There's no need to #undef PREFIX, since we do the token pasting ourselves.

[movit] / deinterlace_effect.frag

// Implicit uniforms:
// uniform int PREFIX(current_field_position);
// uniform float PREFIX(num_lines);
// uniform float PREFIX(self_offset);
// uniform float PREFIX(inv_width);
// uniform float PREFIX(current_offset)[2];
// uniform float PREFIX(other_offset)[3];

// The best explanation of YADIF that I've seen is actually a pseudocode
// reimplementation from the Doom9 forum:
//
//   http://forum.doom9.org/showthread.php?p=980375#post980375
//
// We generally follow its terminology instead of the original C source
// (which I'll refer to as “C YADIF”), although I've used the C source as a
// reference to double-check at times. We're not bit-exact the same as
// C YADIF; in particular, we work in linear light, and left/right edge
// handling might also be a bit different (for top/bottom edge handling,
// C YADIF repeats texels like we do). Also, C YADIF generally works on
// Y', Cb and Cr planes separately, while we work on the entire RGBA triplet
// and do our spatial interpolation decisions based on the pixel as a whole,
// so our decision metric also naturally becomes different.

#define DIFF(s1, s2) dot((s1) - (s2), (s1) - (s2))

vec4 FUNCNAME(vec2 tc) {
        int yi = int(round(tc.y * PREFIX(num_lines) - 0.5f));

        // Figure out if we just want to keep the current line or if
        // we need to interpolate. This branch is obviously divergent,
        // but the very nature of deinterlacing would seem to require that.
        //
        // Note that since we have bottom-left origin, yi % 2 will return 0
        // for bottom and 1 for top.
        if ((yi % 2) != PREFIX(current_field_position)) {
                return INPUT3(vec2(tc.x, tc.y + PREFIX(self_offset)));
        }

        // First, estimate the current pixel from the neighboring pixels in the
        // same field (spatial interpolation). We try first 0 degrees (straight
        // up/down), then ±45 degrees and then finally ±63 degrees. The best of
        // these, as determined by the “spatial score” (basically sum of squared
        // differences in three neighboring pixels), is kept.
        //
        // The C version of YADIF goesn't check +63° unless +45° gave an improvement,
        // and similarly not -63° unless -45° did. The MMX version goes through pains
        // to simulate the same, but notes that it “hurts both quality and speed”.
        // We're not bit-exact the same as the C version anyway, and not sampling
        // ±63° would probably be a rather divergent branch, so we just always do it.

        // a b c d e f g     ↑ y
        //       x           |
        // h i j k l m n     +--> x

        vec2 a_pos = vec2(tc.x - 3.0 * PREFIX(inv_width), tc.y + PREFIX(current_offset)[1]);
        vec2 b_pos = vec2(tc.x - 2.0 * PREFIX(inv_width), a_pos.y);
        vec2 c_pos = vec2(tc.x -       PREFIX(inv_width), a_pos.y);
        vec2 d_pos = vec2(tc.x,                           a_pos.y);
        vec2 e_pos = vec2(tc.x +       PREFIX(inv_width), a_pos.y);
        vec2 f_pos = vec2(tc.x + 2.0 * PREFIX(inv_width), a_pos.y);
        vec2 g_pos = vec2(tc.x + 3.0 * PREFIX(inv_width), a_pos.y);

        vec2 h_pos = vec2(tc.x - 3.0 * PREFIX(inv_width), tc.y + PREFIX(current_offset)[0]);
        vec2 i_pos = vec2(tc.x - 2.0 * PREFIX(inv_width), h_pos.y);
        vec2 j_pos = vec2(tc.x -       PREFIX(inv_width), h_pos.y);
        vec2 k_pos = vec2(tc.x,                           h_pos.y);
        vec2 l_pos = vec2(tc.x +       PREFIX(inv_width), h_pos.y);
        vec2 m_pos = vec2(tc.x + 2.0 * PREFIX(inv_width), h_pos.y);
        vec2 n_pos = vec2(tc.x + 3.0 * PREFIX(inv_width), h_pos.y);

        vec4 a = INPUT3(a_pos);
        vec4 b = INPUT3(b_pos);
        vec4 c = INPUT3(c_pos);
        vec4 d = INPUT3(d_pos);
        vec4 e = INPUT3(e_pos);
        vec4 f = INPUT3(f_pos);
        vec4 g = INPUT3(g_pos);
        vec4 h = INPUT3(h_pos);
        vec4 i = INPUT3(i_pos);
        vec4 j = INPUT3(j_pos);
        vec4 k = INPUT3(k_pos);
        vec4 l = INPUT3(l_pos);
        vec4 m = INPUT3(m_pos);
        vec4 n = INPUT3(n_pos);

        // 0 degrees. Note that pred is actually twice the real spatial prediction;
        // we halve it later to same some arithmetic. Also, our spatial score is not
        // the same as in C YADIF; we use the total squared sum over all four
        // channels instead of deinterlacing each channel separately.
        //
        // Note that there's a small, arbitrary bonus for this first alternative,
        // so that vertical interpolation wins if everything else is equal.
        vec4 pred = d + k;
        float score;
        float best_score = DIFF(c, j) + DIFF(d, k) + DIFF(e, l) - 1e-4;

