Question

I was studying Ray Tracing on http://www.devmaster.net/articles/raytracing_series/part1.php when I came across this piece of code:

void Engine::InitRender()
{
// set first line to draw to
m_CurrLine = 20;
// set pixel buffer address of first pixel
m_PPos = 20 * m_Width;
// screen plane in world space coordinates
m_WX1 = -4, m_WX2 = 4, m_WY1 = m_SY = 3, m_WY2 = -3;
// calculate deltas for interpolation
m_DX = (m_WX2 - m_WX1) / m_Width;
m_DY = (m_WY2 - m_WY1) / m_Height;
m_SY += 20 * m_DY;
// allocate space to store pointers to primitives for previous line
m_LastRow = new Primitive*[m_Width];
memset( m_LastRow, 0, m_Width * 4 );
}

I’m quite confused on how the author map screen coordinates to world coordinates…
Can anyone please tell me how the author derived these lines?
Or tell me how one would map screen coordinates to world coordinates?

// screen plane in world space coordinates
m_WX1 = -4, m_WX2 = 4, m_WY1 = m_SY = 3, m_WY2 = -3;

Thank you in advance!

Answer 1

EDIT: Here is relevant code from raytracer.cpp:

// render scene
vector3 o( 0, 0, -5 );

// initialize timer
int msecs = GetTickCount();

// reset last found primitive pointer
Primitive* lastprim = 0;

// render remaining lines
for(int y = m_CurrLine; y < (m_Height - 20); y++)
{
    m_SX = m_WX1;

    // render pixels for current line
    for ( int x = 0; x < m_Width; x++ )
    {
        // fire primary ray
        Color acc( 0, 0, 0 );
        vector3 dir = vector3( m_SX, m_SY, 0 ) - o;
        NORMALIZE( dir );
        Ray r( o, dir );
        float dist;
        Primitive* prim = Raytrace( r, acc, 1, 1.0f, dist );
        int red = (int)(acc.r * 256);
        int green = (int)(acc.g * 256);
        int blue = (int)(acc.b * 256);
        if (red > 255) red = 255;
        if (green > 255) green = 255;
        if (blue > 255) blue = 255;
        m_Dest[m_PPos++] = (red << 16) + (green << 8) + blue;
        m_SX += m_DX;
    }

    m_SY += m_DY;

    // see if we've been working to long already
    if ((GetTickCount() - msecs) > 100) 
    {
        // return control to windows so the screen gets updated
        m_CurrLine = y + 1;
        return false;
    }
}

return true;

Therefore the camera is at (0,0,-5) and the screen onto which the world is being projected has top-left corner (-4,3,0) and bottom-right corner (4,-3,0).