User:Izak/Design document

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In our world today , we observe that all solid objects are simply combinations of basic ones like cubes,spheres,cones,etc and we frequently have to manipulate objects which have surfaces such as gadgets and devices . With the explosion in ubiquitous technology, Computer-aided design (CAD) software help us create digital content in adverts and movies as well as visualize some solid objects like perfume bottles and shampoo before they are actually manufactured. In this project, we propose that BRL-CAD, which aspires to be the best CAD software, should incorporate a heart, a symbol of love, into its core functionality as one of its basic solid objects in order to increase its customer base and differentiate itself amongst its competitors. Indeed, this heart structure ( also called a heart primitive ) shall be used by those producing cartoons and designing electronic cards, gifts and presents during celebrations such as birthdays, weddings,family reunions, anniversaries and Valentine's day which deeply appeals to many individuals,families and communities.So during the summer, this heart primitive will be included into the raytrace library as a set of routines with corresponding support added to other parts of the source code.


Here, I give a detailed description of how the heart primitive for the BRL-CAD software will be implemented.

  • Firstly, the constants defined in the include/ headers like raytrace.h and rtgeom.h will be edited.
  • Secondly, hrt.c (the heart primitive code), a set of callback functions, will be written and added to the src/librt/primitives/hrt/ directory.
  • Lastly, associated support for the heart primitive will be added to other parts of the BRL-CAD package.

A. Editing include/ headers .


raytrace.h, a header file located in include/ which contains all the data structures and manifest constants necessary for interacting with the BRL-CAD ray-trace library , will be edited to serve the hrt.c file when included as follows;

  • For a chosen (by consensus with mentors) integer K , define ID_HRT and comment.
  • Define ID_HEART after the /*- ADD_BELOW_HERE-*/ comment which is after the ID_CONSTRAINT.
  • ID_MAX_SOLID and ID_MAXIMUM constants will be incremented.


rtgeom.h, a header file located in /include/ which contains details of the internal forms used by librt routines for different solids, will be modified as follows;

  • Add ID_HRT internal section by writing struct rt_heart_internal structure .
  • Write macro RT_HEART_CK_MAGIC(_p) BU_CKMAG(_p, RT_HEART_INTERNAL_MAGIC, "rt_heart_internal")

3.db.h and/or db5.h

db.h and/or db5.h,header files located in /include/ which define the internal database format, will be edited as follows;

  • Add heart solid record #define HRT K to db.h
  • In db.h , write hrt_rec structure containing pad, name , id , etc.
  • Edit db5.h for minor type brlcad heart define K , for a chosen integer K .

4. magic.h

magic.h, the header file in which magic numbers repose serves the BRL-CAD utilities and ray tracing libraries. This file will be edited as follows;

  • Add the RT_HRT_INTERNAL_MAGIC define .

B. Writing src/librt/primitives/hrt/hrt.c .

Ray tracing

A parametric surface can be created first. Starting from a volume of revolution defined by Euler angles φ (rotation around the z-axis) and θ (angle to the z-axis) and distance r defined below by

0° ≤ φ < 360°

0° ≤ θ < 180°

r(φ, θ) = 1 + 6 sin^2 (φ)(1 - θ/π) (θ/π)^2

At φ=0° and φ=180° the cross section is an unit circle.

At φ=90° and φ=270° the cross section is 1+6(1-θ/π)(θ/π)^2, which describes a lobe towards θ=135° .

The surface equations can be rewritten as follows ;

x(φ, θ) = cos(φ) sin(θ) r(φ, θ) = cos(φ) sin(θ) + 6 cos(φ) sin(θ) sin^2(φ) (1 - θ/π) (θ/π)^2 .

y(φ, θ) = sin(φ) sin(θ) r(φ, θ) = sin(φ) sin(θ) + 6 sin(φ) sin(θ) sin(φ)^2 (1 - θ/π) (θ/π)^2

z(φ, θ) = cos(θ) r(φ, θ) = cos(θ) + 6 cos(θ) sin^2(φ) (1 - θ/π) (θ/π)^2

and can be written using patch coordinates as

x(u, v) = cos(2πu) sin(πv) + 6 cos(2πu) sin(πv) sin^2(2πu) (1 - v) v^2

y(u, v) = sin(2πu) sin(πv) + 6 sin(2πu) sin(πv) sin(2πu)^2 (1 - v) v^2

z(u, v) = cos(πv) + 6 cos(πv) sin(2πu)^2 (1 - v) v2

Note: r(u,v) = 1 + 6 sin^2(2πu) (1 - v) v^2 and 0 < u,v < 1 .

