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VDP1 (Saturn)

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VDP1 (Saturn)

Video Display Processor 1 (VDP1) is one of two graphics processing units housed within the Sega Saturn console. It is a custom integrated circuit used only in Saturn hardware, and the Saturn-derived Sega Titan Video arcade hardware.

VDP1 is typically responsible for rendering all "foreground" graphics, such as sprites and polygons (quads) along with texture mapping and Gouraud shading. It works in conjunction with a second graphics processor, VDP2, to render an image onto the screen.


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The Sega Saturn is equipped with dual custom VDP chips for graphics processing. The VDP 1 chip is primarily responsible for sprite generation. Polygon generation is accomplished through manipulation of the sprite engine. Texture mapping and Gouraud shading is also handled by the VDP1. The VDP1 relies on a dual frame buffer that handles rotation and stores textures in special 512K cache.

Both the VDP1 and VDP2 32-bit background and scroll plane video display processor have direct access to the both SH2s, as well as direct memory access (DMA) to both the main and video RAM.

Overview

The VDP1 was based on the Sega Model series, with a quad polygon engine based on the Model 1, along with the Model 2's textured mapping capability. However, the way it maps textures onto quads is different from the Model 2. While the Model 2 maps a texture across multiple polygons, the VDP1 maps a single texture onto a single polygon. This allows the Saturn to render sprites and texture-mapped polygons in the same manner.Media:ST-013-R3-061694.pdf[1] The VDP1's texture-mapped polygons are thus also similar to the Sega System 32's textures.

The VDP1 is capable of drawing more polygons than the Model 1, but less than the Model 2. The VDP1 renders quad polygons, which, despite being used by the most powerful gaming system at the time (Sega Model 2 arcade system), was not industry standard for 3D graphics, compared to the more widely used triangle polygons. The VDP1's quad polygons are drawn with forward texture mapping (a form of perspective correction), bilinear approximation (reduces texture warping), and medium polygon accuracy (resulting in seamless polygons).[2]

The VDP1 had issues with transparency. The VDP1 supports per-pixel transparency between different polygons/sprites in the VDP1 framebuffer, or between VDP1 and VDP2 layers, but not both at the same time, with the VDP2's transparency overriding the VDP1's transparency. In addition, the VDP1 takes six times longer to draw transparent pixels than opaque pixels.Media:ST-013-R3-061694.pdf[3] There are several ways to overcome the VDP1's transparency issues. The most common method used by Saturn games is to fake transparency with dithering, using a mesh that gets blended by a television's Composite or S-Video cable. Another method is to use the VDP2's hardware transparency, by using a VDP2 bitmap layer as an additional transparent framebuffer, copying transparent assets from the VDP1 framebuffer to a VDP2 bitmap framebuffer layer (e.g. the transparent polygons in Burning Rangers). Another method possible is software transparency, programming the CPU with software code, but there are no known games to do so.

The Saturn's VDP1 was the basis for Nvidia's first graphics processor, the NV1, which was one of the first 3D graphics accelerators on PC, released in 1995. Like the Saturn, it uses quad polygons and supports forward texture mapping with limited perspective correction, and several Saturn ports are available for it. However, the NV1 has a fillrate of 12.5 MPixels/s and a rendering performance of 50,000 polygons/sec, less than the VDP1's 28.4375–57.2728 MPixels/s fillrate and 500,000 polygons/sec rendering performance. In comparison, the most powerful PC graphics card of 1995, Yamaha's Tasmania 3D, which was based on triangle polygons, had a 25 MPixels/s fillrate and 300,000 polygons/sec rendering throughput, more than the NV1, but less than the VDP1.

The VDP1's 3D polygons could be programmed to utilize per-vertex colored lighting and RGB point lights, such as in Panzer Dragoon Saga.

Technical specifications

Errata

VDP1 transparency rendering feature causes strips of pixels to be rewritten to framebuffer for 2-point (scaled) and 4-point (quadrangle) "sprites", applying the transparency effect multiple times. It can be seen in a multitude of commercial games (Two examples: Robotica explosions or Smoke, Loaded shadows and a lot of particles.).

The VDP1 supports per-pixel transparency between different polygons/sprites in the VDP1 framebuffer, or between VDP1 and VDP2 layers, but not both at the same time, with the VDP2's transparency overriding the VDP1's transparency. There are several ways to overcome this issue. The most common method used by Saturn games is to fake transparency with dithering, using a mesh that gets blended by a television's Composite or S-Video cable. Another method is using a VDP2 bitmap layer as an additional transparent framebuffer, copying transparent assets from the VDP1 framebuffer to a VDP2 bitmap framebuffer layer (e.g. the transparent polygons in Burning Rangers). Another method possible is software transparency, programming the CPU with software code, but there are no known games to do so.

Another issue is that the VDP1 takes six times longer to draw translucent pixels than opaque pixels.Media:ST-013-R3-061694.pdf[3] The VDP2, in comparison, has no issues with transparency, nor does the use of transparency affect the VDP2's performance. The VDP1 can overcome its transparency performance issue by drawing paletted sprites/polygons, which use the VDP2's palette and transparency, without affecting performance. However, this wouldn't allow multi-layered transparency between sprites/polygons, which would require the use of the methods mentioned above.

