■ 3.4 VDP2

VDP2 displays the scroll screen and determines the priority of the full screen (including sprites). The simultaneous display of the scroll screen has been expanded to a maximum of 5 screens, and the screen can be moved up / down / left / right and rotated. The priority (display priority) can be set programmable for each character.

● System configuration

VRAM is connected to VDP2 and has built-in color RAM. Image data is defined in VRAM and color RAM from the CPU via the SCU.
The data defined in VRAM is read according to the register settings and becomes the image data of each scroll screen. The image data of each scroll screen and the image data sent from VDP1 and the external image circuit are displayed in display priority according to the register settings and become the display image data. Furthermore, the display image data is converted to display color data and output to the TV (Fig. 3.12).

Figure 3.12 VDP2 system configuration

Table 3.7 VDP2 system specifications

No	Item	specification	remarks
1	TV screen	・ Select from horizontal resolution 320, 352, 640, 704 pixels ・ Vertical resolution 224, 240, 256 pixels (when non-interlaced) 448, 480, 512 pixels (when interlaced) Select from	Vertical resolutions 256 and 512 pixels are PAL only
2	charactor	-Select from character size 1 x 1 cell, 2 x 2 cell-Select from 16, 256, 2048, 32768, 16.77 million character colors	Bitmap method is also possible
3	Normal scroll surface	・ Maximum number of simultaneous display screens: 4 screens ・ Vertical and horizontal screen scrolling possible ・ Line scrolling possible ・ Vertical cell scrolling possible ・ 1/4 to 256 times enlargement / reduction possible ・ Mosaic function
Four	Rotating scroll surface	・ Maximum number of simultaneous display screens: 2 ・ Enlargement / reduction rotation is possible ・ Rotation parameters can be switched on the screen ・ Special screen processing using a coefficient table	When two screens are displayed, the normal scroll plane cannot be displayed.
Five	window	・ 2 normal windows ・ 1 sprite window ・ Line window possible
6	priority	-The priority of each screen can be specified programmable.-The priority can be changed in character units and dot units.
7	Screen processing	・ Color calculation for up to 4 screens is possible ・ Color calculation ratio 32 steps ・ Color offset function ・ Shadow function

● Function

VDP2 has a scroll function that controls the scroll screen display and a priority function that determines the display priority.

Figure 3.13 Scrolling, priority function

Scroll function ────┬─ Enlargement / reduction　
├─ Line scroll ├─ Vertical cell scroll ├─ Mosaic processing　
└─ Window　
　
Priority function ──┬─ Color calculation　
├─ Color offset └─ Shadow　

● Scroll screen

Scrolling has a scroll screen for displaying pictures and a window for controlling the display. Furthermore, there are two types of scroll screens: a normal scroll screen that allows you to change the number of screens that can be displayed, and a rotary scroll screen that allows you to rotate and display.
Table 3.8 shows the functions of the normal scroll screen and the rotary scroll screen, and the number of character colors.

Table 3.8 Scroll screen functions

function	Normal scroll screen				Rotating scroll screen
	Screen 0	Screen 1	Screen 2	Screen 3	Screen 0	Screen 1
Scale function	1/4 times to 256 times		none		Possible at any magnification
Rotation function	none				can be
Line scroll function	can be	can be	none	none	none
Vertical cell scroll function	can be	can be	none	none	none
Mosaic processing function	can be				Yes (horizontal only)
Bitmap display	It can be displayed	It can be displayed	Cannot be displayed	Cannot be displayed	It can be displayed	Cannot be displayed
Number of character colors	16 colors 256 colors 2048 colors 32768 colors Choose from 16.77 million colors	16 colors 256 colors 2048 colors Select from 32768 colors	16 colors Select from 256 colors	16 colors Select from 256 colors	16 colors 256 colors 2048 colors 32768 colors Choose from 16.77 million colors	16 colors 256 colors 2048 colors 32768 colors Choose from 16.77 million colors

◆ Enlargement / reduction function

You can scale the entire screen horizontally and vertically. When displaying in a reduced size in the horizontal direction, there are restrictions such as reducing the number of faces.

◆ Line scroll function

With the function of scrolling up / down / left / right for each line and scaling in the horizontal direction, you can give a sense of distance to the road in a drive game, for example.

