int input_size = (direction == VERTICAL) ? input_height : input_width;
- // This is needed because it counteracts the precision issues we get
- // because we sample the input texture with normalized coordinates
- // (especially when the repeat count along the axis is not a power of
- // two); we very rapidly end up in narrowly missing a texel center,
- // which causes precision loss to propagate throughout the FFT.
+ // Because of the memory layout (see below) and because we use offsets,
+ // the support texture values for many consecutive values will be
+ // the same. Thus, we can store a smaller texture (giving a small
+ // performance boost) and just sample it with NEAREST. Also, this
+ // counteracts any precision issues we might get from linear
+ // interpolation.
Node *self = chain->find_node_for_effect(this);
glActiveTexture(chain->get_input_sampler(self, 0));
check_error();
assert((fft_size & (fft_size - 1)) == 0); // Must be power of two.
assert(fft_size % subfft_size == 0);
int stride = fft_size / subfft_size;
- for (int i = 0; i < fft_size; ++i) {
- int k = i / stride; // Element number within this sub-FFT.
- int offset = i % stride; // Sub-FFT number.
+ for (int i = 0; i < subfft_size; i++) {
+ int k = i;
double twiddle_real, twiddle_imag;
if (k < subfft_size / 2) {
// for using offsets and not direct coordinates as in GPUwave
// is that we can have multiple FFTs along the same line,
// and want to reuse the support texture by repeating it.
- int base = k * stride * 2 + offset;
+ int base = k * stride * 2;
int support_texture_index = i;
int src1 = base;
int src2 = base + stride;
+ double sign = 1.0;
if (direction == FFTPassEffect::VERTICAL) {
// Compensate for OpenGL's bottom-left convention.
- support_texture_index = fft_size - support_texture_index - 1;
- src1 = fft_size - src1 - 1;
- src2 = fft_size - src2 - 1;
+ support_texture_index = subfft_size - support_texture_index - 1;
+ sign = -1.0;
}
- tmp[support_texture_index * 4 + 0] = fp64_to_fp16((src1 - support_texture_index) / double(input_size));
- tmp[support_texture_index * 4 + 1] = fp64_to_fp16((src2 - support_texture_index) / double(input_size));
+ tmp[support_texture_index * 4 + 0] = fp64_to_fp16(sign * (src1 - i * stride) / double(input_size));
+ tmp[support_texture_index * 4 + 1] = fp64_to_fp16(sign * (src2 - i * stride) / double(input_size));
tmp[support_texture_index * 4 + 2] = fp64_to_fp16(twiddle_real);
tmp[support_texture_index * 4 + 3] = fp64_to_fp16(twiddle_imag);
}
// which gives a nice speed boost.
//
// Note that the source coordinates become somewhat less accurate too, though.
- glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA16F, fft_size, 1, 0, GL_RGBA, GL_HALF_FLOAT, tmp);
+ glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA16F, subfft_size, 1, 0, GL_RGBA, GL_HALF_FLOAT, tmp);
check_error();
delete[] tmp;