バイキュービック画像補間を実装しようとしています。コードの関連するセクションのみを貼り付けました。画像をバッファにロードしたり、ピクセルからピクセルを読み取ったりするコードをスキップしました。計算が正しいと合理的に確信しています。ただし、出力にひどいアーティファクトが含まれているようです。
すべてのアクションは、resizeメソッドで発生します。
経験豊富なグラフィックプログラマーが、私が間違っている可能性があることについて彼らの勘を共有できるかもしれないことを望んでいます。
以下は、入力の幅と高さを2倍に変更したときに取得する入力画像と出力画像です。
double Interpolator::interpolate(const double p0, const double p1, const double p2, const double p3, const double x){
return (-0.5f*p0+1.5f*p1-1.5f*p2+0.5*p3)*pow(x,3)+
(p0-2.5f*p1+2.f*p2-0.5f*p3)*pow(x,2)+
(-0.5f*p0+0.5f*p2)*x+
p1;
}
bool Image::equals(double a, double b, double threshold){
if(fabs(a-b)<=threshold)
return true;
return false;
}
void Image::interpolate(const Pixel p[], double offset, Pixel& result){
result.r = Interpolator::interpolate(p[0].r,p[1].r,p[2].r,p[3].r,offset);
result.g = Interpolator::interpolate(p[0].g,p[1].g,p[2].g,p[3].g,offset);
result.b = Interpolator::interpolate(p[0].b,p[1].b,p[2].b,p[3].b,offset);
result.a = Interpolator::interpolate(p[0].a,p[1].a,p[2].a,p[3].a,offset);
}
void Image::getSamplingCoords(const int nearest,
const int max,
int coords[]){
coords[0] = nearest-1;
if(coords[0]<0)
coords[0] = nearest;
coords[1] = nearest;
coords[2] = nearest+1;
if(coords[2]>=max)
coords[2] = nearest;
coords[3] = nearest+2;
//The following check should not be necessary
//since this is never expected to occur. Nevertheless...
if(coords[3]>=max)
coords[3] = nearest;
}
void Image::interpolateAlongY(int x, int y, int yMax, double yOffset, Pixel& result){
if(equals(yOffset,0.f,ERROR_THRESHOLD)){
//No interpolation required
getPixel(x,y,result);
return;
}
int yCoords[4];
getSamplingCoords(y, yMax, yCoords);
Pixel interpolants[4];
for(int i=0; i<4; ++i){
getPixel(x, yCoords[i], interpolants[i]);
}
interpolate(interpolants, y, result);
}
void Image::resize(const int newWidth, const int newHeight){
//Ensure that we have a valid buffer already
if(buffer==NULL){
printf("ERROR: Must load an image before resizing it!");
assert(false);
}
//We first need to create a new buffer with the new dimensions
unsigned char* newBuffer = new unsigned char[newWidth*newHeight*channelCount];
for(int j=0; j<newHeight; ++j){
for(int i=0; i<newWidth; ++i){
size_t newIndexOffset = (j*newWidth+i)*channelCount;
//For this pixel in the target image we
//a) Find the nearest pixel in the source image
//b) Find the offset from the aforementioned nearest pixel
int xNear,yNear;
double xOffset,yOffset;
double x = ((double)width/(double)newWidth)*i;
double y = ((double)height/(double)newHeight)*j;
xNear = floor(x);
yNear = floor(y);
xOffset = x-xNear;
yOffset = y-yNear;
//If offset is 0, we don't need any interpolation
//we simply need to sample the source pixel and proceed
// if(equals(xOffset,0.f,ERROR_THRESHOLD) && equals(yOffset,0.f,ERROR_THRESHOLD)){
// Pixel result;
// getPixel(xNear, yNear, result);
// *(newBuffer+newIndexOffset) = result.r;
// *(newBuffer+newIndexOffset+1) = result.g;
// *(newBuffer+newIndexOffset+2) = result.b;
// if(channelCount==4)
// *(buffer+newIndexOffset+3) = result.a;
// continue;
// }
//We make a check that xNear and yNear obtained above
//are always smaller than the edge pixels at the extremeties
if(xNear>=width || yNear>=height){
printf("ERROR: Nearest pixel computation error!");
assert(false);
}
//Next we find four pixels along the x direction around this
//nearest pixel
int xCoords[4];
getSamplingCoords(xNear,width,xCoords);
//For each of these sampling xCoords, we interpolate 4 nearest points
//along Y direction
Pixel yInterps[4];
for(int k=0; k<4; k++){
interpolateAlongY(xCoords[k], yNear, height, yOffset, yInterps[k]);
}
//Finally, the resultant pixel is a cubic interpolation
//on the 4 obtained pixels above
Pixel result;
if(equals(xOffset,0.f,ERROR_THRESHOLD)){
result.r = yInterps[0].r;
result.g = yInterps[0].g;
result.b = yInterps[0].b;
result.a = yInterps[0].a;
}else{
interpolate(yInterps, xOffset, result);
}
*(newBuffer+newIndexOffset) = result.r;
*(newBuffer+newIndexOffset+1) = result.g;
*(newBuffer+newIndexOffset+2) = result.b;
if(channelCount==4)
*(newBuffer+newIndexOffset+3) = result.a;
}
}
//Now we can deallocate the memory of our current buffer
delete [] buffer;
//Reassign our newly sampled buffer to our own
buffer = newBuffer;
//Reset our image dimensions
height = newHeight;
width = newWidth;
}