Computer controlled screen animation

Abstract: Method and apparatus for generating relative movement between a first image and a second image while avoiding display screen flicker is disclosed. The method includes a step of copying to a first buffer a portion of a display memory corresponding to a present portion of the second image wherein a first image will first appear. The method further includes the steps of superposing within the display memory the first image upon the present portion of the second image and copying to a second buffer the portion of the display memory corresponding to the present portion of the second image, including the first image superposed thereon, and a portion of the display memory corresponding to a next portion of the second image wherein the first image will next appear. The method further includes the steps of erasing the first image from the second buffer by copying the present portion from the first buffer to the second buffer and superposing, within the second buffer, the first image upon the next portion of the second image. The method includes a further step of copying the second buffer to the display memory such that the first image disappears from the present portion of the second image while simultaneously appearing within the next portion of the second image, thereby avoiding display screen flicker.

Description: This invention relates computer controlled screen animation and methods and apparatus for displaying an animated sequence of graphical images that effectively prevents the occurrence of display screen flicker due to the relative movement of a portion of the image relative to another portion of the image. In a system that displays an animated sequence of images, wherein an image of an object or objects moves relative to an image of other objects, a particular problem relates to the generation of screen flicker. This flicker results from a conventional approach of erasing the object at a first location within a screen memory and redrawing the object at a second location. As the object grows larger and/or more complex in shape the time required to erase and rewrite the individual image pixels increases, resulting in a more pronounced screen flicker effect. The presence of screen flicker is objectionable at least for the reason that it prevents a realistic, flicker-free animation sequence from being displayed. Electronics International Vol. 53, No. 25, November 1980, pages 123-126 discloses a graphics system in which a first image may be moved relative to a second image upon which it is superimposed. The images are stored in a plurality of superimposed image planes. The present invention provides a method for creating relative movement between first and second images in a data processing system having display memory means coupled to display means whereon the first image is displayed superimposed upon the second image, comprising the steps of: copying to first buffer means a portion of the display memory means corresponding to a present portion of the second image; storing within the display memory means an image comprising the first image superimposed upon the present portion of the second image; characterised by the steps of: copying to second buffer means the contents of a portion of the display memory means corresponding to the present portion of the second image, including the first image superposed thereon and the contents of a portion of the display memory means corresponding to a next portion of the second image wherein the first image is to next appear; erasing the first image from the second buffer means by copying the present portion from the first buffer means to the second buffer means; superposing, within the second buffer means, the first image upon the next portion of the second image; and copying the contents of second buffer means to the display memory means such that the first image disappears from the present portion of the second image while simultaneously appearing within the next portion of the second image. Such an arrangement is thought to provide for displaying on a display screen a relative movement of a first object to a second object without generating a significant amount of screen flicker. One form of such a method and supporting apparatus for displaying relative motion between a first image and a second image is disclosed hereinafter by way of example. The arrangement includes a display memory coupled to a display whereon the first image is displayed to move in relation to the second image. The system further includes a plurality of buffer memories coupled to an animation controller, typically a CPU. The method includes a step of copying to a first buffer a portion of the display memory corresponding to a present portion of the second image wherein the first image will first appear. The method further includes the steps of superposing within the display memory the first image upon the present portion of the second image and copying to a second buffer the portion of the display memory corresponding to the present portion of the second image, including the first image superposed thereon, and a portion of the display memory corresponding to a next portion of the second image wherein the first image will next appear. The method further includes the steps of erasing the first image from the second buffer by copying the present portion from the first buffer to the second buffer and superposing, within the second buffer, the first image upon the next portion of the second image. The method includes a further step of copying the second buffer to the display memory such that the first image disappears from the present portion of the second image while simultaneously appearing within the next portion of the second image, thereby avoiding display screen flicker. The first image is generated and stored within a buffer memory, the image including one or more displayable objects surrounded by transparent "zero" pels. The first image is superposed upon the second image such that the object lies in front of the second image. The present invention will be described further by way of example with reference to an embodiment thereof as illustrated in the accompanying drawings, in which Fig. 1 is a block diagram of an information processing system; Figs. 2a-2e illustrate a display screen and the contents of various buffers during the performance of an animation sequence therein; and Fig. 3 is a flow chart that illustrates the operation sequence thereof. The following description of one embodiment of the invention is made in the context of an audio/visual application processor implemented on an IBM TM PS/2 TM computer system (IBM and PS/2 are trademarks of the IBM Corporation of Armonk, NY). It should be appreciated, though, that the teaching of the invention may be practiced with a number of different types of information processing systems in support of a number of different