LibVideo: Fast-path converting colors by only matrix coefficients

We don't need to run through the whole floating-point color converter
for videos that use sRGB transfer characteristics and BT.709 color
primaries. This commit adds a new templated inlining function to
ColorConverter to do a very fast fixed-point YCbCr to RGB conversion.

With the fast path, frame conversion times go from ~7.8ms down to
~3.7ms. The fast path can benefit a lot more from extra SIMD vector
width, as well.

Userland/Libraries/LibVideo/Color/ColorConverter.cpp

@@ -14,14 +14,12 @@
 
 namespace Video {
 
-DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndependentCodePoints cicp)
+DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndependentCodePoints input_cicp, CodingIndependentCodePoints output_cicp)
 {
     // We'll need to apply tonemapping for linear HDR values.
     bool should_tonemap = false;
-    switch (cicp.transfer_characteristics()) {
+    switch (input_cicp.transfer_characteristics()) {
     case TransferCharacteristics::SMPTE2084:
-        should_tonemap = true;
-        break;
     case TransferCharacteristics::HLG:
         should_tonemap = true;
         break;
@@ -34,7 +32,7 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
     //    float 0..1 range.
     //    This can be done with a 3x3 scaling matrix.
     size_t maximum_value = (1u << bit_depth) - 1;
-    float scale = 1.0 / maximum_value;
+    float scale = 1.0f / maximum_value;
     FloatMatrix4x4 integer_scaling_matrix = {
         scale, 0.0f, 0.0f, 0.0f, // y
         0.0f, scale, 0.0f, 0.0f, // u
@@ -50,7 +48,7 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
     float y_max;
     float uv_min;
     float uv_max;
-    if (cicp.video_full_range_flag() == VideoFullRangeFlag::Studio) {
+    if (input_cicp.video_full_range_flag() == VideoFullRangeFlag::Studio) {
         y_min = 16.0f / 255.0f;
         y_max = 235.0f / 255.0f;
         uv_min = y_min;
@@ -77,7 +75,7 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
     FloatMatrix4x4 color_conversion_matrix;
 
     // https://kdashg.github.io/misc/colors/from-coeffs.html
-    switch (cicp.matrix_coefficients()) {
+    switch (input_cicp.matrix_coefficients()) {
     case MatrixCoefficients::BT709:
         color_conversion_matrix = {
             1.0f, 0.0f, 0.78740f, 0.0f,       // y
@@ -104,7 +102,7 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
         };
         break;
     default:
-        return DecoderError::format(DecoderErrorCategory::Invalid, "Matrix coefficients {} not supported", matrix_coefficients_to_string(cicp.matrix_coefficients()));
+        return DecoderError::format(DecoderErrorCategory::Invalid, "Matrix coefficients {} not supported", matrix_coefficients_to_string(input_cicp.matrix_coefficients()));
     }
 
     // 4. Apply the inverse transfer function to convert RGB values to the
@@ -113,23 +111,21 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
     //    up the conversion.
     auto to_linear_lookup_table = InterpolatedLookupTable<to_linear_size>::create(
         [&](float value) {
-            return TransferCharacteristicsConversion::to_linear_luminance(value, cicp.transfer_characteristics());
+            return TransferCharacteristicsConversion::to_linear_luminance(value, input_cicp.transfer_characteristics());
         });
 
     // 5. Convert the RGB color to CIE XYZ coordinates using the input color
     //    primaries and then to the output color primaries.
     //    This is done with two 3x3 matrices that can be combined into one
     //    matrix multiplication.
-    ColorPrimaries output_cp = ColorPrimaries::BT709;
-    FloatMatrix3x3 color_primaries_matrix = TRY(get_conversion_matrix(cicp.color_primaries(), output_cp));
+    FloatMatrix3x3 color_primaries_matrix = TRY(get_conversion_matrix(input_cicp.color_primaries(), output_cicp.color_primaries()));
 
     // 6. Apply the output transfer function. For HDR color spaces, this
     //    should apply tonemapping as well.
     //    Use a lookup table as with step 3.
-    TransferCharacteristics output_tc = TransferCharacteristics::SRGB;
     auto to_non_linear_lookup_table = InterpolatedLookupTable<to_non_linear_size>::create(
         [&](float value) {
-            return TransferCharacteristicsConversion::to_non_linear_luminance(value, output_tc);
+            return TransferCharacteristicsConversion::to_non_linear_luminance(value, output_cicp.transfer_characteristics());
         });
 
     // Expand color primaries matrix with identity elements.
@@ -152,10 +148,10 @@ DecoderErrorOr<ColorConverter> ColorConverter::create(u8 bit_depth, CodingIndepe
         1.0f, // w
     };
 
-    bool should_skip_color_remapping = output_cp == cicp.color_primaries() && output_tc == cicp.transfer_characteristics();
+    bool should_skip_color_remapping = output_cicp.color_primaries() == input_cicp.color_primaries() && output_cicp.transfer_characteristics() == input_cicp.transfer_characteristics();
     FloatMatrix4x4 input_conversion_matrix = color_conversion_matrix * range_scaling_matrix * integer_scaling_matrix;
 
