agg_span_gouraud_gray.h

//----------------------------------------------------------------------------
// Anti-Grain Geometry - Version 2.4
// Copyright (C) 2002-2005 Maxim Shemanarev (http://www.antigrain.com)
//
// Permission to copy, use, modify, sell and distribute this software 
// is granted provided this copyright notice appears in all copies. 
// This software is provided "as is" without express or implied
// warranty, and with no claim as to its suitability for any purpose.
//
//----------------------------------------------------------------------------
// Contact: mcseem@antigrain.com
//          mcseemagg@yahoo.com
//          http://www.antigrain.com
//----------------------------------------------------------------------------
//
// Adaptation for high precision colors has been sponsored by 
// Liberty Technology Systems, Inc., visit http://lib-sys.com
//
// Liberty Technology Systems, Inc. is the provider of
// PostScript and PDF technology for software developers.
// 
//----------------------------------------------------------------------------

#ifndef AGG_SPAN_GOURAUD_GRAY_INCLUDED
#define AGG_SPAN_GOURAUD_GRAY_INCLUDED

#include <cmath>
#include <cstdlib>
#include "agg_basics.h"
#include "agg_color_gray.h"
#include "agg_dda_line.h"
#include "agg_span_gouraud.h"

namespace agg
{

    //=======================================================span_gouraud_gray
    template<class ColorT> class span_gouraud_gray : public span_gouraud<ColorT>
    {
    public:
        typedef ColorT color_type;
        typedef typename color_type::value_type value_type;
        typedef span_gouraud<color_type> base_type;
        typedef typename base_type::coord_type coord_type;
        enum subpixel_scale_e
        { 
            subpixel_shift = 4, 
            subpixel_scale = 1 << subpixel_shift
        };

    private:
        //--------------------------------------------------------------------
        struct gray_calc
        {
            void init(const coord_type& c1, const coord_type& c2)
            {
                m_x1  = c1.x - 0.5;
                m_y1  = c1.y - 0.5;
                m_dx  = c2.x - c1.x;
                double dy = c2.y - c1.y;
                m_1dy = (std::fabs(dy) < 1e-10) ? 1e10 : 1.0 / dy;
                m_v1 = c1.color.v;
                m_a1 = c1.color.a;
                m_dv = c2.color.v - m_v1;
                m_da = c2.color.a - m_a1;
            }

            void calc(double y)
            {
                double k = (y - m_y1) * m_1dy;
                if(k < 0.0) k = 0.0;
                if(k > 1.0) k = 1.0;
                m_v = m_v1 + iround(m_dv * k);
                m_a = m_a1 + iround(m_da * k);
                m_x = iround((m_x1 + m_dx * k) * subpixel_scale);
            }

            double m_x1;
            double m_y1;
            double m_dx;
            double m_1dy;
            int    m_v1;
            int    m_a1;
            int    m_dv;
            int    m_da;
            int    m_v;
            int    m_a;
            int    m_x;
        };


    public:
        //--------------------------------------------------------------------
        span_gouraud_gray() {}
        span_gouraud_gray(const color_type& c1, 
                          const color_type& c2, 
                          const color_type& c3,
                          double x1, double y1, 
                          double x2, double y2,
                          double x3, double y3, 
                          double d = 0) : 
            base_type(c1, c2, c3, x1, y1, x2, y2, x3, y3, d)
        {}

        //--------------------------------------------------------------------
        void prepare()
        {
            coord_type coord[3];
            base_type::arrange_vertices(coord);

            m_y2 = int(coord[1].y);

            m_swap = cross_product(coord[0].x, coord[0].y, 
                                   coord[2].x, coord[2].y,
                                   coord[1].x, coord[1].y) < 0.0;

            m_c1.init(coord[0], coord[2]);
            m_c2.init(coord[0], coord[1]);
            m_c3.init(coord[1], coord[2]);
        }

        //--------------------------------------------------------------------
        void generate(color_type* span, int x, int y, unsigned len)
        {
            m_c1.calc(y);
            const gray_calc* pc1 = &m_c1;
            const gray_calc* pc2 = &m_c2;

            if(y < m_y2)
            {
                // Bottom part of the triangle (first subtriangle)
                //-------------------------
                m_c2.calc(y + m_c2.m_1dy);
            }
            else
            {
                // Upper part (second subtriangle)
                //-------------------------
                m_c3.calc(y - m_c3.m_1dy);
                pc2 = &m_c3;
            }

            if(m_swap)
            {
                // It means that the triangle is oriented clockwise, 
                // so that we need to swap the controlling structures
                //-------------------------
                const gray_calc* t = pc2;
                pc2 = pc1;
                pc1 = t;
            }

            // Get the horizontal length with subpixel accuracy
            // and protect it from division by zero
            //-------------------------
            int nlen = std::abs(pc2->m_x - pc1->m_x);
            if(nlen <= 0) nlen = 1;

            dda_line_interpolator<14> v(pc1->m_v, pc2->m_v, nlen);
            dda_line_interpolator<14> a(pc1->m_a, pc2->m_a, nlen);

            // Calculate the starting point of the gradient with subpixel 
            // accuracy and correct (roll back) the interpolators.
            // This operation will also clip the beginning of the span
            // if necessary.
            //-------------------------
            int start = pc1->m_x - (x << subpixel_shift);
            v    -= start; 
            a    -= start;
            nlen += start;

            int vv, va;
            enum lim_e { lim = color_type::base_mask };

            // Beginning part of the span. Since we rolled back the 
            // interpolators, the color values may have overflow.
            // So that, we render the beginning part with checking 
            // for overflow. It lasts until "start" is positive;
            // typically it's 1-2 pixels, but may be more in some cases.
            //-------------------------
            while(len && start > 0)
            {
                vv = v.y();
                va = a.y();
                if(vv < 0) vv = 0; if(vv > lim) vv = lim;
                if(va < 0) va = 0; if(va > lim) va = lim;
                span->v = (value_type)vv;
                span->a = (value_type)va;
                v     += subpixel_scale; 
                a     += subpixel_scale;
                nlen  -= subpixel_scale;
                start -= subpixel_scale;
                ++span;
                --len;
            }

            // Middle part, no checking for overflow.
            // Actual spans can be longer than the calculated length
            // because of anti-aliasing, thus, the interpolators can 
            // overflow. But while "nlen" is positive we are safe.
            //-------------------------
            while(len && nlen > 0)
            {
                span->v = (value_type)v.y();
                span->a = (value_type)a.y();
                v    += subpixel_scale; 
                a    += subpixel_scale;
                nlen -= subpixel_scale;
                ++span;
                --len;
            }

            // Ending part; checking for overflow.
            // Typically it's 1-2 pixels, but may be more in some cases.
            //-------------------------
            while(len)
            {
                vv = v.y();
                va = a.y();
                if(vv < 0) vv = 0; if(vv > lim) vv = lim;
                if(va < 0) va = 0; if(va > lim) va = lim;
                span->v = (value_type)vv;
                span->a = (value_type)va;
                v += subpixel_scale; 
                a += subpixel_scale;
                ++span;
                --len;
            }
        }


    private:
        bool      m_swap;
        int       m_y2;
        gray_calc m_c1;
        gray_calc m_c2;
        gray_calc m_c3;
    };


}

#endif