types of application programs that provide a visual display. Referring first to Fig. 1 and to Fig. 2a there is shown a visual display screen 10. Associated with display screen 10 is a read/write display memory 1 wherein stored data is read out and displayed on the screen 10 in a conventional manner. An individual or a group of storage locations within the display memory 1 corresponds to individual pixels or pels of the display screen 10. The display memory 1 is coupled to a central processing unit (CPU) 2, the CPU 2 further being coupled via a bus 3 to a main memory 4 wherein instructions are loaded and stored from a mass storage device 5. Certain of the stored instructions control the CPU 2 to perform a graphics display program, including graphical animation sequences, that calls routines comprised of instructions that implement the steps of the method of the invention. The data stored within the display memory 1 may provide information for displaying a monotone or a colour image, depending upon the characteristics of the particular system display screen 10. For the example described below display screen 10 displays graphical animation sequences including sequences wherein a first object or image moves relative to a second object, such as a background image. It should be appreciated that the teaching of the invention is applicable to a wide variety of possible display or animation subjects and sequences. Referring now to Figs. 2a-2e and Fig. 3 the method of the invention is described in greater detail. Initially, a region of the display screen 10 is determined wherein an image of an object will first appear on the display screen. This region is referred to as the "present region" in the flowchart block 14 of Fig. 3 in that it is the region wherein the object is currently present, as opposed to a "next region" wherein the object will appear next. In the illustrated example the object is determined to first appear in a region designated 10a. The contents of the display memory 1 corresponding to the region 10a of the display screen 10 are copied to a buffer memory A, as indicated by the arrow. Buffer memory A is preferably a region allocated within main memory 4 for this purpose. Buffer memory A functions as a temporary store that preserves or saves a portion of the screen image within a specific portion of the display screen 10. The area of the region 10, that is the number of storage locations required for the buffer memory A, corresponds to that of a Source (S) buffer wherein an image of the object, in this case an aircraft, is initially generated (Fig. 3, block 12). The screen coordinates of the region 10a are thus a function of the size of the S buffer and the desired initial position of the object. The S buffer is also typically an allocated region within the main system memory 4. Next, the contents of the S buffer are combined with the display memory 1 such that the object appears on the display screen 10 within the region 10a (Fig. 3, block 16). The S buffer contents are initially generated such that the object is surrounded by zero value pels, that is, the region surrounding the object is transparent when superposed on the display screen 10. This results in the object being displayed and the portion of the display screen 10 background not covered by the object also appearing within the region 10a. The S buffer is combined with and superposed on the display screen 10 by a process, described below, that is applied to those memory locations within the display memory 1 that correspond to the region 10a. The number of storage locations allocated for the S buffer, and hence the area enclosed by the S buffer, is a function of the size and shape of the object being depicted. The area is sufficient to completely enclose the object outline within a region having the shape of a parallelogram, a shape that is efficiently and rapidly manipulated. It is noted that each pel has a numeric value. One such value is designated as "transparent". By convention, a value of zero may be so designated. When a foreground image is copied over a background image, the process performing the copy first inspects the value of the foreground pel: if "transparent" it does not copy that pel; if non-transparent, it does. The result is that wherever the foreground image had a transparent pel, the corresponding background pel remains visible at that position in the combined image. As can be seen in Fig. 2b, a second region 10b of the display screen 10 is delineated, this second region 10b being the region of the display screen 10 wherein the object next appears as it moves relative to the background image. Further in accordance with the invention both of the regions 10a and 10b, the present and the next regions, are copied to a buffer memory B (Fig. 3, block 18). As can be seen the buffer memory B contains the region 10a, including the image of the object, and the adjoining region 10b wherein the object will next appear. The dashed line shown in the centre of the buffer memory B is illustrated to differentiate the region 10a from the region 10b; it being appreciated that such a dashed line would not generally appear in the image or within the buffer. In Fig. 2d the contents of the buffer memory A are transferred to that portion of buffer memory B that corresponds to the region 10a (Fig. 3, block 20). This causes the image of the object to be overwritten and replaced by the corresponding portion of the original background image, thereby effectively erasing the image of the object from the buffer memory B. Next, the contents of the screen memory 1 that correspond to the adjoining region 10b are copied to the buffer memory A to preserve the appearance of the background image before the object is superposed thereon (Fig. 3, block 22). That is, the original contents of the buffer memory A, corresponding to the image of the region 10a before the appearance of the object, are written back to the region 10a within buffer memory B and are replaced by the image corresponding to the adjoining region 10b. In Fig. 2e it can be seen that the contents of the S buffer, that is the object, are combined with the buffer memory B within the portion corresponding to the region 10b (Fig. 3, block 24). As before, the object pels overwrite the background portion of the image while the pels surrounding the object are transparent. This results in the object appearing to lie in front of the original portion of the image within the region 10b. Next, the content of the buffer memory B is copied to the display memory 1 such that the regions 10a and 10b are both replaced by the contents of the B buffer (Fig. 3, block 26). Comparing Fig. 2e to Fig. 2b it can be seen that this operation results in an apparent movement of the object, relative to the borders of the display screen and to the background image objects, from the region 10a to the region 10b. Furthermore, it can be appreciated that