Copy voxel of 3D oblique slice to 2d array

Question

for(int x = 0; x < 256; ++x){
   for(int y = 0; y < 256; y++){
        for(int z = 0; z < 256; z++){
           float zero = planeEquation(x,y,z);
             if((zero < 0.5) && (zero > -0.5)){
             pVolumeTemp[x*256 + y] = datasetRaw[x][y][z];          
           } 
        }
   }
}

It works only when the plane is vertical or horizontal. Sure, I'm overwriting pVolumeTemp[x,y] when there is one more z point on the plane. But I don't know how to solve it. Someone can help me?? I want 2d slice image