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# explaining a piece of code

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### #1AhmedCoeia  Members   -  Reputation: 510

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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: 11353

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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.