Chapter II - The Playfield Hardware

INTRODUCTION

There are two basic parts to the display which you see on the screen:

• Things which move, called "sprites", and

• Things which don't move, called "playfields" or playfield objects.
  

A playfield is the backdrop against which the sprites may be displayed or with which the sprites can interact. A playfield object is simply a smaller subsection of the playfield which is considered by the software to be an object of some kind.

Even though playfield objects are classed as non-moving object, these objects can appear to move by using a technique called playfield animation. The chapter titled "THE BLITTER" shows that playfield objects can be rapidly redrawn on the screen, while saving and restoring the background onto which they are drawn, to give the illusion of motion. Much of the demonstration software which you may have already seen uses this animation technique.

This chapter concentrates on the playfield hardware itself. It shows you what defines a playfield, how its colors are specified, how it is controlled, and how other parts of the computer interact with the playfield hardware.

WHAT TOPICS ARE COVERED IN THIS CHAPTER

This chapter covers:

• How to define playfield resolution

• How to select the color of the playfield elements using one or more bit-planes

• How Hold & Modify mode increases your choice of onscreen colors available

• How to use the bit-planes as a single playfield, or how to split the static display into two independently controllable playfields.

• How to control the size of the static display.

• How to determine how much memory space is used by the display

• What part of the display memory actually appears on the screen

• How to smoothly scroll (move the playfield) in either a horizontal or vertical direction

• How the playfield display interacts with other parts of the Amiga Personal Computer

PLAYFIELD HARDWARE

There are two different operating modes for the playfield display:

• Normal Resolution Mode
• High Resolution Mode
  

NORMAL RESOLUTION MODE

In normal resolution mode, there are 320 picture elements or "pixels" which form each horizontal line of the screen display. This is for the resolution which is generally used for the standard home television screen. It allows clear text at 40 columns per line as well as good resolution while displaying multi-colored images.

You can define a "color palette", essentially a set of paint pots, which contains 32 out of a possible 4096 available colors. Normal resolution mode allows you to select any one of these 32 colors which you have defined for any one of the pixel elements which make up the overall picture. This allows you to create detailed multi-colored pictures on the screen.

A special operating mode, called Hold & Modify, allows you to control the color even more closely. This mode, shown later in this chapter, lets you write up to 3616 colors on the screen at the same time on a standard television set, or up to 4096 different colors on an RGB monitor.

HIGH RESOLUTION MODE

In high resolution mode, there are 640 pixels which form each horizontal line of the screen display. In this mode, each pixel can be any one of 16 colors which you have defined. High resolution pictures are normally only produced on a high resolution monochrome or RGB monitor. (A standard television set does not have a high enough resolution to respond correctly to the high resolution signal.)

NON-INTERLACE MODE

In non-interlace mode, you have about 200 lines per screen in a vertical direction.

INTERLACE MODE

In interlace mode, you have about 400 lines per screen in the vertical direction. Note that this does NOT mean you are using more screen area in the interlace mode than in non-interlace mode. It simply means that the video scanning circuitry, on one frame (roughly once each 1/60th of a second) displays 200 lines. Then on the next display frame, it interlaces (places a line in between two existing lines) the next picture frame along with the existing one onscreen. This provides additional vertical resolution.

TEXT ON A GRAPHICS SCREEN

The normal operating mode of this system is GRAPHICS. Text is simply treated as a special form of graphics element.

On a 320 element horizontal line (normal resolution mode) you can generally expect to use 40 characters of text. In this mode, each character could be defined as a series of dots (pixels) in an 8 by 8 square matrix.

On a 640 element horizontal line (high resolution mode), you can use 80 characters of text per line. Again the same character set may be used.

Because text is simply a special case of graphics, you can easily mix multi-colored graphics and text on the same screen. You may use either the built-in character-set or your own special font. Likewise, the placement for text need not be limited to a hard-and-fast 40 or 80 positions on 25 fixed lines. A special purpose DMA channel called the blitter allows you to place any kind of graphics element anywhere on the screen.

This lets you use subscripts, superscripts, underlines, embedded special characters or graphics, proportional spacing and many other features for a what-you-see-is-what-you-get presentation to the computer user.