C++ Image interpolation with Bicubic method - c++

Question

I am just trying to smoothing the image by BiCubic interpolation. I got some code which is used to interpolate the RGB image. I have changed the code to work for Grayscale image. But in result i only got fully black image. Considered input and output image size are same. The code is pasted below. Please help me. Thanks in advance.
inline Uint16 saturate(float x, unsigned max_pixel)
{
return x > max_pixel ? max_pixel
: x < 0.0f ? 0
: Uint16(x);
}
inline float get_subpixel(const Uint16* in, std::size_t dest_width, std::size_t dest_height, unsigned x, unsigned y)
{
if (x < dest_width && y < dest_height)
return in[(y * dest_width) + x];
return 0;
}
void interpolate(unsigned dest_width, unsigned dest_height, unsigned bits_allocated, const Uint16* src, Uint16** dest)
{
const double tx = 1;
const double ty = 1;
float C[5] = { 0 };
unsigned max_bit = pow(2, bits_allocated);
for (unsigned i = 0; i < dest_height; ++i)
{
for (unsigned j = 0; j < dest_width; ++j)
{
const float x = float(tx * j);
const float y = float(ty * i);
const float dx = tx * j - x, dx2 = dx * dx, dx3 = dx2 * dx;
const float dy = ty * i - y, dy2 = dy * dy, dy3 = dy2 * dy;
for (int jj = 0; jj < 4; ++jj)
{
const int idx = y - 1 + jj;
float a0 = get_subpixel(src, dest_width, dest_height, x, idx);
float d0 = get_subpixel(src, dest_width, dest_height, x - 1, idx) - a0;
float d2 = get_subpixel(src, dest_width, dest_height, x + 1, idx) - a0;
float d3 = get_subpixel(src, dest_width, dest_height, x + 2, idx) - a0;
float a1 = -(1.0f / 3.0f) * d0 + d2 - (1.0f / 6.0f) * d3;
float a2 = 0.5f * d0 + 0.5f * d2;
float a3 = -(1.0f / 6.0f) * d0 - 0.5f * d2 + (1.0f / 6.0f) * d3;
C[jj] = a0 + a1 * dx + a2 * dx2 + a3 * dx3;
d0 = C[0] - C[1];
d2 = C[2] - C[1];
d3 = C[3] - C[1];
a0 = C[1];
a1 = -(1.0f / 3.0f) * d0 + d2 - (1.0f / 6.0f) * d3;
a2 = 0.5f * d0 + 0.5f * d2;
a3 = -(1.0f / 6.0f) * d0 - 0.5f * d2 + (1.0f / 6.0f) * d3;
(*dest)[i * dest_width + j] = saturate(a0 + a1 * dy + a2 * dy2 + a3 * dy3, max_bit);
}
}
}
}

Answer 1

How can you have this? The c's havent been computed until the jj loop ends the brace should be above the d's - I'm not considering if the method is correct otherwise.
for (int jj = 0; jj < 4; ++jj)
{
const int idx = y - 1 + jj;
float a0 = get_subpixel(src, dest_width, dest_height, x, idx);
float d0 = get_subpixel(src, dest_width, dest_height, x - 1, idx) - a0;
float d2 = get_subpixel(src, dest_width, dest_height, x + 1, idx) - a0;
float d3 = get_subpixel(src, dest_width, dest_height, x + 2, idx) - a0;
float a1 = -(1.0f / 3.0f) * d0 + d2 - (1.0f / 6.0f) * d3;
float a2 = 0.5f * d0 + 0.5f * d2;
float a3 = -(1.0f / 6.0f) * d0 - 0.5f * d2 + (1.0f / 6.0f) * d3;
C[jj] = a0 + a1 * dx + a2 * dx2 + a3 * dx3;
// } // end jj
d0 = C[0] - C[1];
d2 = C[2] - C[1];
d3 = C[3] - C[1];
a0 = C[1];
a1 = -(1.0f / 3.0f) * d0 + d2 - (1.0f / 6.0f) * d3;
a2 = 0.5f * d0 + 0.5f * d2;
a3 = -(1.0f / 6.0f) * d0 - 0.5f * d2 + (1.0f / 6.0f) * d3;
(*dest)[i * dest_height + j] = saturate(a0 + a1 * dy + a2 * dy2 + a3 * dy3, max_bit);
} // end jj move his above
}
}

Answer 2

I wanted to share great link
cubic splines