JPH01166179A - Method for drawing clipped straight line - Google Patents

Abstract

PURPOSE:To enable the coordinate of a grid point being an object to be displayed to be obtained by executing a DDA algorithm by making a point on a virtual outer frame provided on the outside of a clip frame to be a starting point when a line segment is displayed within the clip frame and only arithmetic-operating an integer number value. CONSTITUTION:A CPU 13 executes graphic display sequence according to a program stored in a memory 14. The DDA algorithm in this case is entrusted to a DDA circuit 15 which is exclusively designed. As the result, the DDA algorithm can be executed at high speed. Besides, when the clipped line segment is displayed within the clip frame 1, the decision sequence of the grid point only by the integer number value is previously executed by using this invented method.

Description

[Detailed Description of the Invention] [Summary] Regarding the method of drawing clipped straight lines when displaying line segments within a clipping frame, the coordinates of grid points to be displayed can be determined by only integer value operations. For the purpose of this, we assume a frame one dot outside the clipping frame, find the intersection between this outer frame and a straight line, use the DDA algorithm to find the intersection with the clipping frame, and create a line segment. Configure to display.

[Industrial Field of Application] The present invention relates to a method of drawing a clipped straight line when displaying a line segment within a clipping frame.

In a graphic display device, when performing multi-window display on the display, each display portion is displayed surrounded by a clip frame. When displaying images surrounded by a clip frame, if there is a straight line that cuts through the clip frame as shown in Figure 6,
The problem is how to find the coordinates of grid points on the clipping frame.

[Prior Art] In a graphic display device, a so-called DDA algorithm is used to display line segments.

FIG. 7 is an explanatory diagram of the DDA algorithm.

The ODA algorithm is an algorithm related to line segment display, and when drawing a line segment l in a diagram, if the X coordinate is increased by 1 from the X coordinate of the starting point x 1, the X coordinate is made equal to the current y coordinate. The display point at the lattice point is determined as indicated by the dots while sequentially determining from the start point to the end point whether the value increases by 1 according to the distance in the y direction from the line segment l. Line segments can be displayed by allocating dots to grid point positions using such an ODA algorithm.

The case shown in FIG. 7 will be explained as an example. Starting point (xi, yl
) to the end point (x2.y2), how much can you create a line segment that approximates line segment l using the grid points that exist between the start and end points of this line segment? The DDA algorithm pursues this. Assume that the starting point is exactly at the grid point 1 (xl, yl). To determine whether the next X coordinate at the point where x+i is the current y coordinate y1 or y1+1 above by 1,
As shown in the figure, a line segment 4' parallel to the X axis is drawn exactly between yl and y1+1. Looking at the vertical relationship between the x1+1 points of this line segment l' and the line segment l, 4' is higher. Therefore, the X coordinate at the point xl +1 is set to the current y coordinate yl. As a result, 2 is (xi
+1. yl). Next, find the X coordinate at the point x1+2. , line segment at the point ×1+2! ' and line segment l
Looking at the hierarchical relationship with , this time l is above l'. Therefore, the next grid point 3 is not the current y-coordinate position, but the point y1+1 above by 1 (xl +2.yl
+1). By applying the same operation to all the X coordinates, a straight line approximated by lattice points as shown in FIG. 7 is obtained.

If such a straight line is to be displayed within the clip frame, it is necessary to display only the points included within the clip frame among the lattice points representing the straight line, as shown in FIG. FIG. 8 is a diagram showing a display example of clipped straight lines. The example shown in the figure shows a display example of a straight line clipped at the X coordinate yc.

[Problems to be Solved by the Invention] In the example shown in Fig. 8, the following method is used to obtain the X coordinate (indicated by XC in the figure) on the clip frame to be displayed. ing. That is, as described above, when the X coordinate increases, it is necessary to choose between increasing the X coordinate or leaving it unchanged. Straight line! If the value of is smaller by 0.5 than the value of y on the clipping frame, the X coordinate as the coordinate of the grid point is set to yc, which is increased by 1. Therefore, the corresponding X coordinate value must also correspond to an X coordinate value larger than yc-0, 5.

Therefore, conventionally it is a straight line! In the formula (for example, y=ax+b),
yc minus 0.5 from yc indicating the X coordinate value on the clip frame - 0,5 yb in the figure
The corresponding X coordinate value was obtained by substituting the X coordinate value of the intermediate position between and yc. However, when such a calculation method is used, a floating point iI mechanism that handles numerical values including decimal numbers is required, making the hardware circuit complicated. On the other hand, when floating point arithmetic is not used, when dealing with such a decimal point, it is treated as if there is a decimal point between bit 0 and pit 1, so the representable coordinate values are halved. There was a problem.

In order to eliminate such problems, the DDA algorithm should be performed from the starting point of the straight line.

However, in this method, the ODA algorithm operation until the straight line l approaches the clipping frame is a waste of time, and it takes a long time.

The present invention has been made in view of these points, and
It is an object of the present invention to provide a method for drawing clipped straight lines that can determine the coordinates of grid points to be displayed by only calculating integer values.