These can be simplified and even approximated using Taylor series expansions of sine and cosine .These functions are continuously differentiable over their domains.

I intend to compute the normal vector (ray) using the cross product of the partial derivatives of the coordinate functions. That is,

∂ux(u,v) = ∂x(u,v)/∂u = -2π sin(2πu) sin(πv) (1 + 6 v^2 - 18 v^2 cos^2(2πu) - 6 v^3 + 18 v^3 cos(2πu)) ---------------------(i)

∂vx(u,v) = ∂x(u,v)/∂v = π cos(2πu) cos(πv) + 6π cos(2πu)cos(πv) sin^2(2πu) (1-v) v^2 - 6 cos(2πu) sin(πv) sin^2(2πu) v^2 + 12 cos(2πu) sin(πv) sin^2(2πu) (1-v) v --------(ii)

∂uy(u,v) = ∂y(u,v)/∂u = 2πcos(2πu)sin(πv) + 48 π^2cos^2(2πu)sin(πv)sin(2πu)(1 – v)v^2 ------------------------------ (iii)

∂vy(u,v) = ∂y(u,v)/∂v = - πcos(πv)cos(2πu) – 6πsin^3(2πu)cos(πv)(1 – v)v^2 + 6sin^3(2πu)sin(πv)(2 – 3v)v -------------------- (iv)

∂uz(u,v) = ∂z(u,v)/∂u = cos(πv)( 1 + 12πsin(4πu)(1 – v)v^2) ------------- (v)

∂vz(u,v) = ∂z(u,v)/∂v = - πsin(πv)( 1 + 6sin^2(2πu)( 1 – v)v^2) + 6sin^2(2πu)cos(πv)(2 – 3v)v ------------------- (vi)

where the vectors tu(u,v) = <∂ux(u,v) , ∂uy(u,v) , ∂uz(u,v)> and tv(u, v) = <∂vx(u,v) , ∂vy(u,v) , ∂vz(u,v)> are tangents to the surface of the heart . Thus, we obtain the vector perpendicular to the tangent ( which is the normal to the surface of the heart ) which is N(u, v) = p(u, v) / || p(u, v) || where p(u, v) = tu(u,v) X tv(u,v). The normal to the surface N(u,v) is the ray touching the heart from a particular point in 3D.


Here, I discuss how I intend to do visualization (geometric representation and analysis). I intend to do this by finding the intersection points between the line (light ray ) and the heart. We simply will write a line-heart test which tells us if a point on the heart surface intersects with the line. The points of intersection are those that satisfy the line equation and the equation for the surface of the heart at the same coordinates x, y, z. Here , we equate the heart equation and the equation of the line given below by X = o + dL where d > 0 , X = (x,y,z) is in R^3 and L is the direction of the line. To search for points that are on the line and on the heart means combining the equations and solving for d. When equations are combined , expanded and rearranged , we get a quadratic equation ad^2 + bd + c = 0 which can be solved using the quadratic formula. d = -b + sqrt(b^2 - 4 * a * c) / 2*a and d = -b - sqrt(b^2 - 4*a*c) / 2*a. If the value of b^2 - 4*a*c is less than zero, then it is clear that no solutions exist, that is, the line misses the heart. If it is zero, then exactly one solution exists, that is, the line just touches the heart at one point. If it is greater than zero, two solutions exist, and thus the line touches the heart in two points (the entry and exit point).

C. Additional support to other files.

1. table.c

table.c , a file located in the src/librt/primitives directory ,contains tables for the BRL-CAD Package ray-tracing library ( librt ). The following will be done to accommodate the heart primitive in table.c;

  • Declare a raytrace interface for the heart by RT_DECLARE_INTERFACE(hrt) .
  • Edit the rt_functab[] array , which indexes the different callback functions in hrt.c, by providing an entry for the heart primitive. That is, add RT_FUNCTAB_MAGIC, "ID_HRT", "hrt",rt_heart_*, et cetera .
  • Edit the idmap[] array which maps database objects to internal ones by adding ID_HRT .
  • Edit the rt_id_solid() function which determines the appropriate function subscripts by adding a case for the ID_HRT .

