Figure 3.2 VDP1 system configuration

• Drawing of parts (character lines)
• Color mode specification
• Color calculation
• Mesh processing
• Specifying clipping and relative coordinates
• Frame buffer display control
• Display method

 Classification

 Part name

 function

 Definite law

 parts

 Texture parts

 Fixed sprite

 charactor,
 Up / down, left / right inversion

 Read direction of 1 vertex

 Scale sprite

 charactor,
 Up / down, left / right inversion,
 Can be enlarged / reduced and expanded / contracted

 Read direction of 2 vertices, or fixed point, width and read direction

 Deformed sprite

 charactor,
 Up / down, left / right inversion,
 Enlargement / reduction, expansion / contraction,
 Can be rotated and twisted

 4 Vertex read directions

 Non-textured parts

 polygon

 Rectangle,
 Fill the inside

 4 vertices

 Polyline

 Rectangle

 4 vertices

 line

 Straight line

 Start and end points

▲

◆ Texture parts

Sprites draw character patterns. Character patterns define pixel data as a character pattern table in VRAM.

• Standard sprite This is an ordinary rectangular sprite. You can flip the original picture pattern up, down, left, and right. In addition, it can be flipped vertically and horizontally in any sprite mode (Fig. 3.3).

  Figure 3.3 Fixed form sprite
  

• Scale sprite A sprite that can be scaled up and down, and can be zoomed only in the vertical and horizontal directions (Fig. 3.4).

  Figure 3.4 Scale sprites
  

• Deformed sprite A deformable sprite. By specifying the coordinates of the four vertices of the character, the original picture can be transformed into any shape, so you can enlarge, reduce, rotate, and invert as you like. If you look at this as a polygon, you can say that it is the same as a texture-mapped polygon.

▲
◆ Non-texture parts

• Polygon 4 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 one with the original picture is the sprite, and the one without it is the polygon.
• It is a quadrangle with four poly lines connected.
• Line to draw the line in a single color in between the two points.

  Figure 3.6 Polygons, polylines, lines
  

▲
● Color mode specification There are color bank method, RGB code method, and color lookup method for color mode specification of texture parts. The color specification for non-textured parts has pixel data.

• The color is referenced in the color RAM of VDP2 by combining the color bank with the palette code of 16, 64, 128, 256 colors.
• Select the VDP2 color palette in the color bank and select a color from the color palette in the palette code.
• Within one character, you can represent 16, 64, 128, 256 colors.
• The data written by the color bank method is processed by dividing it into the function bits of color calculation, priority, color bank, and palette code on the VDP2 side.

Figure 3.7 Color bank system configuration

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

◆ RGB code generation

Color is expressed by 5 bits of RGB (red, green, blue) brightness. 32,768 colors can be expressed in one character.

◆ Color look-up table

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

▲
● Color calculation With VDP1, you can specify Gouraud shading, shadow, semi-brightness, and semi-transparent color calculation.
Table 3.5 shows the types of color operations.

 kinds

 Contents

 Half brightness

 The brightness of the original picture is halved and drawn in the frame buffer. Since the background is overwritten, the background cannot be seen and the brightness of the original picture is halved.

 Translucent

 Add the one with the background brightness halved and the one with the original picture halved,
 Draw the result in the framebuffer.

 Shadow

 The background brightness is halved and redrawn in the frame buffer. In this case, a shadow in the shape of the character in the original picture is created. Also, the characters in the original picture are used only for the shadow shape, and the color data is ignored.

 Gouraud shading

 The original picture with Gouraud shading applied is drawn in the frame buffer.

 Gouraud shading translucent

 Gouraud shading is applied to the original picture, but the brightness is further reduced by half.
 Add the background brightness halved. Draw the result in the framebuffer.

 Gouraud shading half brightness

 Gouraud shading is applied to the original picture, but the brightness is further reduced by half.
 Draws in the framebuffer.

◆ Gouraud shading Gouraud shading can be applied only to parts drawn in RGB. Gouraud shading is a method of interpolating colors between the vertices of polygons to make a plane look like a curved surface.
Brightness correction values are given to each of the four vertices of the polygon, and Gouraud shading is applied between those four vertices to make it look like a curved surface. Gouraud shading can be applied not only to polygons but also to polylines and lines. Figure 3.8 shows an example of Gouraud shading.

Figure 3.8 Example of Gouraud shading

▲
● Mesh processing All parts can be meshed. When you apply the mesh, the parts are drawn in a grid pattern every other dot.
If "X coordinate value + Y coordinate value" is even, it will be drawn, and if it is odd, it will not be drawn and will be skipped. (Fig. 3.9)

Figure 3.9 Example of mesh processing

◆ System clipping

System clipping is always enabled when drawing, and the inside of the set area is drawn.
The setting is fixed at the upper left coordinate (0,0) of the screen, and can be set by specifying the lower right coordinate. (Fig. 3.10)

Figure 3.10 System clipping

◆ User clipping

User clipping can be selected by software, and user clipping can be enabled for each part, or the effective area for user clipping can be selected outside or inside the setting area.
The user clipping area can be set by specifying each vertex of the upper left coordinate and the lower right coordinate. (Fig. 3.11).

Figure 3.11 User Clipping

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

▲
● Display method For display, a TV, which is a general display device, is used. The domestic and American TV standards are NTSC. Europe is PAL system.
TV display is performed by reading data from the beginning of the frame buffer every frame (1 frame is 1/60 second).
Normally, one frame is one field, but you can make one frame into two fields by interlacing and double the vertical resolution (1 frame is 1/30 second). Interlaces include double-dense interlaces and single-dense interlaces, as shown in Table 3.6 (Table 3.6).