[movit] / resample_effect.cpp

// Three-lobed Lanczos, the most common choice.
#define LANCZOS_RADIUS 3.0

#include <math.h>
#include <assert.h>

#include "resample_effect.h"
#include "effect_chain.h"
#include "util.h"
#include "opengl.h"

namespace {

float sinc(float x)
{
        if (fabs(x) < 1e-6) {
                return 1.0f - fabs(x);
        } else {
                return sin(x) / x;
        }
}

float lanczos_weight(float x, float a)
{
        if (fabs(x) > a) {
                return 0.0f;
        } else {
                return sinc(M_PI * x) * sinc(M_PI * x / a);
        }
}

// Euclid's algorithm, from Wikipedia.
unsigned gcd(unsigned a, unsigned b)
{
        while (b != 0) {
                unsigned t = b;
                b = a % b;
                a = t;
        }
        return a;
}

unsigned combine_samples(float *src, float *dst, unsigned num_src_samples, unsigned max_samples_saved)
{
        unsigned num_samples_saved = 0;
        for (unsigned i = 0, j = 0; i < num_src_samples; ++i, ++j) {
                // Copy the sample directly; it will be overwritten later if we can combine.
                if (dst != NULL) {
                        dst[j * 2 + 0] = src[i * 2 + 0];
                        dst[j * 2 + 1] = src[i * 2 + 1];
                }

                if (i == num_src_samples - 1) {
                        // Last sample; cannot combine.
                        continue;
                }
                if (num_samples_saved == max_samples_saved) {
                        // We could maybe save more here, but other rows can't, so don't bother.
                        continue;
                }

                float w1 = src[i * 2 + 0];
                float w2 = src[(i + 1) * 2 + 0];
                if (w1 * w2 < 0.0f) {
                        // Differing signs; cannot combine.
                        continue;
                }

                float pos1 = src[i * 2 + 1];
                float pos2 = src[(i + 1) * 2 + 1];
                assert(pos2 > pos1);

                float offset, total_weight;
                combine_two_samples(w1, w2, &offset, &total_weight);

                // OK, we can combine this and the next sample.
                if (dst != NULL) {
                        dst[j * 2 + 0] = total_weight;
                        dst[j * 2 + 1] = pos1 + offset * (pos2 - pos1);
                }

                ++i;  // Skip the next sample.
                ++num_samples_saved;
        }
        return num_samples_saved;
}

}  // namespace

ResampleEffect::ResampleEffect()
        : input_width(1280),
          input_height(720)
{
        register_int("width", &output_width);
        register_int("height", &output_height);

        // The first blur pass will forward resolution information to us.
        hpass = new SingleResamplePassEffect(this);
        hpass->set_int("direction", SingleResamplePassEffect::HORIZONTAL);
        vpass = new SingleResamplePassEffect(NULL);
        vpass->set_int("direction", SingleResamplePassEffect::VERTICAL);

        update_size();
}

void ResampleEffect::rewrite_graph(EffectChain *graph, Node *self)
{
        Node *hpass_node = graph->add_node(hpass);
        Node *vpass_node = graph->add_node(vpass);
        graph->connect_nodes(hpass_node, vpass_node);
        graph->replace_receiver(self, hpass_node);
        graph->replace_sender(self, vpass_node);
        self->disabled = true;
} 

// We get this information forwarded from the first blur pass,
// since we are not part of the chain ourselves.
void ResampleEffect::inform_input_size(unsigned input_num, unsigned width, unsigned height)
{
        assert(input_num == 0);
        assert(width != 0);
        assert(height != 0);
        input_width = width;
        input_height = height;
        update_size();
}
                
void ResampleEffect::update_size()
{
        bool ok = true;
        ok |= hpass->set_int("input_width", input_width);
        ok |= hpass->set_int("input_height", input_height);
        ok |= hpass->set_int("output_width", output_width);
        ok |= hpass->set_int("output_height", input_height);

        ok |= vpass->set_int("input_width", output_width);
        ok |= vpass->set_int("input_height", input_height);
        ok |= vpass->set_int("output_width", output_width);
        ok |= vpass->set_int("output_height", output_height);

        assert(ok);
}

bool ResampleEffect::set_float(const std::string &key, float value) {
        if (key == "width") {
                output_width = value;
                update_size();
                return true;
        }
        if (key == "height") {
                output_height = value;
                update_size();
                return true;
        }
        return false;
}

