.6 According to this embodiment, the 3
Since a part of the dimensional shape can be specified by picking a point on the screen, there is an advantage that there is no need to specify a segment by inputting a segment identification number in the editing process of the three-dimensional shape. In addition, in pick segment detection processing, polygons that obviously do not include pick points can be excluded from detection targets by simple calculations using a combination of subtraction and logical product, which has the effect of speeding up the detection processing. .

Further, in the above embodiment, the check flag is checked after clipping the segment data. However, polygons that are excluded from display by clipping processing.

Since it is included in the set of polygons that are excluded by checking with the check flag, the clipping process can be omitted. By omitting the above, it is possible to speed up the pick segment I/detection process.

Furthermore, in the above embodiment, Yp - Yv , Yv - Yp , Xp -X for each vertex of the polygon
v , Xv −Xp * Zp −Zv e Zv−Z
The calculation result of p has not been considered. If the sign bits of the calculation result are all zero, the pick point coordinates and the vertex coordinates are equal, so the subsequent check process can be omitted. By omitting the above, it is possible to speed up the pick segment detection process.

Furthermore, in the embodiment described above, the detection process was unconditionally applied to pick segment 1 for all segments. However, for each segment, the minimum and maximum coordinate values of a three-dimensional area that surrounds the entire segment as shown in Figure 6 are stored in the segment buffer, so that the three-dimensional area does not include pick points. Regarding the segment,
It is possible to omit the pick segment detection process.

By omitting the above, it is possible to speed up the pick segment detection process.

Further, in the above embodiment, the segment identification number of a portion of the picked three-dimensional shape was detected, but it goes without saying that the present invention can also be used to detect the pick identifier in the segment buffer.

〔Effect of the invention〕

According to the present invention, a part of a three-dimensional shape displayed on a graphic screen can be specified by picking a single point on the screen, and a picked segment can be detected by a simple calculation that combines subtraction and logical product. Since it is possible to simplify the process, there is an effect that editing processing of three-dimensional shape data can be performed interactively and at high speed.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG. 2 is a block diagram of an embodiment of the present invention.
PAD showing the processing contents of the pick processing section in the figure, and FIG. 3 is a relationship diagram between pick points and Z buffer. FIG. 4 is a model diagram of a picked segment detection method applying the Cohen-Sutherland clipping technique, and FIG. 5 is a state diagram of check flags in the detection method. FIG. 6 is a diagram showing a three-dimensional area surrounding a segment. 101... Pick processing unit, 102... Pick device, 103... Z buffer, 104... Segment buffer, 105... Display processing unit, 106...
Central processing unit. Figure 30'1.4 P1.7ri1x, coordinate T! ! ,! Degree standard 1st; ZP ((j 2v > Zp

Claims (1)

[Claims] 1. Picking a part of a three-dimensional shape displayed on a graphics terminal by performing hidden surface removal using a Z buffer, which is composed of segments that are a collection of primitives mainly consisting of polygons. In the dimensional shape pick method,
The three-dimensional coordinates of the picked point are calculated from the picked X, Y coordinate values on the screen and the Z coordinate values left in the area corresponding to the above X, Y coordinate values of the Z buffer when displayed by hidden surface removal. After obtaining the values, from the X, Y, and Z coordinate values of each vertex of each polygon in the primitive set,
, Y, Z coordinate values of the pick point and the X, Y, Z coordinate values of the above vertices, respectively.
, the sign bit of the result of subtracting the Z coordinate value is stored in a predetermined bit of a flag consisting of a 6-bit area, and the logical product of the above flags for all vertices of each polygon is calculated, and the logical product is Non-zero polygons are clearly excluded as not containing pick points, and only for polygons for which the above logical product is zero, it is possible to determine whether or not they contain pick points by finding the plane equation of the plane containing the polygons. A three-dimensional shape picking method characterized by performing a more detailed check. 2. In the above three-dimensional shape picking method, the minimum and maximum coordinate values of the three-dimensional area surrounding each segment are stored in a segment buffer, and the minimum and maximum coordinate values of each segment and the three-dimensional shape of the pick point are The method according to claim 1, wherein the coordinate values are compared and the presence or absence of a polygon that includes the pick point is checked only for segments that have a three-dimensional area that includes the pick point. 3D shape picking method.