From c2e19c0fb1952bb7f91ac9ce3b4d25f06b6d2ad6 Mon Sep 17 00:00:00 2001 From: "Steinar H. Gunderson" Date: Sat, 22 Mar 2014 17:09:36 +0100 Subject: [PATCH] Use a smaller support texture for the FFT. Many of the rows in the support texture are exactly the same, so don't store the duplicates; gives a small performance boost. In a sense, this is exactly the same property that GPUwave uses with drawing multiple quads at the lower level. --- fft_pass_effect.cpp | 30 +++++++++++++++--------------- 1 file changed, 15 insertions(+), 15 deletions(-) diff --git a/fft_pass_effect.cpp b/fft_pass_effect.cpp index 4069be9..deb0a1c 100644 --- 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) { @@ -116,18 +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 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); } @@ -152,7 +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; -- 2.39.2