        // -45 degrees.
        score = DIFF(b, k) + DIFF(c, l) + DIFF(d, m);
        if (score < best_score) {
                pred = c + l;
                best_score = score;
        }

        // -63 degrees.
        score = DIFF(a, l) + DIFF(b, m) + DIFF(c, n);
        if (score < best_score) {
                pred = b + m;
                best_score = score;
        }

        // +45 degrees.
        score = DIFF(d, i) + DIFF(e, j) + DIFF(f, k);
        if (score < best_score) {
                pred = e + j;
                best_score = score;
        }

        // +63 degrees.
        score = DIFF(e, h) + DIFF(f, i) + DIFF(g, j);
        if (score < best_score) {
                pred = f + i;
                // best_score isn't used anymore.
        }

        pred *= 0.5f;

        // Now we do a temporal prediction (p2) of this pixel based on the previous
        // and next fields. The spatial prediction is clamped so that it is not
        // too far from this temporal prediction, where “too far” is based on
        // the amount of local temporal change. (In other words, the temporal prediction
        // is the safe choice, and the question is how far away from that we'll let
        // our spatial choice run.) Note that here, our difference metric
        // _is_ the same as C YADIF, namely per-channel abs.
        //
        // The sample positions look like this; in order to avoid variable name conflicts
        // with the spatial interpolation, we use uppercase names. x is, again,
        // the current pixel we're trying to estimate.
        //
        //     C   H      ↑ y
        //   A   F   K    |
        //     D x I      |
        //   B   G   L    |
        //     E   J      +-----> time
        //
        vec2 AFK_pos = d_pos;
        vec2 BGL_pos = k_pos;
        vec4 A = INPUT1(AFK_pos);
        vec4 B = INPUT1(BGL_pos);
        vec4 F = d;
        vec4 G = k;
        vec4 K = INPUT5(AFK_pos);
        vec4 L = INPUT5(BGL_pos);

        vec2 CH_pos = vec2(tc.x, tc.y + PREFIX(other_offset)[2]);
        vec2 DI_pos = vec2(tc.x, tc.y + PREFIX(other_offset)[1]);
        vec2 EJ_pos = vec2(tc.x, tc.y + PREFIX(other_offset)[0]);

        vec4 C = INPUT2(CH_pos);
        vec4 D = INPUT2(DI_pos);
        vec4 E = INPUT2(EJ_pos);

        vec4 H = INPUT4(CH_pos);
        vec4 I = INPUT4(DI_pos);
        vec4 J = INPUT4(EJ_pos);

        // Find temporal differences around this line, using all five fields.
        // tdiff0 is around the current field, tdiff1 is around the previous one,
        // tdiff2 is around the next one.
        vec4 tdiff0 = abs(D - I);
        vec4 tdiff1 = abs(A - F) + abs(B - G);  // Actually twice tdiff1.
        vec4 tdiff2 = abs(K - F) + abs(L - G);  // Actually twice tdiff2.
        vec4 diff = max(tdiff0, 0.5f * max(tdiff1, tdiff2));

        // The following part is the spatial interlacing check, which loosens up the
        // allowable temporal change. (See also the comments in the .h file.)
        // It costs us four extra loads (C, E, H, J) and a few extra ALU ops;
        // we're already very load-heavy, so the extra ALU is effectively free.
        // It costs about 18% performance in some benchmarks, which squares
        // well with going from 20 to 24 loads (a 20% increase), although for
        // total overall performance in longer chains, the difference is nearly zero.
        //
        // The basic idea is seemingly to allow more change if there are large spatial
        // vertical changes, even if there are few temporal changes. These differences
        // are signed, though, which make it more tricky to follow, although they seem
        // to reduce into some sort of pseudo-abs. I will not claim to understand them
        // very well.
        //
        // We start by temporally interpolating the current vertical line (p0–p4):
        //
        //     C p0 H      ↑ y
        //   A   p1   K    |
        //     D p2 I      |
        //   B   p3   L    |
        //     E p4 J      +-----> time
        //
        // YADIF_ENABLE_SPATIAL_INTERLACING_CHECK will be #defined to 1
        // if the check is enabled. Otherwise, the compiler should
        // be able to remove the dependent code quite easily.
        vec4 p0 = 0.5f * (C + H);
        vec4 p1 = F;
        vec4 p2 = 0.5f * (D + I);
        vec4 p3 = G;
        vec4 p4 = 0.5f * (E + J);

#if YADIF_ENABLE_SPATIAL_INTERLACING_CHECK
        vec4 max_ = max(max(p2 - p3, p2 - p1), min(p0 - p1, p4 - p3));
        vec4 min_ = min(min(p2 - p3, p2 - p1), max(p0 - p1, p4 - p3));
        diff = max(diff, max(min_, -max_));
#endif

        return clamp(pred, p2 - diff, p2 + diff);
}

#undef DIFF
#undef YADIF_ENABLE_SPATIAL_INTERLACING_CHECK