[Means for Solving the Problems] FIG. 1 is a flowchart showing the principle of the method of the present invention. In the present invention, a frame is assumed to be one dot outside the clip frame (step [1]), and the intersection point between this outer frame and a straight line is found (
Step [2]) From the obtained intersection point, use the DDA algorithm to find the intersection point with the clipping frame and display the line segment (
Step ■).

[Operation] According to the present invention, since the ODA algorithm is used from a point on the virtual frame that is one dot outside the clipping frame, the coordinate points of the grid points to be displayed can be determined only by calculation of integer values.

[Example] FIG. 2 is a flowchart showing an example of the method of the present invention. First, draw a frame (outer frame) one dot outside the clip frame.
Assume (step ■). FIG. 3 is an explanatory diagram of the method of the present invention. In the figure, 1 is a clip frame, and 2 is an outer frame assumed to be one dot outside of the clip frame 1. If we assume outer frame 2, this outer frame and straight Ii! /Determine which intersection yb (step ■). In the example of FIG. 3, yb = yc -1.

Next, one grid point representing a line segment in yb is found (step ■). Specifically, the equation of straight line 4 (for example, y=
ax+b) by substituting y=yb for the corresponding X coordinate value x
Find b (the point marked x in the figure) and the remainder. In this way, the coordinates (xb, yb) of the grid point are found, but if (remainder) x 2 ≧ (xe - xs), where xe is the coordinate of the starting point of straight line l, and xs is the coordinate of the ending point of straight line l. In this case, xb=xb+1. Such a correction is necessary in a case where the straight line l and the frame are arranged as shown in FIG. This correction is necessary when the remainder is too large, so the explanation will be continued assuming that the X coordinate value is xb.

Once the grid point (xb, yb) is found, the DDA algorithm is executed using this point as a new starting point.

In other words, find the DD△ parameter at the found grid point from the above remainder (step ■), xb+1, xb +
2. ..., and the corresponding X coordinate value is determined according to the DDA algorithm (step ■).

The grid points of clipping frame 1 can be determined by finding the X coordinate value at the point yb+1. and,
This point becomes the starting point of the line segment displayed within the clipping frame. According to the present invention, the grid points on the outer frame are first determined, and then the ODA algorithm is applied to display the line segments within the clip frame, so there is no need to perform decimal point calculations. .

FIG. 5 is a system configuration diagram for implementing the method of the present invention. In the figure, 11 is a clip frame.

A CRT for displaying straight lines, etc., 12 a frame memory (FM) for storing information to be displayed on the CRT 11, 13 a CPLI for performing various calculations, 14 a memory for storing programs, etc., and 15 for executing a DDA algorithm. A DDA circuit 16 is a bus connecting these components. In a system configured in this way, CP
U13 executes a graphic display sequence according to a program stored in memory 14. D in this case
The DA algorithm is left to a specially designed DDA circuit 15. As a result, the DDΔ algorithm can be executed at high speed. When displaying a line segment clipped within a clipping frame, a grid point determination sequence using only integer values is performed in advance using the method of the present invention as described above.

[Effects of the Invention] As described in detail above, according to the present invention, when displaying a line segment within a clip frame, a DDA algorithm is executed using a point on a virtual outer frame provided outside the clip frame as a starting point. As a result, it is possible to provide a clipped straight line drawing method that can determine the coordinates of a grid point to be displayed by only using the calculations of &fi&, which has a great practical effect.

[Brief explanation of the drawing]

FIG. 1 is a flow chart showing the principle of the present invention, FIG. 2 is a flow chart showing an embodiment of the method of the present invention, FIG. 3 is an explanatory diagram of the method of the present invention, FIG. 4 is a diagram showing correction of xb, Fig. 5 is a system configuration diagram for implementing the method of the present invention, Fig. 6 is a diagram that does not show the relationship between the clipping frame and the straight line, Fig. 7 is an explanatory diagram of the DD△ algorithm, and Fig. 8 is a diagram showing the clipped It is a figure which shows the example of a display of a straight line. In FIG. 5, 11 is a CRT. 12 is fresh memory, 13 is CPtJ. 14 is a memory, 15 is an ODA circuit, and 16 is a bus. Flowchart illustrating the principles of the method of the present invention Figure 1 Figure 7 Flowchart illustrating an example of the present invention 7!
Figure 2: Yusakaki Volume 3 Figure showing the correction of Xb Figure 4 Figure 5 Figure showing the fusion of the clip frame and the straight line Figure 650 Kanowarana (
U Kaikai 1 iXI! l! s force 1 praise y coordinate t
DDA algorithm when increasing by 1 ■Explanatory diagram Figure 7 Figure 8

Claims (1)

[Claims] A frame is assumed to be one dot outside the clip frame (step [1
]), find the intersection between this outer frame and the straight line (step [2]), use the DDA algorithm to find the intersection with the clipping frame, and display the line segment (step [3]). A method for drawing clipped straight lines.