Android 上實作水波特效二--優化

Android 上實作水波特效二--優化   

羅朝輝 (http://www.cnblogs.com/kesalin/) 本文遵循“署名-非商業用途-保持一緻”創作公用協定   

    在上一篇文章《Android 上實作水波特效》中對水波波幅的計算是針對每一個像素的，效率比較低，尤其是在手機上運作，相當緩慢。我們可以利用線性插值進行優化，這樣可以将計算減少一半（MeshSize 為 2）或減少四分之三（MeshSize 為 4），效率得以大大提升，即使是在水機上也能較為流暢地運作。

在下面的代碼中，為了充分使用移位運算替代乘除法，MeshSize 必須為 2 的整次幂，MeshShift 就是其幂數，表示計算時的移位位數。代碼下載下傳連結：http://www.cppblog.com/Files/kesalin/RippleDemo_opt.zip

線性插值優化之後的水波擴散代碼如下：

static final int MeshSize = 2;

static final int MeshShift = 1;  

int m_meshWidth;

int m_meshHeight;

m_meshWidth = m_width / MeshSize + 1;

m_meshHeight = m_height / MeshSize + 1;;

void rippleSpread()

{
m_waveFlag = false;

int i = 0, offset = 0;

for (int y = 1; y < m_meshHeight - 1; ++y) {

offset = y * m_meshWidth;

for (int x = 1; x < m_meshWidth - 1; ++x) {

i = offset + x;

m_buf2[i] = (short)(((m_buf1[i - 1] + m_buf1[i + 1]
                    + m_buf1[i - m_meshWidth]
                    + m_buf1[i + m_meshWidth]) >> 1) -m_buf2[i]);

            m_buf2[i] -= (m_buf2[i] >> 5);
            m_waveFlag |= (m_buf2[i] != 0);
       }
    }


if (m_waveFlag){

       m_waveFlag = false;

for (int y = 1; y < m_meshHeight - 1; ++y) {

          offset = y * m_meshWidth;

for (int x = 1; x < m_meshWidth - 1; ++x) {
             i = offset + x;
             m_bufDiffX[i] = (short)((m_buf2[i + 1] - m_buf2[i - 1]) >> 3);
             m_bufDiffY[i] = (short)((m_buf2[i + m_meshWidth] -m_buf2[i - m_meshWidth]) >> 3);
             m_waveFlag |= (m_bufDiffX[i] != 0 || m_bufDiffY[i] != 0);
          }
       }
   }      

//交換波能資料緩沖區
short[] temp = m_buf1;
    m_buf1 = m_buf2;
   m_buf2 = temp;
}      

既然波幅計算使用了線性插值，描繪時的代碼也許相應進行更改：

Point p1, p2, p3, p4;
Point pRowStart, pRowEnd, p, rowStartInc, rowEndInc, pInc;

void rippleRender()
{ 
int px = 0, py = 0, dx = 0, dy = 0;
int index = 0, offset = 0;

for (int j = 1; j < m_meshHeight; ++j) {
      offset = j * m_meshWidth;
for (int i = 1; i < m_meshWidth; ++i) {    
         index = offset + i;
         p1.x = m_bufDiffX[index - m_meshWidth - 1];
         p1.y = m_bufDiffY[index - m_meshWidth - 1];
         p2.x = m_bufDiffX[index - m_meshWidth];
         p2.y = m_bufDiffY[index - m_meshWidth];
         p3.x = m_bufDiffX[index - 1];
         p3.y = m_bufDiffY[index - 1];
         p4.x = m_bufDiffX[index];
         p4.y = m_bufDiffY[index];

         pRowStart.x = p1.x << MeshShift;
         pRowStart.y = p1.y << MeshShift;
         rowStartInc.x = p3.x - p1.x;
         rowStartInc.y = p3.y - p1.y;

         pRowEnd.x = p2.x << MeshShift;
         pRowEnd.y = p2.y << MeshShift;
         rowEndInc.x = p4.x - p2.x;
         rowEndInc.y = p4.y - p2.y;

         py = (j - 1) << MeshShift;
for (int y = 0; y < MeshSize; ++y) {
            p.x = pRowStart.x;
            p.y = pRowStart.y;

// scaled by MeshSize times
            pInc.x = (pRowEnd.x - pRowStart.x) >> MeshShift;
            pInc.y = (pRowEnd.y - pRowStart.y) >> MeshShift;

            px = (i - 1) << MeshShift;
for (int x = 0; x < MeshSize; ++x) {
                dx = px + p.x >> MeshShift;
                dy = py + p.y >> MeshShift;

if ((dx >= 0) && (dy >= 0) && (dx < m_width) && (dy < m_height) ) {
                    m_bitmap2[py * m_width + px] = m_bitmap1[dy * m_width + dx];
                 }
else {
                    m_bitmap2[py * m_width + px] = m_bitmap1[py * m_width + px];
                 }

                 p.x += pInc.x;
                 p.y += pInc.y;
                 ++px;
             }

             pRowStart.x += rowStartInc.x;
             pRowStart.y += rowStartInc.y;
             pRowEnd.x += rowEndInc.x;
             pRowEnd.y += rowEndInc.y;
             ++py;

         }
      }
   }
}