path: root/video/out/opengl/video_shaders.c

/*
 * This file is part of mpv.
 *
 * mpv is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * mpv is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <math.h>

#include "video_shaders.h"
#include "video.h"

#define GLSL(x) gl_sc_add(sc, #x "\n");
#define GLSLF(...) gl_sc_addf(sc, __VA_ARGS__)
#define GLSLH(x) gl_sc_hadd(sc, #x "\n");
#define GLSLHF(...) gl_sc_haddf(sc, __VA_ARGS__)

// Set up shared/commonly used variables and macros
void sampler_prelude(struct gl_shader_cache *sc, int tex_num)
{
    GLSLF("#undef tex\n");
    GLSLF("#define tex texture%d\n", tex_num);
    GLSLF("vec2 pos = texcoord%d;\n", tex_num);
    GLSLF("vec2 size = texture_size%d;\n", tex_num);
    GLSLF("vec2 pt = pixel_size%d;\n", tex_num);
}

static void pass_sample_separated_get_weights(struct gl_shader_cache *sc,
                                              struct scaler *scaler)
{
    gl_sc_uniform_tex(sc, "lut", scaler->gl_target, scaler->gl_lut);
    // Define a new variable to cache the corrected fcoord.
    GLSLF("float fcoord_lut = LUT_POS(fcoord, %d.0);\n", scaler->lut_size);

    int N = scaler->kernel->size;
    if (N == 2) {
        GLSL(vec2 c1 = texture(lut, vec2(0.5, fcoord_lut)).rg;)
        GLSL(float weights[2] = float[](c1.r, c1.g);)
    } else if (N == 6) {
        GLSL(vec4 c1 = texture(lut, vec2(0.25, fcoord_lut));)
        GLSL(vec4 c2 = texture(lut, vec2(0.75, fcoord_lut));)
        GLSL(float weights[6] = float[](c1.r, c1.g, c1.b, c2.r, c2.g, c2.b);)
    } else {
        GLSLF("float weights[%d];\n", N);
        for (int n = 0; n < N / 4; n++) {
            GLSLF("c = texture(lut, vec2(1.0 / %d.0 + %d.0 / %d.0, fcoord_lut));\n",
                    N / 2, n, N / 4);
            GLSLF("weights[%d] = c.r;\n", n * 4 + 0);
            GLSLF("weights[%d] = c.g;\n", n * 4 + 1);
            GLSLF("weights[%d] = c.b;\n", n * 4 + 2);
            GLSLF("weights[%d] = c.a;\n", n * 4 + 3);
        }
    }
}

// Handle a single pass (either vertical or horizontal). The direction is given
// by the vector (d_x, d_y). If the vector is 0, then planar interpolation is
// used instead (samples from texture0 through textureN)
void pass_sample_separated_gen(struct gl_shader_cache *sc, struct scaler *scaler,
                               int d_x, int d_y)
{
    int N = scaler->kernel->size;
    bool use_ar = scaler->conf.antiring > 0;
    bool planar = d_x == 0 && d_y == 0;
    GLSL(color = vec4(0.0);)
    GLSLF("{\n");
    if (!planar) {
        GLSLF("vec2 dir = vec2(%d.0, %d.0);\n", d_x, d_y);
        GLSL(pt *= dir;)
        GLSL(float fcoord = dot(fract(pos * size - vec2(0.5)), dir);)
        GLSLF("vec2 base = pos - fcoord * pt - pt * vec2(%d.0);\n", N / 2 - 1);
    }
    GLSL(vec4 c;)
    if (use_ar) {
        GLSL(vec4 hi = vec4(0.0);)
        GLSL(vec4 lo = vec4(1.0);)
    }
    pass_sample_separated_get_weights(sc, scaler);
    GLSLF("// scaler samples\n");
    for (int n = 0; n < N; n++) {
        if (planar) {
            GLSLF("c = texture(texture%d, texcoord%d);\n", n, n);
        } else {
            GLSLF("c = texture(tex, base + pt * vec2(%d.0));\n", n);
        }
        GLSLF("color += vec4(weights[%d]) * c;\n", n);
        if (use_ar && (n == N/2-1 || n == N/2)) {
            GLSL(lo = min(lo, c);)
            GLSL(hi = max(hi, c);)
        }
    }
    if (use_ar)
        GLSLF("color = mix(color, clamp(color, lo, hi), %f);\n",
              scaler->conf.antiring);
    GLSLF("}\n");
}