SingleResamplePassEffect::SingleResamplePassEffect(ResampleEffect *parent)
        : parent(parent),
          direction(HORIZONTAL),
          input_width(1280),
          input_height(720),
          last_input_width(-1),
          last_input_height(-1),
          last_output_width(-1),
          last_output_height(-1)
{
        register_int("direction", (int *)&direction);
        register_int("input_width", &input_width);
        register_int("input_height", &input_height);
        register_int("output_width", &output_width);
        register_int("output_height", &output_height);

        glGenTextures(1, &texnum);
}

SingleResamplePassEffect::~SingleResamplePassEffect()
{
        glDeleteTextures(1, &texnum);
}

std::string SingleResamplePassEffect::output_fragment_shader()
{
        char buf[256];
        sprintf(buf, "#define DIRECTION_VERTICAL %d\n", (direction == VERTICAL));
        return buf + read_file("resample_effect.frag");
}

// Using vertical scaling as an example:
//
// Generally out[y] = w0 * in[yi] + w1 * in[yi + 1] + w2 * in[yi + 2] + ...
//
// Obviously, yi will depend on y (in a not-quite-linear way), but so will
// the weights w0, w1, w2, etc.. The easiest way of doing this is to encode,
// for each sample, the weight and the yi value, e.g. <yi, w0>, <yi + 1, w1>,
// and so on. For each y, we encode these along the x-axis (since that is spare),
// so out[0] will read from parameters <x,y> = <0,0>, <1,0>, <2,0> and so on.
//
// For horizontal scaling, we fill in the exact same texture;
// the shader just interprets is differently.
void SingleResamplePassEffect::update_texture(GLuint glsl_program_num, const std::string &prefix, unsigned *sampler_num)
{
        unsigned src_size, dst_size;
        if (direction == SingleResamplePassEffect::HORIZONTAL) {
                assert(input_height == output_height);
                src_size = input_width;
                dst_size = output_width;
        } else if (direction == SingleResamplePassEffect::VERTICAL) {
                assert(input_width == output_width);
                src_size = input_height;
                dst_size = output_height;
        } else {
                assert(false);
        }


        // For many resamplings (e.g. 640 -> 1280), we will end up with the same
        // set of samples over and over again in a loop. Thus, we can compute only
        // the first such loop, and then ask the card to repeat the texture for us.
        // This is both easier on the texture cache and lowers our CPU cost for
        // generating the kernel somewhat.
        num_loops = gcd(src_size, dst_size);
        slice_height = 1.0f / num_loops;
        unsigned dst_samples = dst_size / num_loops;

        // Sample the kernel in the right place. A diagram with a triangular kernel
        // (corresponding to linear filtering, and obviously with radius 1)
        // for easier ASCII art drawing:
        //
        //                *
        //               / \                      |
        //              /   \                     |
        //             /     \                    |
        //    x---x---x   x   x---x---x---x
        //
        // Scaling up (in this case, 2x) means sampling more densely:
        //
        //                *
        //               / \                      |
        //              /   \                     |
        //             /     \                    |
        //   x-x-x-x-x-x x x x-x-x-x-x-x-x-x
        //
        // When scaling up, any destination pixel will only be influenced by a few
        // (in this case, two) neighboring pixels, and more importantly, the number
        // will not be influenced by the scaling factor. (Note, however, that the
        // pixel centers have moved, due to OpenGL's center-pixel convention.)
        // The only thing that changes is the weights themselves, as the sampling
        // points are at different distances from the original pixels.
        //
        // Scaling down is a different story:
        //
        //                *
        //               / \                      |
        //              /   \                     |
        //             /     \                    |
        //    --x------ x     --x-------x--
        //
        // Again, the pixel centers have moved in a maybe unintuitive fashion,
        // although when you consider that there are multiple source pixels around,
        // it's not so bad as at first look:
        //
        //            *   *   *   *
        //           / \ / \ / \ / \              |
        //          /   X   X   X   \             |
        //         /   / \ / \ / \   \            |
        //    --x-------x-------x-------x--
        //
        // As you can see, the new pixels become averages of the two neighboring old
        // ones (the situation for Lanczos is of course more complex).
        //
        // Anyhow, in this case we clearly need to look at more source pixels
        // to compute the destination pixel, and how many depend on the scaling factor.
        // Thus, the kernel width will vary with how much we scale.
        float radius_scaling_factor = std::min(float(dst_size) / float(src_size), 1.0f);
        int int_radius = lrintf(LANCZOS_RADIUS / radius_scaling_factor);
        int src_samples = int_radius * 2 + 1;
        float *weights = new float[dst_samples * src_samples * 2];
        for (unsigned y = 0; y < dst_samples; ++y) {
                // Find the point around which we want to sample the source image,
                // compensating for differing pixel centers as the scale changes.
                float center_src_y = (y + 0.5f) * float(src_size) / float(dst_size) - 0.5f;
                int base_src_y = lrintf(center_src_y);