2. mirror.c

mirror.c, a file in src/librt/primitives which contains routines to mirror objects about some axis, will be hacked to support src/librt/primitives/hrt.c by doing the following.

  • Write RT_DECLARE_MIRROR(hrt) .
  • Edit rt_mirror() function by adding case ID_HRT .

3. db_scan.c

db_scan.c,a file in src/librt/,will be edited as follows;

  • Add the ID_HRT case to db_scan() function .


  • Edit librt_nil_la_SOURCES by adding primitives/hrt/hrt.c


  • Add necessary defines for ECMD_HEART_* .

6. mged/tedit.c

  • Add case ID_HRT to the writesolid() function which writes numerical parameters of a solid to a file.
  • Add case ID_HRT to the readsolid() function which reads numerical parameters of a solid from a file.


Pre mid term evaluation period

  • Edit include/ header files like raytrace.h, rtgeom.h, magic.h and db.h /db5.h
  • Write src/librt/primitives/hrt/hrt.c

Post mid term evaluation period

  • Add support to other files in BRL-CAD like primitives/table.c, primitives/mirror.c, src/librt/db_scan.c, mged/tedit.c, mged/sedit.h and .


June 17th to July 26th : Work period (Pre-mid term evaluation)

(1 week)

  • Edit include/ header files like raytrace.h, rtgeom.h, magic.h and db.h /db5.h .

(1 week)

  • Write hrt_specific structure .
  • Write rt_hrt_prep() function to determine if system is dealing with a valid heart .
  • Write rt_hrt_print() function to display some properties of the heart .
  • Write rt_hrt_shot() function to intersect a ray with a hrt .
  • Testing and debugging functions in hrt.c

(1 week)

  • Write rt_hrt_norm() function to return the normal, entry and exit points of a ray .
  • Write rt_hrt_curve() function to return th curvature of the heart .
  • Write rt_hrt_uv() function to return the polar cooordinates of a point hit by a ray .
  • Write rt_hrt_free() function .
  • Testing and debugging routines in hrt.c.

(1 week)

  • Write rt_hrt_plot() function .
  • Write rt_hrt_import() function from database format to internal format .
  • Write rt_hrt_export5() function to export from internal format to external format .
  • Testing and debugging routines in hrt.c.

(1 week)

  • Write rt_hrt_tess() function .
  • Write rt_hrt_describe() function to present the heart in human-readable format.
  • Write rt_hrt_params() function .
  • Testing and debugging routines in hrt.c

(1 week)

  • Submission of hrt.c to mentors for final review .
  • Final corrections of hrt.c .

June 29th to August 2nd : Mid-term evaluation

(1 week)

  • Submission of preliminary hrt.c to Google .

Aug 5th to Sept 13th : Work period (Post mid-term evaluation)

(2 weeks)

  • Edit the table to support librt in table.c .
  • Edit mirror.c to help mirror objects across some axis.

(2 weeks)

  • Write database interaction support in db_scan.c .
  • Edit to include hrt support .

(2 weeks)

  • Create necessary support for heart primitive in src/mged/?edit.? files .

Sep 16th to Sep 27th: Testing and Documentation period

(1 week)

  • Final testing and debugging of src/librt/primitives/hrt/hrt.c code .
  • Documenting heart primitive in BRL-CAD .
  • Final review of hrt.c by BRL-CAD mentors .
  • Final corrections of hrt.c .

(1 week)

  • Final submission of hrt.c code to Google.


As proposed in the project proposal -, the sextic equation of the heart can be used alongside differential calculus to add the heart primitive to the BRL-CAD package. A possible deviation from this approach will be to implement a pseudoprimitive that is, a primitive created using other primitives but usable as a CSG primitive e.g. a ell created using a sph. We could use a metaball to create a heart by mapping the heart's properties like lobe size and aspect ratio as metaball properties.



Determining which points are inside / outside the heart . Computing the surface of intersection between the heart and other BRL-CAD primitives.


  • Taubin, G. "An Accurate Algorithm for Rasterizing Algebraic Curves." In Second ACM/IEEE Symposium on Solid Modeling and Applications Proceedings. 221-230, May 1993.
  • Nordstrand, T. "Heart."
  • "A Generalization of Algebraic Surface drawing" by James F. Blinn.
  • Wolfram mathworld's sextic equation:
  • Solving sextic equations by division , Raghavendra G. Kulkarni.
  • Solving sextic equations by decomposition , Raghavendra G. Kulkarni.