void pass_sample_polar(struct gl_shader_cache *sc, struct scaler *scaler)
{
    double radius = scaler->kernel->f.radius;
    int bound = (int)ceil(radius);
    bool use_ar = scaler->conf.antiring > 0;
    GLSL(color = vec4(0.0);)
    GLSLF("{\n");
    GLSL(vec2 fcoord = fract(pos * size - vec2(0.5));)
    GLSL(vec2 base = pos - fcoord * pt;)
    GLSL(vec4 c;)
    GLSLF("float w, d, wsum = 0.0;\n");
    if (use_ar) {
        GLSL(vec4 lo = vec4(1.0);)
        GLSL(vec4 hi = vec4(0.0);)
    }
    gl_sc_uniform_tex(sc, "lut", scaler->gl_target, scaler->gl_lut);
    GLSLF("// scaler samples\n");
    for (int y = 1-bound; y <= bound; y++) {
        for (int x = 1-bound; x <= bound; x++) {
            // Since we can't know the subpixel position in advance, assume a
            // worst case scenario
            int yy = y > 0 ? y-1 : y;
            int xx = x > 0 ? x-1 : x;
            double dmax = sqrt(xx*xx + yy*yy);
            // Skip samples definitely outside the radius
            if (dmax >= radius)
                continue;
            GLSLF("d = length(vec2(%d.0, %d.0) - fcoord)/%f;\n", x, y, radius);
            // Check for samples that might be skippable
            if (dmax >= radius - M_SQRT2)
                GLSLF("if (d < 1.0) {\n");
            if (scaler->gl_target == GL_TEXTURE_1D) {
                GLSLF("w = texture1D(lut, LUT_POS(d, %d.0)).r;\n",
                      scaler->lut_size);
            } else {
                GLSLF("w = texture(lut, vec2(0.5, LUT_POS(d, %d.0))).r;\n",
                      scaler->lut_size);
            }
            GLSL(wsum += w;)
            GLSLF("c = texture(tex, base + pt * vec2(%d.0, %d.0));\n", x, y);
            GLSL(color += vec4(w) * c;)
            if (use_ar && x >= 0 && y >= 0 && x <= 1 && y <= 1) {
                GLSL(lo = min(lo, c);)
                GLSL(hi = max(hi, c);)
            }
            if (dmax >= radius - M_SQRT2)
                GLSLF("}\n");
        }
    }
    GLSL(color = color / vec4(wsum);)
    if (use_ar)
        GLSLF("color = mix(color, clamp(color, lo, hi), %f);\n",
              scaler->conf.antiring);
    GLSLF("}\n");
}

static void bicubic_calcweights(struct gl_shader_cache *sc, const char *t, const char *s)
{
    // Explanation of how bicubic scaling with only 4 texel fetches is done:
    //   http://www.mate.tue.nl/mate/pdfs/10318.pdf
    //   'Efficient GPU-Based Texture Interpolation using Uniform B-Splines'
    // Explanation why this algorithm normally always blurs, even with unit
    // scaling:
    //   http://bigwww.epfl.ch/preprints/ruijters1001p.pdf
    //   'GPU Prefilter for Accurate Cubic B-spline Interpolation'
    GLSLF("vec4 %s = vec4(-0.5, 0.1666, 0.3333, -0.3333) * %s"
                " + vec4(1, 0, -0.5, 0.5);\n", t, s);
    GLSLF("%s = %s * %s + vec4(0, 0, -0.5, 0.5);\n", t, t, s);
    GLSLF("%s = %s * %s + vec4(-0.6666, 0, 0.8333, 0.1666);\n", t, t, s);
    GLSLF("%s.xy *= vec2(1, 1) / vec2(%s.z, %s.w);\n", t, t, t);
    GLSLF("%s.xy += vec2(1.0 + %s, 1.0 - %s);\n", t, s, s);
}

