Computer Graphics Foundations 2010.1 first assignment

Students:Alexandre Valdetaro - xvaldetaro@gmail.com

Gustavo Nunes – xnunes@msn.com

Professor:Marcelo Gattass

Project Objectives:

• Visualization in the color spaces:CIE XYZ, CIE Lab, sRGB e XYZProjected, of the following color groups:– Visible Colors.– sRGB space representable colors– Munsell– Kodak Minilab

Implemented Software

• A software for visualization and conversion of all the desired color groups in all of the color spaces.

• XNA framework has been used.• All the screenshots from the slides have been

taken from the software.

Visible Colors

• The raw data of the spectral pure colors has been converted to CIE XYZ in two different ways:– Setting Y as a fixed value– Setting Y as y*(fixed value)

*The first conversion has been called the XYZ and the second XySlashZ

Visible Colors• With the 401 pure color points a fan

tesselation with the black and the white was done.

Fan tesselation

Visible Colors• With the fan tesselated mesh, a shoestring

type tesselation refined the mesh

Visible Colors

• With the densely tesselated mesh, the conversion to other color spaces used the formulas from:

www.brucelindbloom.com

sRGB representable colors

• A Cube mesh has been created in sRGB space, with its vertices: (0,0,0), (1,0,0),(0,1,0),(0,0,1),(1,1,0),(1,0,1),(0,1,1),(1,1,1)

• The cube has been then tesselated acording to the refine level available in the user interface

• The densely tesselated mesh has then been converted to other color spaces using www.brucelindbloom.com

sRGB representable colors

Simple refinement using baricentric coordinates

Munsell colors

• The raw data was a Point Cloud in CIE Lab space.

• Only the Hull is needed to make the mesh.• To Obtain the hull, the segment maxima

algorithm[Morovic and Luo 2000] has been used with discretization 16x8

• The mesh produced with the hull was then converted to other spaces using www.brucelindbloom.com

Munsell Colors• The point cloud and the aproximation of the

segment maxima as the wireframe mesh

Kodak Minilab

• Used the same heuristic of Munsell, except that the color used to render each vertex of the mesh came with the Kodak raw data in sRGB, so the Lab position was only used for geometry.

Kodak Minilab• The point cloud and the aproximation of the

segment maxima as the wireframe mesh

XYZProjected• Represents the XYZ values projected on the plane

X + Y + Z• Widely used representation of the visible

spectre(Horseshoe)

Known Problems• The conversion from XYZ to lab can cause artifacts if the mesh

has very thin triangles. • Suppose one has a triangle made with the white point and to

pure color points A and B that are so close to each other that the triangle can be considered a line(happens in the edges of the spectrum)

• When refining the tesselation, suppose LA the line between the white and the point A, and LB between white and point B.

• It is desired to segment these lines and triangulate them.• For sake of clarity the LA point will be represented with an x

and LB points with an o.

Known Problems• After segmented, the points are connected

clockwisely

Known Problems• When converting to Lab supposing the points are so

close that LA = LB, the lines would look coarsely like this:

XYZ Lab

Known Problems• When connecting the point some counter clockwise triangles

will appear. • Ex: the triangle LA3,LB4,LA4 is clockwise in XYZ and gets

counter clockwise in Lab

Known Problems

• sRGB will have negative valors for the full visible spectrum.

• By the formula, any value below a fixed constant close to zero should use a linear function.

• So negative values with module greater than the constant should be treated not linearly

Known Problems

• The Lab values from the Kodak Minilab raw data does not match with the sRGB values also provided in the raw data.

Known Problems• The segment maxima algorithm can generate artifacts

depending on the distribution of the points.• The first image is the expected result when connecting

the points of each segment• The second image shows a counter clockwise triangle

END