this movement occurs without generating screen flicker in that writing the buffer B image to the screen memory 1 causes the object to be totally erased from the region 10a while simultaneously being written to the region 10b. The buffer B contents are preferably written in a conventional manner to the screen memory 1 during a time that the screen is blanked during vertical retrace. It is next determined if the object is to continue to move across the display screen (Fig. 3, block 28). If not the method halts (Fig. 3, block 30). If further movement is desired a third region 10c is delineated, the region 10c being that region wherein the object is next to appear. Region 10b becomes associated with the present region and region 10c with the next region (Fig. 3, block 32). At this time the regions 10b and 10c are copied to the buffer memory B as previously described in relation to Fig. 2c (Fig. 3, block 18). The method thereafter repeats the steps shown in Figs. 2c, 2d and 2e as the object is moved across the display screen 10. It is noted that in some embodiments of the invention that the content of the S buffer is replaced with a slightly different image between iterations of the loop. The example shows an aircraft flying across the screen at the same distance from the viewer. For such an animation sequence the originally generated S buffer is used repetitively. However, if the animation sequence were of a bird approaching from a distance it will be appreciated that in each iteration the bird would present a slightly larger image, and the orientation of the wings would differ. This animation sequence requires multiple "next" images of the bird to be generated with all of the different wing positions and of varying sizes. The method as previously described is used, with the added step shown in block 34 that the "next" image of the bird is generated within or copied to the S buffer. The disclosed embodiment can be modified in various ways, including the provision of a dedicated animation co-processor having local buffer memories or by special purpose hardware embodied in, by example, a gate array. Also, certain steps of the method may be combined or accomplished in other than the order described while still obtaining the desired result.

Claims: A method for creating relative movement between first and second images in a data processing system having display memory means (1) coupled to display means (10) whereon the first image is displayed superimposed upon the second image, comprising the steps of: copying (14) to first buffer means (A) a portion of the display memory means corresponding to a present portion of the second image; storing (16) within the display memory means an image comprising the first image superimposed upon the present portion of the second image; characterised by the steps of copying (18) to second buffer means (B) the contents of a portion of the display memory means corresponding to the present portion of the second image, including the first image superposed thereon and the contents of a portion of the display memory means corresponding to a next portion of the second image wherein the first image is to next appear; erasing the first image from the second buffer means by copying (20) the present portion from the first buffer means to the second buffer means; superposing (24), within the second buffer means, the first image upon the next portion of the second image; and copying (26) the contents of second buffer means to the display memory means such that the first image disappears from the present portion of the second image while simultaneously appearing within the next portion of the second image. A method as claimed in Claim 1, and comprising an initial step of generating (12) within a source buffer means a representation of the first image, the first image being surrounded by transparent pels. A method as claimed in Claim 1 or Claim 2, wherein the first and the second steps of superposing are accomplished by combining the contents of the source buffer means with the contents of the display memory means and with the contents of the second buffer means, respectively. A method as claimed in any preceding claim, and including the step of identifying (32) the next portion of the second image with the present portion of the second image and in which the steps are repeated to create further movement between the first and the second images. A method as claimed in any preceding claim, wherein a size of the first and the second image portions corresponds to a size of the first image and the surrounding transparent background. A method as claimed in any preceding claim, wherein a plurality of representating of the first image is generated (34) and superposed in sequence during successive iterations. A data processing system adapted to perform the method of any preceding claim and comprising display memory means (1) coupled to display means (10), and apparatus for generating relative motion between a first displayed image and a second displayed image, comprising: first buffer means (A) for storing a first portion of the display memory means (1) corresponding to the second image; second buffer means (B) for storing a first portion of the display memory means corresponding to the second image having the first image combined therewith and also for storing a second portion of the display memory means corresponding to the second image at a location where the first image is next to be combined; and control means (2), coupled to the display memory means and to the plurality of buffer memory means and including logic to control copying to the first buffer means (A) a portion of the display memory means (1) corresponding to a present portion of the second image; storing within the display memory (1) means an image comprising the first image superimposed upon the present portion of the second image; copying to second buffer means (B) the contents of a portion of the display memory means (1) corresponding to the present portion of the second image, including the first image superposed thereon, and the contents of a portion of the display memory means (1) corresponding to a next portion of the second image wherein the first image is to next appear; erasing the first image from the second buffer means (B) by copying the present portion from the first buffer means (A) to the second buffer means (B); superposing, within the second buffer means (B), the first image upon the next portion of the second image; and copying the second buffer means (B) to the display memory means (1) such that the first image disappears from the present portion of the second image while simultaneously appearing within the next portion of the second image.

Inventors: BEITEL BRADLEY JAMES, GORDON ROBERT DOUGLAS, BEITEL, BRADLEY JAMES, GORDON, ROBERT DOUGLAS
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G06T13
IPC codes: G06T15, G06T13, G09G5, H04N5