void pass_sample_bicubic_fast(struct gl_shader_cache *sc)
{
    GLSLF("{\n");
    GLSL(vec2 fcoord = fract(pos * size + vec2(0.5, 0.5));)
    bicubic_calcweights(sc, "parmx", "fcoord.x");
    bicubic_calcweights(sc, "parmy", "fcoord.y");
    GLSL(vec4 cdelta;)
    GLSL(cdelta.xz = parmx.rg * vec2(-pt.x, pt.x);)
    GLSL(cdelta.yw = parmy.rg * vec2(-pt.y, pt.y);)
    // first y-interpolation
    GLSL(vec4 ar = texture(tex, pos + cdelta.xy);)
    GLSL(vec4 ag = texture(tex, pos + cdelta.xw);)
    GLSL(vec4 ab = mix(ag, ar, parmy.b);)
    // second y-interpolation
    GLSL(vec4 br = texture(tex, pos + cdelta.zy);)
    GLSL(vec4 bg = texture(tex, pos + cdelta.zw);)
    GLSL(vec4 aa = mix(bg, br, parmy.b);)
    // x-interpolation
    GLSL(color = mix(aa, ab, parmx.b);)
    GLSLF("}\n");
}

void pass_sample_oversample(struct gl_shader_cache *sc, struct scaler *scaler,
                                   int w, int h)
{
    GLSLF("{\n");
    GLSL(vec2 pos = pos + vec2(0.5) * pt;) // round to nearest
    GLSL(vec2 fcoord = fract(pos * size - vec2(0.5));)
    // Determine the mixing coefficient vector
    gl_sc_uniform_vec2(sc, "output_size", (float[2]){w, h});
    GLSL(vec2 coeff = fcoord * output_size/size;)
    float threshold = scaler->conf.kernel.params[0];
    threshold = isnan(threshold) ? 0.0 : threshold;
    GLSLF("coeff = (coeff - %f) / %f;\n", threshold, 1.0 - 2 * threshold);
    GLSL(coeff = clamp(coeff, 0.0, 1.0);)
    // Compute the right blend of colors
    GLSL(color = texture(tex, pos + pt * (coeff - fcoord));)
    GLSLF("}\n");
}

// Common constants for SMPTE ST.2084 (HDR)
static const float HDR_M1 = 2610./4096 * 1./4,
                   HDR_M2 = 2523./4096 * 128,
                   HDR_C1 = 3424./4096,
                   HDR_C2 = 2413./4096 * 32,
                   HDR_C3 = 2392./4096 * 32;

// Common constants for ARIB STD-B67 (Hybrid Log-gamma)
static const float B67_A = 0.17883277,
                   B67_B = 0.28466892,
                   B67_C = 0.55991073;

// Common constants for Panasonic V-Log
static const float VLOG_B = 0.00873,
                   VLOG_C = 0.241514,
                   VLOG_D = 0.598206,
                   VLOG_R = 46.085527; // nominal peak

// Linearize (expand), given a TRC as input. This corresponds to the EOTF
// in ITU-R terminology.
void pass_linearize(struct gl_shader_cache *sc, enum mp_csp_trc trc)
{
    if (trc == MP_CSP_TRC_LINEAR)
        return;