Foototes

  1. 57.27272 MB/s per bus
  2. 28.63636 MHz texture cache VRAM, 28.63636 MHz draw/render framebuffer, 28.63636 MHz display/erase framebufferMedia:ST-013-R3-061694.pdf[10]
  3. 1 cycle per pixelMedia:ST-013-R3-061694.pdf[7]Media:ST-013-R3-061694.pdf[32]Media:ST-013-R3-061694.pdf[22]
  4. 28.63636 MPixels/s draw, 14.418 MPixels/s erase/writeMedia:ST-013-R3-061694.pdf[33]Media:ST-013-R3-061694.pdf[34]
  5. 164,576 Gouraud-shaded 10×10 polygons/sec: 57.27272 million parallel bus cycles/sec, 248 cycles overhead per polygon (16 cycles command table fetch,Media:TUTORIAL.pdf[35] 232 cycles Gouraud shading),Media:TUTORIAL.pdf[36] 348 cycles per polygon (100 cycles drawing per 100-pixel polygon)[37]
  6. Takes six times longer when using VDP1's RGB mode.Media:ST-013-R3-061694.pdf[3] When using VDP2 palette mode, the VDP1 draws shadowed/translucent objects at full speed.
  7. Drawing process is asynchronous,Media:TUTORIAL.pdf[35] commands/textures read from texture cache and pixels/texels written to rendering framebuffer in parallel (57.27272 million parallel bus cycles/sec),Media:ST-013-R3-061694.pdf[38] 148 textured 504×255 polygons/sec (386,905 parallel cycles per polygon), 136,363 textured 10×10 polygons/sec (420 parallel cycles per polygon), 189,644 textured 8×8 polygons/sec (302 parallel cycles per polygon)Media:TUTORIAL.pdf[36]
  8. 57.27272 million parallel bus cycles/sec, 147 textured 504×255 polygons/sec (387,137 parallel cycles per polygon), 87,841 textured 10×10 polygons/sec (652 parallel cycles per polygon)Media:TUTORIAL.pdf[36][37]
  9. Flat shading: 16 cycles per polygon in 28.63636 MHz texture cache,Media:TUTORIAL.pdf[35] 1 cycle per pixel in 28.63636 MHz framebuffer
  10. 57.27272 million parallel bus cycles/sec, 248 cycles overhead per polygon (16 cycles command table fetch,Media:TUTORIAL.pdf[35] 232 cycles Gouraud shading),Media:TUTORIAL.pdf[36] 32 cycles drawing per 32-pixel polygon[37]
  11. 57.27272 million parallel bus cycles/sec, 171 parallel cycles per polygonMedia:TUTORIAL.pdf[36]
  12. 200,000 texture-mapped polygons/sec,[39][5] 57.27272 million parallel bus cycles/sec, 285 parallel cycles per polygonMedia:TUTORIAL.pdf[36]
  13. 57.27272 million parallel bus cycles/sec, 403 parallel cycles per polygonMedia:TUTORIAL.pdf[36][37]

References

  1. 1.0 1.1 1.2 1.3 File:ST-013-R3-061694.pdf
  2. 2.0 2.1 Sega Saturn 3D Capabilities
  3. 3.0 3.1 3.2 3.3 3.4 3.5 File:ST-013-R3-061694.pdf, page 110
  4. 4.0 4.1 4.2 File:Sega Service Manual - Sega Saturn (PAL) - 013-1 - June 1995.pdf
  5. 5.0 5.1 5.2 5.3 5.4 File:NextGeneration US 24.pdf, page 64
  6. Sega Saturn (MAME)
  7. 7.0 7.1 7.2 7.3 File:ST-013-R3-061694.pdf, page 52
  8. Sega Saturn hardware notes (2004-04-27)
  9. File:Sega Service Manual - Sega Saturn (PAL) - 013-1 - June 1995.pdf, page 34
  10. File:ST-013-R3-061694.pdf, page 6
  11. File:TUTORIAL.pdf, page 11
  12. 12.0 12.1 File:ST-TECH.pdf, page 147
  13. 13.0 13.1 13.2 STV VDP1 (MAME)
  14. File:ST-TECH.pdf, page 135
  15. File:ST-013-R3-061694.pdf, page 34
  16. File:ST-238-R1-051795.pdf
  17. 17.0 17.1 File:ST-238-R1-051795.pdf, page 232
  18. ST-013-R3-061694.pdf
  19. File:ST-013-R3-061694.pdf, page 24
  20. File:ST-058-R2-060194.pdf, page 177
  21. File:ST-013-R3-061694.pdf, page 149
  22. 22.0 22.1 22.2 File:ST-013-R3-061694.pdf, page 64
  23. Sega Saturn Tech Specs
  24. Sega System 24 Hardware Notes (2013-06-16)
  25. File:ST-013-R3-061694.pdf, page 119
  26. File:ST-013-R3-061694.pdf, page 18
  27. File:ST-013-R3-061694.pdf, page 29
  28. File:ST-013-R3-061694.pdf, page 81
  29. 29.0 29.1 File:ST-013-R3-061694.pdf, page 40
  30. 30.0 30.1 File:ST-013-R3-061694.pdf, page 75
  31. File:ST-013-R3-061694.pdf, page 39
  32. File:ST-013-R3-061694.pdf, page 35
  33. File:ST-013-R3-061694.pdf, page 65
  34. File:ST-013-R3-061694.pdf, page 61
  35. 35.0 35.1 35.2 35.3 File:TUTORIAL.pdf, page 15
  36. 36.0 36.1 36.2 36.3 36.4 36.5 36.6 File:TUTORIAL.pdf, page 8
  37. 37.0 37.1 37.2 37.3 37.4 Saturn VDP1 hardware notes (2003-05-17)
  38. File:ST-013-R3-061694.pdf, page 44
  39. 39.0 39.1 File:SegaVisions US 24.pdf, page 14
  40. File:ST-013-R3-061694.pdf, page 41