Level of Detail How to make games run fast and look good

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Level of Detail(How to make games run fast and
look good)
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Remember this?
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What went wrong?

• Simple answer
• Aliasing due to undersampling.
• Can be fixed/ameliorated by supersampling
• Somewhat longer answer
• Ratio between pixels and texels changes as we
  move backwards.
• In effect, the texels get too big.

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The result
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Mipmaps

• Why Mipmaps?
• map is the easy part vide colour maps,
  bumpmaps, texture maps, etc.
• Mip multum in parvo (many/much in little).

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Storing Mipmaps
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So, what has changed?

• Well, that depends.
• If you make the smaller maps naively, then not
  much.
• If you filter the maps as you create them then
  you only need to do it once, and on a restricted
  area.

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Filtering Mipmaps

• Simple approach is to filter the mipmaps as you
  create them.
• Depending on the approach, you can either create
  each successive level from the one before, or
  derive them all from the original (L0) texture.

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Mapping pixels to texels

• Nearest texel leads to pixelation and jagged
  edges.
• Calculate average of 2x2 square surrounding texel
  position.
• Linearly interpolate between those same 4 pixels.

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Using Mipmaps

• L log2 (texture size / polygon size).
• If L lt 0.0, texture smaller than polygon and
  magnification filter is used.
• If L gt 0.0, texture is larger than polygon and
  mipmapping is used where L specifies mipmap level.

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Choosing Mipmap Level

• Can use previous formula.
• Can also interpolate between mipmap levels. This
  leads to tri-linear filtering.

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Mipmaps and OpenGL (1)

• First load textures for every mipmap level using
  glTexImage2D().
• Be careful if you forget one of the levels,
  mip-mapping is disabled by OpenGL without any
  warning.
• Level 0 has the highest resolution.

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Mipmaps and OpenGL (2)

• Generating smaller versions potentially produces
  aliasing ? filtering.
• GLU offers automatic mipmap generation with
  filtering.
• gluBuildMipMaps() constructs the pyramid and
  calls glTexImage() for each level to load the
  image.

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Mipmaps and OpenGL(3)

• OpenGL tries to pick best mipmap level and texel
  within that level.
• GL_NEAREST (Point Sampling) pick the texel with
  nearest centre.
• GL_LINEAR (Bilinear Sampling) weighted average
  of 2x2 closest texels.

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Mipmaps and OpenGL(4)

• GL_NEAREST_MIPMAP_LINEAR average nearest texels
  from two mipmap levels.
• GL_LINEAR_MIPMAP_LINEAR (Trilinear) average two
  averaged texels from two mipmaps.

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