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Histopyramid

Histopyramid aka Histogram Pyramid provides an efficient way of converting sparse matrix (represented by a texture) into list of coordinates (points). Where each active cell in the matrix can contribute to 1 or more points. All operations of the algorithm can be executed in parallel hence are performed on the GPU.

Following set of methods implement Histopyramid operations as described in High‐speed marching cubes using histopyramids by Dyken C, Ziegler G, Theobalt C and Seidel H Link to the paper: http://olmozavala.com/Custom/OpenGL/Tutorials/OpenGL4_Examples/MarchingCubes_Dyken/Dyken_et_al-2008-Computer_Graphics_Forum.pdf

Methods​

buildHistopyramidBaseLevel(gl : WebGLRenderingContext, opts : Object) : Object​

Takes an input texture, and builds base level (level 0) histogram pyramid. Also packs 4 weights of a 2X2 area into a single RGBA channel of base level texture.

  • gl - WebGL context.
  • opts (Object=) - options
    • texture (Texture2D) - Source texture for which base histopyramid level to be built.
    • channel (String, Optional, Default: r) - Pixel channel of source texture to be used in building histopyramid.

Returns an object with following fields.

  • baseTexture (Texture2D) - Base level texture object.
  • flatPyramidTexture (Texture2D) - Texture with enough size, that can hold all levels of histopyramid. Base level texture is already copied at xy offset of (0, 0).
  • textureData (ArrayBuffer, Optional) - Array containing data of base level texture. Returned only when base level texture size is 1X1.

getHistoPyramid(gl : WebGLRenderingContext, opts : Object) : Object​

Takes an input texture, and builds all levels of histogram pyramid. All levels are RGBA textues, where each pixels packs 4 weights of a 2X2 texture region.

  • gl - WebGL context.
  • opts (Object=) - options
    • texture (Texture2D) - Source texture for which base histopyramid level to be built.
    • channel (String, Optional, Default: r) - Pixel channel of source texture to be used in building histopyramid.

Returns an object with following fields.

  • pyramidTextures (Array) - Array of Texture2D objects representing all mip levels of the histopyramid.
  • flatPyramidTexture (Texture2D) - Texture2D object with all the mip levels laid out horizontally, with base level texture at the origin. Size of this texture is same as input texture.
  • levelCount (Number) - Number of mip levels generated.
  • topLevelData (ArrayBuffer) - Array containing data from the last (1X1) mip level. This contains the number of points that can be generated from this texture.

histoPyramidGenerateIndices(gl : WebGLRenderingContext, opts : Object) : Object​

Takes an input texture, and generates a Buffer object with list of points that correspond to non zero values of the texture.

  • gl - WebGL context.
  • opts (Object=) - options
    • texture (Texture2D) - Source texture for which base histopyramid level to be built.
    • channel (String, Optional, Default: r) - Pixel channel of source texture to be used in building histopyramid.

Returns an object with following fields.

  • locationAndIndexBuffer (Buffer): Buffer object contains coordinate data of generated points. Each point is represented 4 floats (XYZW). Where X contains x coordinate, Y contains y coordinate and Z contains local key-index and W contains key-index. x and y coordiantes of the point are with in ((0,0) to (width, height)] range, where width and height are dimensions of source texture object.