◆ Vertical scroll function

It is a function that allows you to scroll up and down in units of cells in the horizontal direction, and you can add depth to, for example, vertical scrolling games.

◆ Mosaic function

Each scroll screen is divided into the specified horizontal and vertical directions, and the color of the dot on the upper left of each area is displayed on all the dots in that area.

Figure 3.14 Mosaic pattern

◆ Rotation function

• Rotation display The rotation scroll plane has a rotation of the coordinate axes (X, Y, Z axes) and a rotation of the screen axis perpendicular to the TV screen. Also, up to 2 screens can be displayed at the same time.

  Figure 3.15 Image change due to rotation axis
  

  Figure 3.16 Image change due to screen axis
  

• Rotation calculation Since the hardware performs the rotation calculation according to the specified parameters, it is possible to display the rotation without putting too much load on the CPU. In addition, since the coordinates can be calculated by multiplying each dot by a different value, it is possible to display a distorted image.

• Simultaneous display by splitting the screen With this function, you can display an image that looks like it is displayed on two sides just by displaying the rotating scroll surface on one side.

◆ Scroll screen configuration

There are two screen formats for scrolling screens: cell format and bitmap format. The cell format is a method of displaying cells side by side in the same way as a conventional home-use game machine, and the bitmap format is a method of displaying a picture corresponding to each dot on the screen, similar to a personal computer.

• Cell format The cell format scroll screen is a "cell" that is a picture pattern of 8 dots horizontally and 8 dots vertically, a "character pattern" that arranges cells, a "page" that arranges character patterns, and a "plain" that arranges pages. , It consists of a "map" of planes. Figure 3.17 shows the configuration of the cell-type scroll screen.

  Figure 3.17 Cell-style scroll screen
  

• Bitmap format The bitmap format scroll screen consists of bitmap patterns measuring 512 dots or 1024 dots horizontally and 256 dots or 512 dots vertically. Figure 3.18 shows the configuration of the scroll screen in bitmap format.

Figure 3.18 Relationship between bitmap format scroll screen and data settings

512 horizontal dots x 256 vertical dots Horizontal 512 dots x Vertical 512 dots Horizontal 1024 dots x Vertical 256 dots Horizontal 1024 dots x Vertical 512 dots

◆ Window

Windows are classified into three types according to the area specification (coordinate specification) method.

• Normal rectangular window Specified by the coordinates of two points, the start point and the end point

• Normal line window Specified by the coordinates of the start point and end point of each line

• Sprite window Specified by sprite character pattern

  Figure 3.19 window
  

● Priority function The display priority of sprites and scroll screens is determined by the 3-bit priority number. You can set up to eight values for the sprite priority number, one of which is specified on a character-by-character basis.

◆ Judgment of priority

The priority number of the scroll screen is usually specified in units of faces. (If you use the special priority function, you can change it in character units or dot units.)

Figure 3.20 Priority function

◆ Special priority function

The priority number corresponding to each scroll screen can be changed in character units or dot units. By using this function to change the priority of only a certain part of the scroll screen, you can make one scroll screen look like multiple screens.

◆ Color calculation function

The color calculation function adds the color data of multiple screens to create the effect that the back screen can be seen through the front screen. Normally, it is performed on two screens, the top image and the second image, but if you use the extended color calculation function, you can perform up to four screens.

Figure 3.21 Color calculation function

◆ Insert line color screen

The line color screen is a screen that forcibly inserts the specified screen as the second image of the part that is the top image and performs color calculation. At that time, in the part where the line color screen is inserted, the second image before insertion becomes the third image, and the third image becomes the image below it (force image). Figure 3.22 shows the insertion of a line color screen.

Figure 3.22 Inserting a line color screen

◆ Shadow function

The shadow function is a function that casts a shadow in the shape of a sprite character on each screen.

Figure 3.23 Shadow function

◆ Blur calculation function

The blur calculation function is a function that adds the horizontal color data of a specified screen at a fixed rate, and can produce the effect of blurring a distant background.

Figure 3.24 Blurring calculation function

For the area where the specified screen is the top image or the second image, the addition result of the color data is forcibly set to the second image. By performing color calculation on this second image and top image, you can display a picture that has been blurred.

◆ Color offset function

The color offset function is a function that adds (subtracts) the offset value to the color data on the screen and displays it, and is used for fade-in and fade-out. You can specify whether to use the color offset function for each screen.