    // Note that this clamp may technically violate the definition of
    // ITU-R BT.2100, which allows for sub-blacks and super-whites to be
    // displayed on the display where such would be possible. That said, the
    // problem is that not all gamma curves are well-defined on the values
    // outside this range, so we ignore it and just clip anyway for sanity.
    GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)

    switch (trc) {
    case MP_CSP_TRC_SRGB:
        GLSL(color.rgb = mix(color.rgb / vec3(12.92),
                             pow((color.rgb + vec3(0.055))/vec3(1.055), vec3(2.4)),
                             lessThan(vec3(0.04045), color.rgb));)
        break;
    case MP_CSP_TRC_BT_1886:
        // We don't have an actual black point, so we assume a perfect display
        GLSL(color.rgb = pow(color.rgb, vec3(2.4));)
        break;
    case MP_CSP_TRC_GAMMA18:
        GLSL(color.rgb = pow(color.rgb, vec3(1.8));)
        break;
    case MP_CSP_TRC_GAMMA22:
        GLSL(color.rgb = pow(color.rgb, vec3(2.2));)
        break;
    case MP_CSP_TRC_GAMMA28:
        GLSL(color.rgb = pow(color.rgb, vec3(2.8));)
        break;
    case MP_CSP_TRC_PRO_PHOTO:
        GLSL(color.rgb = mix(color.rgb / vec3(16.0),
                             pow(color.rgb, vec3(1.8)),
                             lessThan(vec3(0.03125), color.rgb));)
        break;
    case MP_CSP_TRC_SMPTE_ST2084:
        GLSLF("color.rgb = pow(color.rgb, vec3(1.0/%f));\n", HDR_M2);
        GLSLF("color.rgb = max(color.rgb - vec3(%f), vec3(0.0)) \n"
              "             / (vec3(%f) - vec3(%f) * color.rgb);\n",
              HDR_C1, HDR_C2, HDR_C3);
        GLSLF("color.rgb = pow(color.rgb, vec3(1.0/%f));\n", HDR_M1);
        break;
    case MP_CSP_TRC_ARIB_STD_B67:
        GLSLF("color.rgb = mix(vec3(4.0) * color.rgb * color.rgb,\n"
              "                exp((color.rgb - vec3(%f)) / vec3(%f)) + vec3(%f),\n"
              "                lessThan(vec3(0.5), color.rgb));\n",
              B67_C, B67_A, B67_B);
        // Since the ARIB function's signal value of 1.0 corresponds to
        // a peak of 12.0, we need to renormalize to prevent GL textures
        // from clipping. (In general, mpv's internal conversions always
        // assume 1.0 is the maximum brightness, not the reference peak)
        GLSL(color.rgb /= vec3(12.0);)
        break;
    case MP_CSP_TRC_V_LOG:
        GLSLF("color.rgb = mix((color.rgb - vec3(0.125)) / vec3(5.6), \n"
              "    pow(vec3(10.0), (color.rgb - vec3(%f)) / vec3(%f)) \n"
              "              - vec3(%f),                              \n"
              "    lessThanEqual(vec3(0.181), color.rgb));            \n",
              VLOG_D, VLOG_C, VLOG_B);
        // Same deal as with the B67 function, renormalize to texture range
        GLSLF("color.rgb /= vec3(%f);\n", VLOG_R);
        GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)
        break;
    default:
        abort();
    }
}

// Delinearize (compress), given a TRC as output. This corresponds to the
// inverse EOTF (not the OETF) in ITU-R terminology.
void pass_delinearize(struct gl_shader_cache *sc, enum mp_csp_trc trc)
{
    if (trc == MP_CSP_TRC_LINEAR)
        return;

    GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)
    switch (trc) {
    case MP_CSP_TRC_SRGB:
        GLSL(color.rgb = mix(color.rgb * vec3(12.92),
                             vec3(1.055) * pow(color.rgb, vec3(1.0/2.4))
                                 - vec3(0.055),
                             lessThanEqual(vec3(0.0031308), color.rgb));)
        break;
    case MP_CSP_TRC_BT_1886:
        GLSL(color.rgb = pow(color.rgb, vec3(1.0/2.4));)
        break;
    case MP_CSP_TRC_GAMMA18:
        GLSL(color.rgb = pow(color.rgb, vec3(1.0/1.8));)
        break;
    case MP_CSP_TRC_GAMMA22:
        GLSL(color.rgb = pow(color.rgb, vec3(1.0/2.2));)
        break;
    case MP_CSP_TRC_GAMMA28:
        GLSL(color.rgb = pow(color.rgb, vec3(1.0/2.8));)
        break;
    case MP_CSP_TRC_PRO_PHOTO:
        GLSL(color.rgb = mix(color.rgb * vec3(16.0),
                             pow(color.rgb, vec3(1.0/1.8)),
                             lessThanEqual(vec3(0.001953), color.rgb));)
        break;
    case MP_CSP_TRC_SMPTE_ST2084:
        GLSLF("color.rgb = pow(color.rgb, vec3(%f));\n", HDR_M1);
        GLSLF("color.rgb = (vec3(%f) + vec3(%f) * color.rgb) \n"
              "             / (vec3(1.0) + vec3(%f) * color.rgb);\n",
              HDR_C1, HDR_C2, HDR_C3);
        GLSLF("color.rgb = pow(color.rgb, vec3(%f));\n", HDR_M2);
        break;
    case MP_CSP_TRC_ARIB_STD_B67:
        GLSL(color.rgb *= vec3(12.0);)
        GLSLF("color.rgb = mix(vec3(0.5) * sqrt(color.rgb),\n"
              "                vec3(%f) * log(color.rgb - vec3(%f)) + vec3(%f),\n"
              "                lessThan(vec3(1.0), color.rgb));\n",
              B67_A, B67_B, B67_C);
        break;
    case MP_CSP_TRC_V_LOG:
        GLSLF("color.rgb *= vec3(%f);\n", VLOG_R);
        GLSLF("color.rgb = mix(vec3(5.6) * color.rgb + vec3(0.125),   \n"
              "                vec3(%f) * log(color.rgb + vec3(%f))   \n"
              "                    + vec3(%f),                        \n"
              "                lessThanEqual(vec3(0.01), color.rgb)); \n",
              VLOG_C / M_LN10, VLOG_B, VLOG_D);
        break;
    default:
        abort();
    }
}

// Tone map from a known peak brightness to the range [0,1]
static void pass_tone_map(struct gl_shader_cache *sc, float ref_peak,
                          enum tone_mapping algo, float param)
{
    GLSLF("// HDR tone mapping\n");

    switch (algo) {
    case TONE_MAPPING_CLIP:
        GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)
        break;

    case TONE_MAPPING_REINHARD: {
        float contrast = isnan(param) ? 0.5 : param,
              offset = (1.0 - contrast) / contrast;
        GLSLF("color.rgb = color.rgb / (color.rgb + vec3(%f));\n", offset);
        GLSLF("color.rgb *= vec3(%f);\n", (ref_peak + offset) / ref_peak);
        break;
    }

    case TONE_MAPPING_HABLE: {
        float A = 0.15, B = 0.50, C = 0.10, D = 0.20, E = 0.02, F = 0.30;
        GLSLHF("vec3 hable(vec3 x) {\n");
        GLSLHF("return ((x * (%f*x + %f)+%f)/(x * (%f*x + %f) + %f)) - %f;\n",
               A, C*B, D*E, A, B, D*F, E/F);
        GLSLHF("}\n");

        GLSLF("color.rgb = hable(color.rgb) / hable(vec3(%f));\n", ref_peak);
        break;
    }

    case TONE_MAPPING_GAMMA: {
        float gamma = isnan(param) ? 1.8 : param;
        GLSLF("color.rgb = pow(color.rgb / vec3(%f), vec3(%f));\n",
              ref_peak, 1.0/gamma);
        break;
    }

    case TONE_MAPPING_LINEAR: {
        float coeff = isnan(param) ? 1.0 : param;
        GLSLF("color.rgb = vec3(%f) * color.rgb;\n", coeff / ref_peak);
        break;
    }

    default:
        abort();
    }
}

// Map colors from one source space to another. These source spaces
// must be known (i.e. not MP_CSP_*_AUTO), as this function won't perform
// any auto-guessing.
void pass_color_map(struct gl_shader_cache *sc,
                    struct mp_colorspace src, struct mp_colorspace dst,
                    enum tone_mapping algo, float tone_mapping_param)
{
    GLSLF("// color mapping\n");

