基本原理：

计算四个点的增长斜率，使用双线性插值实现像素填充。

废话也懒得说啦，自己看代码吧，我从一个地方抄袭+修改了一下

源来的代码，原因是原来的代码太乱了，也太让人费解了。

运行效果：

滤镜源代码：

package com.gloomyfish.filter.study;  import java.awt.Rectangle; import java.awt.image.BufferedImage;  /**  *   * @author gloomy-fish 2013-04-25  *  */ public class PerspectiveFilter extends AbstractBufferedImageOp {  private float x0, y0, x1, y1, x2, y2, x3, y3;   private float dx1, dy1, dx2, dy2, dx3, dy3;     private float A, B, C, D, E, F, G, H, I;    private float a11, a12, a13, a21, a22, a23, a31, a32, a33;     private boolean scaled;     private int width;     private int height;       @Override  public BufferedImage filter(BufferedImage src, BufferedImage dest) {         width = src.getWidth();         height = src.getHeight();         if ( dest == null )             dest = createCompatibleDestImage( src, null );      A = a22*a33 - a32*a23;         B = a31*a23 - a21*a33;         C = a21*a32 - a31*a22;         D = a32*a13 - a12*a33;         E = a11*a33 - a31*a13;         F = a31*a12 - a11*a32;         G = a12*a23 - a22*a13;         H = a21*a13 - a11*a23;         I = a11*a22 - a21*a12;         if ( !scaled ) {             float invWidth = 1.0f/width;             float invHeight = 1.0f/height;              A *= invWidth;             D *= invWidth;             G *= invWidth;             B *= invHeight;             E *= invHeight;             H *= invHeight;         }                int[] inPixels = getRGB( src, 0, 0, width, height, null );         int srcWidth = width;      int srcHeight = height;        int srcWidth1 = width-1;       int srcHeight1 = height-1;         int outX=0, outY=0;       Rectangle transformedSpace = new Rectangle(0, 0, width, height);       transformSpace(transformedSpace);       outX = transformedSpace.x;         outY = transformedSpace.y;         int outWidth = transformedSpace.width;         int outHeight = transformedSpace.height;       // int index = 0;      int[] outPixels = new int[transformedSpace.width];         float[] out = new float[2];        for (int y = 0; y < outHeight; y++) {             for (int x = 0; x < outWidth; x++) {              transformInverse(outX+x, outY+y, out);                int srcX = (int)Math.floor( out[0] );              int srcY = (int)Math.floor( out[1] );              float xWeight = out[0]-srcX;               float yWeight = out[1]-srcY;               int nw, ne, sw, se;                 if ( srcX >= 0 && srcX < srcWidth1 && srcY >= 0 && srcY < srcHeight1) {                   // Easy case, all corners are in the image                  int i = srcWidth*srcY + srcX;                     nw = inPixels[i];                  ne = inPixels[i+1];                   sw = inPixels[i+srcWidth];                    se = inPixels[i+srcWidth+1];                 } else {                    // Some of the corners are off the image                    nw = getPixel( inPixels, srcX, srcY, srcWidth, srcHeight );                    ne = getPixel( inPixels, srcX+1, srcY, srcWidth, srcHeight );                     sw = getPixel( inPixels, srcX, srcY+1, srcWidth, srcHeight );                     se = getPixel( inPixels, srcX+1, srcY+1, srcWidth, srcHeight );              }               outPixels[x] = ImageMath.bilinearInterpolate(xWeight, yWeight, nw, ne, sw, se);            }           setRGB( dest, 0, y, transformedSpace.width, 1, outPixels );         }               return dest;    }       protected void transformSpace( Rectangle rect ) {       if ( scaled ) {             rect.x = (int)Math.min( Math.min( x0, x1 ), Math.min( x2, x3 ) );             rect.y = (int)Math.min( Math.min( y0, y1 ), Math.min( y2, y3 ) );             rect.width = (int)Math.max( Math.max( x0, x1 ), Math.max( x2, x3 ) ) - rect.x;             rect.height = (int)Math.max( Math.max( y0, y1 ), Math.max( y2, y3 ) ) - rect.y;             return;         }    }   final private int getPixel( int[] pixels, int x, int y, int width, int height ) {       if (x < 0 || x >= width || y < 0 || y >= height) {            return pixels[(ImageMath.clamp(y, 0, height-1) * width) + ImageMath.clamp(x, 0, width-1)] & 0x00ffffff;        }       return pixels[ y*width+x ];    }       public PerspectiveFilter() {        this( 0, 0, 1, 0, 1, 1, 0, 1);  }       public PerspectiveFilter(float x0, float y0, float x1, float y1, float x2, float y2, float x3, float y3) {      unitSquareToQuad(x0, y0, x1, y1, x2, y2, x3, y3);   }   protected void transformInverse( int x, int y, float[] out ) {      out[0] = width * (A*x+B*y+C)/(G*x+H*y+I);      out[1] = height * (D*x+E*y+F)/(G*x+H*y+I);     }       public void unitSquareToQuad( float x0, float y0, float x1, float y1, float x2, float y2, float x3, float y3 ) {        this.x0 = x0;      this.y0 = y0;      this.x1 = x1;      this.y1 = y1;      this.x2 = x2;      this.y2 = y2;      this.x3 = x3;      this.y3 = y3;              dx1 = x1-x2;       dy1 = y1-y2;       dx2 = x3-x2;       dy2 = y3-y2;       dx3 = x0-x1+x2-x3;        dy3 = y0-y1+y2-y3;                if (dx3 == 0 && dy3 == 0) {             a11 = x1-x0;           a21 = x2-x1;           a31 = x0;          a12 = y1-y0;           a22 = y2-y1;           a32 = y0;          a13 = a23 = 0;        } else {            a13 = (dx3*dy2-dx2*dy3)/(dx1*dy2-dy1*dx2);             a23 = (dx1*dy3-dy1*dx3)/(dx1*dy2-dy1*dx2);             a11 = x1-x0+a13*x1;           a21 = x3-x0+a23*x3;           a31 = x0;          a12 = y1-y0+a13*y1;           a22 = y3-y0+a23*y3;           a32 = y0;      }         a33 = 1;         scaled = false;    }       public void setCorners(float x0, float y0, float x1, float y1, float x2, float y2, float x3, float y3) {        unitSquareToQuad( x0, y0, x1, y1, x2, y2, x3, y3 );         scaled = true;     }      } 

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