Figure 3.2 VDP1 system configuration

• Drawing parts (character lines)
• Specifying color mode
• color calculation
• mesh processing
• Specifying clipping and relative coordinates
• Frame buffer display control
• Display method

 Classification

 Part name

 function

 Definition method

 parts

 texture parts

 shaped sprite

 charactor,
 Flip vertically and horizontally

 1 vertex read direction

 Scaling sprite

 charactor,
 Up/down, left/right flip,
 Can be scaled and expanded

 2 vertices readout direction or immobile
 Point, width and readout direction

 deformed sprite

 charactor,
 Up/down, left/right flip,
 Scaling, stretching,
 Can be rotated and twisted

 4 vertices readout direction

 Non-texture parts

 polygon

 square,
 Fill in the inside

 4 vertices

 polyline

 square

 4 vertices

 line

 straight line

 start and end points

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◆Texture parts

Sprites draw character patterns. Character patterns are defined in VRAM using pixel data as a character pattern table.

• stylized sprite
  It's a normal rectangular sprite. You can flip the original picture pattern vertically and horizontally. In addition, it is possible to flip vertically and horizontally in any sprite mode (Figure 3.3).

  Figure 3.3 Regular sprite
  

• Scaling sprite
  It is a sprite that can be enlarged or reduced, and it is possible to zoom only in the vertical and horizontal directions (Figure 3.4).

  Figure 3.4 Scaling sprite
  

• deformed sprite
  A deformable sprite. By specifying the coordinates of the four vertices of the character, you can transform the original picture into any shape you want, so you can enlarge, reduce, rotate, and flip it as you wish. If you look at this as a polygon, you can say it is the same as a texture-mapped polygon.

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◆Non-texture parts

• polygon
  It is a polygon with 4 vertices. The difference from sprites is that the inside of the plane surrounded by four points is filled with a single color. The ones with the original picture are sprites, and the ones without are polygons.
• polyline
  It is a rectangle with four connected lines.
• line
  Draw a line between two points in a single color.

  Figure 3.6 Polygon, polyline, line
  

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●Color mode specification
There are three ways to specify the color mode of texture parts: color bank method, RGB code method, or color lookup method. Color specifications for non-texture parts include pixel data.

• Combine the 16, 64, 128, and 256 color palette codes with color banks to refer to the colors stored in VDP2's color RAM.
• Select the VDP2 color palette in the color bank, and select a color from the color palette with the palette code.
• 16, 64, 128, and 256 colors can be expressed within one character.
• Data written using the color bank method is processed on the VDP2 side by dividing it into function bits: color calculation, priority, color bank, and palette code.

Figure 3.7 Configuration of color bank method

MSB LSB
┌──┬──┬──────────┬──────────────┐
│  │  │Color bank│ Palette code │
└──┴──┴──────────┴──────────────┘
 │  │　
 │  └─→Priority　
 ↓　
color calculation　

◆RGB code generation

Colors are expressed using 5 bits of brightness for each RGB (red, green, blue). 32,768 colors can be expressed within one character.

◆Color lookup table

Define 16 colors in the color lookup table and select a color from among them. One color is 16 bits and can be either an RGB code or a color bank code.

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●Color calculation
VDP1 allows you to specify Gouraud shading, shadow, semi-brightness, and translucent color operations.
Table 3.5 shows the types of color operations.

 kinds

 Contents

 half brightness

 Draws the original image with half the brightness to the frame buffer. On the base
 Since it is overwritten, the background will not be visible and the brightness of the original image will be halved.

 Translucent

 Add the background brightness halved and the original image brightness halved,
 Draw the result to the framebuffer.

 shadow

 Redraw the background with half the brightness to the frame buffer. this
 In this case, a shadow will be created in the shape of the character in the original picture. Also, the characters in the original picture are shadows.
 Only used for the shape, color data is ignored.

 gouraud shading

 Draw the original image with Gouraud shading to the frame buffer.
 To do.

 gouraud shading
 Translucent

 Gouraud shading was applied to the original picture, but the brightness was further reduced to half,
 The brightness of the background is halved and added. the result in the frame buffer
 Draw on.

 gouraud shading
 half brightness

 I applied Gouraud shading to the original picture, but further reduced the brightness to half,
 Draw to the framebuffer.

◆Gouraud shading
Gouraud shading can only be applied to parts drawn in RGB. Gouraud shading is a method that interpolates colors between polygon vertices to make a plane look like a curved surface.
A brightness correction value is given to each of the four vertices of the polygon, and Gouraud shading is applied between these four vertices to make it look like a curved surface. In addition to polygons, you can also apply Gouraud shading to polylines and lines. Figure 3.8 shows an example of Gouraud shading.

Figure 3.8 Example of Gouraud shading

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●Mesh processing
You can apply a mesh to all parts. When you apply a mesh, the part is drawn in a grid pattern every other dot.
If the "X coordinate value + Y coordinate value" is an even number, it will be drawn, and if it is an odd number, it will not be drawn and will be skipped. (Figure 3.9)

Figure 3.9 Example of mesh processing

◆System clipping

System clipping is always enabled when drawing, and the area inside the set area is drawn.
The settings are fixed at the top left coordinates (0,0) of the screen, and can be set by specifying the bottom right coordinates. (Figure 3.10)

Figure 3.10 System clipping

◆User clipping

User clipping can be selected in the software, and you can enable user clipping for each part or select the effective area for user clipping to be outside or inside the set area.
The user clipping area can be set by specifying each vertex of the upper left coordinate and lower right coordinate. (Figure 3.11).

Figure 3.11 User clipping

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●Frame buffer
The frame buffer is divided into two parts: the display frame buffer and the drawing frame buffer.
Read/write access from the SCU to the frame buffer is performed only to the drawing frame buffer. The display frame buffer is the back bank and cannot be accessed.
Also, by reading the frame buffer, the entire frame buffer plane can be enlarged, reduced, or rotated by giving displacement in the X and Y directions that specifies the read start coordinates and where the next read dot should be. .

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●Display method
For display, a TV, which is a common display device, is used. The TV standard in Japan and America is NTSC. Europe uses PAL system.
TV display is performed by reading data from the beginning of the frame buffer every frame (one frame is 1/60 seconds).
Normally, one frame is one field, but with interlacing, one frame can be made into two fields, and the vertical resolution can be doubled (one frame is 1/30 second). There are two types of interlace: double-dense interlace and single-dense interlace, as shown in Table 3.6.