Use a smaller support texture for the FFT.

[movit] / fft_pass_effect.cpp

diff --git a/fft_pass_effect.cpp b/fft_pass_effect.cpp
index 9c8aeb058e71fa33435901d4f5a2588a38ccb4f3..deb0a1cd94db7373cddbe40abe326ed891fc29f3 100644 (file)
--- a/fft_pass_effect.cpp
+++ b/fft_pass_effect.cpp
@@ -41,11 +41,12 @@ void FFTPassEffect::set_gl_state(GLuint glsl_program_num, const string &prefix,
 
        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();
@@ -91,9 +92,8 @@ void FFTPassEffect::set_gl_state(GLuint glsl_program_num, const string &prefix,
        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) {
@@ -116,16 +116,18 @@ void FFTPassEffect::set_gl_state(GLuint glsl_program_num, const string &prefix,
                // 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 support_texture_index;
+               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 - i - 1;
-               } else {
-                       support_texture_index = i;
+                       support_texture_index = subfft_size - support_texture_index - 1;
+                       sign = -1.0;
                }
-               tmp[support_texture_index * 4 + 0] = fp64_to_fp16((base - support_texture_index) / double(input_size));
-               tmp[support_texture_index * 4 + 1] = fp64_to_fp16((base + stride - 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);
        }
@@ -150,7 +152,7 @@ void FFTPassEffect::set_gl_state(GLuint glsl_program_num, const string &prefix,
        // 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;