    // All operations from here on require linear light as a starting point,
    // so we linearize even if src.gamma == dst.gamma when one of the other
    // operations needs it
    bool need_gamma = src.gamma != dst.gamma ||
                      src.primaries != dst.primaries ||
                      src.nom_peak != dst.nom_peak ||
                      src.sig_peak > dst.nom_peak;

    if (need_gamma)
        pass_linearize(sc, src.gamma);

    // NOTE: When src.gamma = MP_CSP_TRC_ARIB_STD_B67, we would technically
    // need to apply the reference OOTF as part of the EOTF (which is what we
    // implement with pass_linearize), since HLG considers OOTF to be part of
    // the display's EOTF (as opposed to the camera's OETF). But since this is
    // stupid, complicated, arbitrary, and more importantly depends on the
    // target display's signal peak (which is != the nom_peak in the case of
    // HDR displays, and mpv already has enough target-specific display
    // options), we just ignore its implementation entirely. (Plus, it doesn't
    // even really make sense with tone mapping to begin with.) But just in
    // case somebody ends up complaining about HLG looking different from a
    // reference HLG display, this comment might be why.

    // Stretch the signal value to renormalize to the dst nominal peak
    if (src.nom_peak != dst.nom_peak)
        GLSLF("color.rgb *= vec3(%f);\n", src.nom_peak / dst.nom_peak);

    // Tone map to prevent clipping when the source signal peak exceeds the
    // encodable range.
    if (src.sig_peak > dst.nom_peak)
        pass_tone_map(sc, src.sig_peak / dst.nom_peak, algo, tone_mapping_param);

    // Adapt to the right colorspace if necessary
    if (src.primaries != dst.primaries) {
        struct mp_csp_primaries csp_src = mp_get_csp_primaries(src.primaries),
                                csp_dst = mp_get_csp_primaries(dst.primaries);
        float m[3][3] = {{0}};
        mp_get_cms_matrix(csp_src, csp_dst, MP_INTENT_RELATIVE_COLORIMETRIC, m);
        gl_sc_uniform_mat3(sc, "cms_matrix", true, &m[0][0]);
        GLSL(color.rgb = cms_matrix * color.rgb;)
    }

    if (need_gamma)
        pass_delinearize(sc, dst.gamma);
}

// Wide usage friendly PRNG, shamelessly stolen from a GLSL tricks forum post.
// Obtain random numbers by calling rand(h), followed by h = permute(h) to
// update the state. Assumes the texture was hooked.
static void prng_init(struct gl_shader_cache *sc, AVLFG *lfg)
{
    GLSLH(float mod289(float x)  { return x - floor(x / 289.0) * 289.0; })
    GLSLH(float permute(float x) { return mod289((34.0*x + 1.0) * x); })
    GLSLH(float rand(float x)    { return fract(x / 41.0); })

    // Initialize the PRNG by hashing the position + a random uniform
    GLSL(vec3 _m = vec3(HOOKED_pos, random) + vec3(1.0);)
    GLSL(float h = permute(permute(permute(_m.x)+_m.y)+_m.z);)
    gl_sc_uniform_f(sc, "random", (double)av_lfg_get(lfg) / UINT32_MAX);
}

struct deband_opts {
    int enabled;
    int iterations;
    float threshold;
    float range;
    float grain;
};

const struct deband_opts deband_opts_def = {
    .iterations = 1,
    .threshold = 64.0,
    .range = 16.0,
    .grain = 48.0,
};

