Calculate I_x and I_y for variational refinement.

diff --git a/derivatives.frag b/derivatives.frag
new file mode 100644 (file)
index 0000000..f5394c3
--- /dev/null
+++ b/derivatives.frag
@@ -0,0 +1,23 @@
+#version 450 core
+
+in vec2 tc;
+out vec2 derivatives;
+
+uniform sampler2D tex;
+uniform vec2 inv_image_size;
+
+void main()
+{
+       float x_m2 = texture(tex, vec2(tc.x - 2.0 * inv_image_size.x), tc.y).x;
+       float x_m1 = texture(tex, vec2(tc.x -       inv_image_size.x), tc.y).x;
+       float x_p1 = texture(tex, vec2(tc.x +       inv_image_size.x), tc.y).x;
+       float x_p2 = texture(tex, vec2(tc.x + 2.0 * inv_image_size.x), tc.y).x;
+
+       float y_m2 = texture(tex, vec2(tc.x, tc.y - 2.0 * inv_image_size.y)).x;
+       float y_m1 = texture(tex, vec2(tc.x, tc.y -       inv_image_size.y)).x;
+       float y_p1 = texture(tex, vec2(tc.x, tc.y +       inv_image_size.y)).x;
+       float y_p2 = texture(tex, vec2(tc.x, tc.y + 2.0 * inv_image_size.y)).x;
+
+       derivatives.x = (x_p1 - x_m1) * (2.0/3.0) + (x_m2 - x_p2) * (1.0/12.0);
+       derivatives.y = (y_p1 - y_m1) * (2.0/3.0) + (y_m2 - y_p2) * (1.0/12.0);
+}

diff --git a/flow.cpp b/flow.cpp
index 87cc06f1a510f7af39d21283d0cd4f53bf243bfc..d5e73eccf93c47a16d3ef943612873428bd80402 100644 (file)
--- a/flow.cpp
+++ b/flow.cpp
@@ -465,6 +465,64 @@ void Prewarp::exec(GLuint tex0_view, GLuint tex1_view, GLuint flow_tex, GLuint I
        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
 }
 
+// From I, calculate the partial derivatives I_x and I_y. We use a four-tap
+// central difference filter, since apparently, that's tradition (I haven't
+// measured quality versus a more normal 0.5 (I[x+1] - I[x-1]).)
+// The coefficients come from
+//
+//   https://en.wikipedia.org/wiki/Finite_difference_coefficient
+class Derivatives {
+public:
+       Derivatives();
+       void exec(GLuint input_tex, GLuint output_tex, int level_width, int level_height);
+
+private:
+       GLuint derivatives_vs_obj;
+       GLuint derivatives_fs_obj;
+       GLuint derivatives_program;
+       GLuint derivatives_vao;
+
+       GLuint uniform_tex;
+       GLuint uniform_inv_image_size;
+};
+
+Derivatives::Derivatives()
+{
+       derivatives_vs_obj = compile_shader(read_file("vs.vert"), GL_VERTEX_SHADER);
+       derivatives_fs_obj = compile_shader(read_file("derivatives.frag"), GL_FRAGMENT_SHADER);
+       derivatives_program = link_program(derivatives_vs_obj, derivatives_fs_obj);
+
+       // Set up the VAO containing all the required position/texcoord data.
+       glCreateVertexArrays(1, &derivatives_vao);
+       glBindVertexArray(derivatives_vao);
+       glBindBuffer(GL_ARRAY_BUFFER, vertex_vbo);
+
+       GLint position_attrib = glGetAttribLocation(derivatives_program, "position");
+       glEnableVertexArrayAttrib(derivatives_vao, position_attrib);
+       glVertexAttribPointer(position_attrib, 2, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0));
+
+       uniform_tex = glGetUniformLocation(derivatives_program, "tex");
+       uniform_inv_image_size = glGetUniformLocation(derivatives_program, "inv_image_size");
+}
+
+void Derivatives::exec(GLuint input_tex, GLuint output_tex, int level_width, int level_height)
+{
+       glUseProgram(derivatives_program);
+
+       bind_sampler(derivatives_program, uniform_tex, 0, input_tex, nearest_sampler);
+       glProgramUniform2f(derivatives_program, uniform_inv_image_size, 1.0f / level_width, 1.0f / level_height);
+
+       GLuint derivatives_fbo;  // TODO: cleanup
+       glCreateFramebuffers(1, &derivatives_fbo);
+       glNamedFramebufferTexture(derivatives_fbo, GL_COLOR_ATTACHMENT0, output_tex, 0);
+
+       glViewport(0, 0, level_width, level_height);
+       glDisable(GL_BLEND);
+       glBindVertexArray(derivatives_vao);
+       glBindFramebuffer(GL_FRAMEBUFFER, derivatives_fbo);
+       glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+}
+
 int main(void)
 {
        if (SDL_Init(SDL_INIT_EVERYTHING) == -1) {
@@ -526,6 +584,7 @@ int main(void)
        MotionSearch motion_search;
        Densify densify;
        Prewarp prewarp;
+       Derivatives derivatives;
 
        GLuint query;
        glGenQueries(1, &query);
@@ -589,6 +648,18 @@ int main(void)
                glTextureStorage2D(I_t_tex, 1, GL_R16F, level_width, level_height);
                prewarp.exec(tex0_view, tex1_view, dense_flow_tex, I_tex, I_t_tex, level_width, level_height);
 
+               // Derivatives of the images. We're only calculating first derivatives;
+               // the others will be taken on-the-fly in order to sample from fewer
+               // textures overall, since sampling from the L1 cache is cheap.
+               // (TODO: Verify that this is indeed faster than making separate
+               // double-derivative textures.)
+
+               // Calculate I_x and I_y.
+               GLuint I_x_y_tex;
+               glCreateTextures(GL_TEXTURE_2D, 1, &I_x_y_tex);
+               glTextureStorage2D(I_x_y_tex, 1, GL_RG16F, level_width, level_height);
+               derivatives.exec(I_tex, I_x_y_tex, level_width, level_height);
+
                prev_level_flow_tex = dense_flow_tex;
        }
        glEndQuery(GL_TIME_ELAPSED);