                // Now sample <int_radius> pixels on each side around that point.
                for (int i = 0; i < src_samples; ++i) {
                        int src_y = base_src_y + i - int_radius;
                        float weight = lanczos_weight(radius_scaling_factor * (src_y - center_src_y), LANCZOS_RADIUS);
                        weights[(y * src_samples + i) * 2 + 0] = weight * radius_scaling_factor;
                        weights[(y * src_samples + i) * 2 + 1] = (src_y + 0.5) / float(src_size);
                }
        }

        // Now make use of the bilinear filtering in the GPU to reduce the number of samples
        // we need to make. This is a bit more complex than BlurEffect since we cannot combine
        // two neighboring samples if their weights have differing signs, so we first need to
        // figure out the maximum number of samples. Then, we downconvert all the weights to
        // that number -- we could have gone for a variable-length system, but this is simpler,
        // and the gains would probably be offset by the extra cost of checking when to stop.
        //
        // The greedy strategy for combining samples is optimal.
        src_bilinear_samples = 0;
        for (unsigned y = 0; y < dst_samples; ++y) {
                unsigned num_samples_saved = combine_samples(weights + (y * src_samples) * 2, NULL, src_samples, UINT_MAX);
                src_bilinear_samples = std::max<int>(src_bilinear_samples, src_samples - num_samples_saved);
        }

        // Now that we know the right width, actually combine the samples.
        float *bilinear_weights = new float[dst_samples * src_bilinear_samples * 2];
        for (unsigned y = 0; y < dst_samples; ++y) {
                unsigned num_samples_saved = combine_samples(
                        weights + (y * src_samples) * 2,
                        bilinear_weights + (y * src_bilinear_samples) * 2,
                        src_samples,
                        src_samples - src_bilinear_samples);
                assert(int(src_samples) - int(num_samples_saved) == src_bilinear_samples);
        }       

        // Encode as a two-component texture. Note the GL_REPEAT.
        glActiveTexture(GL_TEXTURE0 + *sampler_num);
        check_error();
        glBindTexture(GL_TEXTURE_2D, texnum);
        check_error();
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        check_error();
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
        check_error();
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
        check_error();
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RG32F, src_bilinear_samples, dst_samples, 0, GL_RG, GL_FLOAT, bilinear_weights);
        check_error();

        delete[] weights;
        delete[] bilinear_weights;
}

void SingleResamplePassEffect::set_gl_state(GLuint glsl_program_num, const std::string &prefix, unsigned *sampler_num)
{
        Effect::set_gl_state(glsl_program_num, prefix, sampler_num);

        if (input_width != last_input_width ||
            input_height != last_input_height ||
            output_width != last_output_width ||
            output_height != last_output_height) {
                update_texture(glsl_program_num, prefix, sampler_num);
                last_input_width = input_width;
                last_input_height = input_height;
                last_output_width = output_width;
                last_output_height = output_height;
        }

        glActiveTexture(GL_TEXTURE0 + *sampler_num);
        check_error();
        glBindTexture(GL_TEXTURE_2D, texnum);
        check_error();

        set_uniform_int(glsl_program_num, prefix, "sample_tex", *sampler_num);
        ++sampler_num;
        set_uniform_int(glsl_program_num, prefix, "num_samples", src_bilinear_samples);
        set_uniform_float(glsl_program_num, prefix, "num_loops", num_loops);
        set_uniform_float(glsl_program_num, prefix, "slice_height", slice_height);

        // Instructions for how to convert integer sample numbers to positions in the weight texture.
        set_uniform_float(glsl_program_num, prefix, "sample_x_scale", 1.0f / src_bilinear_samples);
        set_uniform_float(glsl_program_num, prefix, "sample_x_offset", 0.5f / src_bilinear_samples);

        // We specifically do not want mipmaps on the input texture;
        // they break minification.
        glActiveTexture(GL_TEXTURE0);
        check_error();
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        check_error();
}