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# explaining a piece of code

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### #1AhmedCoeia  Members   -  Reputation: 477

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Posted 21 May 2012 - 02:45 PM

The following code is a calculation of a fish that is drawn by OpenGL, I would like someone to explain to me please.

GoldfishGeometry::GoldfishGeometry() :
m_BodySegments(50),
m_CircleSegments(25),
m_StripSegments(50)
{
m_InnerCurve.p0 = Vec2f(0.0f, 0.0f);
m_InnerCurve.p1 = Vec2f(0.8f, 0.1f);
m_InnerCurve.p2 = Vec2f(0.2f, 0.9f);
m_InnerCurve.p3 = Vec2f(0.0f, 1.0f);

m_OuterCurve.p0 = Vec2f(0.0f, 0.0f);
m_OuterCurve.p1 = Vec2f(0.8f, 0.1f);
m_OuterCurve.p2 = Vec2f(1.0f, 0.5f);
m_OuterCurve.p3 = Vec2f(1.0f, 1.0f);

m_EndCurve.p0 = Vec2f(0.0f, 0.0f);
m_EndCurve.p1 = Vec2f(5.0f, 0.1f);
m_EndCurve.p2 = Vec2f(5.0f, 0.5f);
m_EndCurve.p3 = Vec2f(0.0f, 1.0f);

std::vector< float > cylVerts;
cylVerts.reserve(((int)m_CircleSegments+1)*3*2*(int)m_BodySegments);

std::vector< float > cylNormals;
cylNormals.reserve(((int)m_CircleSegments+1)*3*2*(int)m_BodySegments);

std::vector< float > cylTexcoords;
cylTexcoords.reserve(((int)m_CircleSegments+1)*2*2*(int)m_BodySegments);

float x, y0, y1, z, u, v, angle;
float hStep = 1.0f / m_BodySegments;
float angleStep = ( 360.0f/(float)(m_CircleSegments) ) * 3.14159265f / 180.0f;
float texcoordStep = 1.0f/(float)(m_CircleSegments);

unsigned int hOffsetVerts = ((int)m_CircleSegments + 1)*3;
unsigned int hOffsetTex = ((int)m_CircleSegments + 1)*2;

for ( unsigned int h=0; h<(int)m_BodySegments; ++h )
{
for (unsigned int c=0; c<=(int)m_CircleSegments; ++c )
{
unsigned int offsetVerts = c*3;
unsigned int offsetTex = c*2;

angle = c*angleStep;

x = sin(angle);
z = cos(angle);
y0 = h*hStep; y1 = (h+1)*hStep;
u = c*texcoordStep;

cylVerts.push_back(x);
cylVerts.push_back(y0);
cylVerts.push_back(z);

cylNormals.push_back(x);
cylNormals.push_back(0.0f);
cylNormals.push_back(z);

cylTexcoords.push_back(u);
cylTexcoords.push_back(y0);

cylVerts.push_back(x);
cylVerts.push_back(y1);
cylVerts.push_back(z);

cylNormals.push_back(x);
cylNormals.push_back(0.0f);
cylNormals.push_back(z);

cylTexcoords.push_back(u);
cylTexcoords.push_back(y1);
}
}

std::vector< float > stripVerts;
std::vector< float > stripNormals;
std::vector< float > stripTexcoords;

stripVerts.reserve(((int)m_StripSegments)*3*2*(int)m_BodySegments);
stripNormals.reserve(((int)m_StripSegments)*3*2*(int)m_BodySegments);
stripTexcoords.reserve(((int)m_StripSegments)*2*2*(int)m_BodySegments);

float stripStep = 1.0f/(float)(m_StripSegments);

for ( unsigned int h=0; h<(int)m_BodySegments; ++h )
{
for (unsigned int v=0; v<(int)m_StripSegments; ++v )
{
u = v * stripStep;
y0 = h*hStep; y1 = (h+1)*hStep;
x = v * stripStep;

stripVerts.push_back(x);
stripVerts.push_back(y0);
stripVerts.push_back(0.0f);

stripNormals.push_back(0.0f);
stripNormals.push_back(0.0f);
stripNormals.push_back(1.0f);

stripTexcoords.push_back(u);
stripTexcoords.push_back(y0);

stripVerts.push_back(x);
stripVerts.push_back(y1);
stripVerts.push_back(0.0f);

stripNormals.push_back(0.0f);
stripNormals.push_back(0.0f);
stripNormals.push_back(1.0f);

stripTexcoords.push_back(u);
stripTexcoords.push_back(y1);
}
}

glGenVertexArrays(2, &m_VAO[0]);
glGenBuffers(6, &m_VBO[0]);

glBindVertexArray(m_VAO[0]);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[0]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_CircleSegments+1)*3*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&cylVerts[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)0);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[1]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_CircleSegments+1)*2*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&cylTexcoords[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)1, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)1);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[2]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_CircleSegments+1)*3*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&cylNormals[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)2, 3, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)2);

glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);

glBindVertexArray(m_VAO[1]);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[3]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_StripSegments)*3*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&stripVerts[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)0);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[4]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_StripSegments)*2*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&stripTexcoords[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)1, 2, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)1);

glBindBuffer(GL_ARRAY_BUFFER, m_VBO[5]);
glBufferData(GL_ARRAY_BUFFER, ((int)m_StripSegments)*3*2*((int)m_BodySegments)*sizeof(float), (const GLvoid *)&stripNormals[0], GL_STATIC_DRAW);
glVertexAttribPointer((GLuint)2, 3, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray((GLuint)2);

glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);

}

### #2Bacterius  Crossbones+   -  Reputation: 10042

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Posted 21 May 2012 - 03:19 PM

It looks like this code is supposed to create a fish from some geometry (by starting with a few default lines and "shaping" the fish by pushing vertices back and forth to approximate a goldfish's outline, using a sinuosidal pattern), and then displays it via OpenGL (apparently just using solid lines). In immediate mode.

The slowsort algorithm is a perfect illustration of the multiply and surrender paradigm, which is perhaps the single most important paradigm in the development of reluctant algorithms. The basic multiply and surrender strategy consists in replacing the problem at hand by two or more subproblems, each slightly simpler than the original, and continue multiplying subproblems and subsubproblems recursively in this fashion as long as possible. At some point the subproblems will all become so simple that their solution can no longer be postponed, and we will have to surrender. Experience shows that, in most cases, by the time this point is reached the total work will be substantially higher than what could have been wasted by a more direct approach.

- Pessimal Algorithms and Simplexity Analysis

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