#define OPT_BASE_STRUCT struct deband_opts
const struct m_sub_options deband_conf = {
    .opts = (const m_option_t[]) {
        OPT_INTRANGE("iterations", iterations, 0, 1, 16),
        OPT_FLOATRANGE("threshold", threshold, 0, 0.0, 4096.0),
        OPT_FLOATRANGE("range", range, 0, 1.0, 64.0),
        OPT_FLOATRANGE("grain", grain, 0, 0.0, 4096.0),
        {0}
    },
    .size = sizeof(struct deband_opts),
    .defaults = &deband_opts_def,
};

// Stochastically sample a debanded result from a hooked texture.
void pass_sample_deband(struct gl_shader_cache *sc, struct deband_opts *opts,
                        AVLFG *lfg)
{
    // Initialize the PRNG
    GLSLF("{\n");
    prng_init(sc, lfg);

    // Helper: Compute a stochastic approximation of the avg color around a
    // pixel
    GLSLHF("vec4 average(float range, inout float h) {\n");
        // Compute a random rangle and distance
        GLSLH(float dist = rand(h) * range;     h = permute(h);)
        GLSLH(float dir  = rand(h) * 6.2831853; h = permute(h);)
        GLSLH(vec2 o = dist * vec2(cos(dir), sin(dir));)

        // Sample at quarter-turn intervals around the source pixel
        GLSLH(vec4 ref[4];)
        GLSLH(ref[0] = HOOKED_texOff(vec2( o.x,  o.y));)
        GLSLH(ref[1] = HOOKED_texOff(vec2(-o.y,  o.x));)
        GLSLH(ref[2] = HOOKED_texOff(vec2(-o.x, -o.y));)
        GLSLH(ref[3] = HOOKED_texOff(vec2( o.y, -o.x));)

        // Return the (normalized) average
        GLSLH(return (ref[0] + ref[1] + ref[2] + ref[3])/4.0;)
    GLSLHF("}\n");

    // Sample the source pixel
    GLSL(color = HOOKED_tex(HOOKED_pos);)
    GLSLF("vec4 avg, diff;\n");
    for (int i = 1; i <= opts->iterations; i++) {
        // Sample the average pixel and use it instead of the original if
        // the difference is below the given threshold
        GLSLF("avg = average(%f, h);\n", i * opts->range);
        GLSL(diff = abs(color - avg);)
        GLSLF("color = mix(avg, color, greaterThan(diff, vec4(%f)));\n",
              opts->threshold / (i * 16384.0));
    }

    // Add some random noise to smooth out residual differences
    GLSL(vec3 noise;)
    GLSL(noise.x = rand(h); h = permute(h);)
    GLSL(noise.y = rand(h); h = permute(h);)
    GLSL(noise.z = rand(h); h = permute(h);)
    GLSLF("color.xyz += %f * (noise - vec3(0.5));\n", opts->grain/8192.0);
    GLSLF("}\n");
}

// Assumes the texture was hooked
void pass_sample_unsharp(struct gl_shader_cache *sc, float param) {
    GLSLF("// unsharp\n");
    GLSLF("{\n");
    GLSL(float st1 = 1.2;)
    GLSL(vec4 p = HOOKED_tex(HOOKED_pos);)
    GLSL(vec4 sum1 = HOOKED_texOff(st1 * vec2(+1, +1))
                   + HOOKED_texOff(st1 * vec2(+1, -1))
                   + HOOKED_texOff(st1 * vec2(-1, +1))
                   + HOOKED_texOff(st1 * vec2(-1, -1));)
    GLSL(float st2 = 1.5;)
    GLSL(vec4 sum2 = HOOKED_texOff(st2 * vec2(+1,  0))
                   + HOOKED_texOff(st2 * vec2( 0, +1))
                   + HOOKED_texOff(st2 * vec2(-1,  0))
                   + HOOKED_texOff(st2 * vec2( 0, -1));)
    GLSL(vec4 t = p * 0.859375 + sum2 * -0.1171875 + sum1 * -0.09765625;)
    GLSLF("color = p + t * %f;\n", param);
    GLSLF("}\n");
}