-    return ColorConverter(bit_depth, cicp, should_skip_color_remapping, should_tonemap, input_conversion_matrix, to_linear_lookup_table, color_primaries_matrix_4x4, to_non_linear_lookup_table);
+    return ColorConverter(bit_depth, input_cicp, should_skip_color_remapping, should_tonemap, input_conversion_matrix, to_linear_lookup_table, color_primaries_matrix_4x4, to_non_linear_lookup_table);
 }
 
 }

Userland/Libraries/LibVideo/Color/ColorConverter.h

@@ -104,10 +104,10 @@ private:
     }
 
 public:
-    static DecoderErrorOr<ColorConverter> create(u8 bit_depth, CodingIndependentCodePoints cicp);
+    static DecoderErrorOr<ColorConverter> create(u8 bit_depth, CodingIndependentCodePoints input_cicp, CodingIndependentCodePoints output_cicp);
 
     // Referencing https://en.wikipedia.org/wiki/YCbCr
-    ALWAYS_INLINE Gfx::Color convert_yuv_to_full_range_rgb(u16 y, u16 u, u16 v) const
+    ALWAYS_INLINE Gfx::Color convert_yuv(u16 y, u16 u, u16 v) const
     {
         auto max_zero = [](FloatVector4 vector) {
             return FloatVector4(max(0.0f, vector.x()), max(0.0f, vector.y()), max(0.0f, vector.z()), vector.w());
@@ -150,6 +150,99 @@ public:
         return Gfx::Color(r, g, b);
     }
 
+    // Fast conversion of 8-bit YUV to full-range RGB.
+    template<MatrixCoefficients MC, VideoFullRangeFlag FR, Unsigned T>
+    static ALWAYS_INLINE Gfx::Color convert_simple_yuv_to_rgb(T y_in, T u_in, T v_in)
+    {
+        static constexpr i32 bit_depth = 8;
+        static constexpr i32 maximum_value = (1 << bit_depth) - 1;
+        static constexpr i32 one = 1 << 14;
+        static constexpr auto fraction = [](i32 numerator, i32 denominator) constexpr {
+            auto temp = static_cast<i64>(numerator) * one;
+            return static_cast<i32>(temp / denominator);
+        };
+        static constexpr auto coef = [](i32 hundred_thousandths) constexpr {
+            return fraction(hundred_thousandths, 100'000);
+        };
+        static constexpr auto multiply = [](i32 a, i32 b) constexpr {
+            return (a * b) / one;
+        };
+
+        struct RangeFactors {
+            i32 y_offset, y_scale;
+            i32 uv_offset, uv_scale;
+        };
+
+        constexpr auto range_factors = [] {
+            RangeFactors range_factors;
+
+            i32 min = 0;
+            i32 y_max = 255;
+            i32 uv_max = 255;
+
+            if constexpr (FR == VideoFullRangeFlag::Studio) {
+                min = 16;
+                y_max = 235;
+                uv_max = 240;
+            }
+
+            range_factors.y_offset = -min * maximum_value / 255;
+            range_factors.y_scale = fraction(255, y_max - min);
+            range_factors.uv_offset = -((min + uv_max) * maximum_value) / (255 * 2);
+            range_factors.uv_scale = fraction(255, uv_max - min) * 2;
+
+            range_factors.y_scale = multiply(range_factors.y_scale, fraction(255, maximum_value));
+            range_factors.uv_scale = multiply(range_factors.uv_scale, fraction(255, maximum_value));
+
+            return range_factors;
+        }();
+
+        i32 y = y_in + range_factors.y_offset;
+        i32 u = u_in + range_factors.uv_offset;
+        i32 v = v_in + range_factors.uv_offset;
+
+        i32 red;
+        i32 green;
+        i32 blue;
+
+        constexpr i32 y_scale = range_factors.y_scale;
+        constexpr i32 uv_scale = range_factors.uv_scale;
+
+        // The equations below will have the following effects:
+        //  - Scale the Y, U and V values into the range 0...maximum_value*one for these fixed-point operations.
+        //  - Scale the values by the color range defined by VideoFullRangeFlag.
+        //  - Scale the U and V values by 2 to put them in the actual YCbCr coordinate space.
+        //  - Multiply by the YCbCr coefficients to convert to RGB.
+        if constexpr (MC == MatrixCoefficients::BT709) {
+            red = y * y_scale + v * multiply(coef(78740), uv_scale);
+            green = y * y_scale + u * multiply(coef(-9366), uv_scale) + v * multiply(coef(-23406), uv_scale);
+            blue = y * y_scale + u * multiply(coef(92780), uv_scale);
+        }
+
+        if constexpr (MC == MatrixCoefficients::BT601) {
+            red = y * y_scale + v * multiply(coef(70100), uv_scale);
+            green = y * y_scale + u * multiply(coef(-17207), uv_scale) + v * multiply(coef(-35707), uv_scale);
+            blue = y * y_scale + u * multiply(coef(88600), uv_scale);
+        }
+
+        if constexpr (MC == MatrixCoefficients::BT2020ConstantLuminance) {
+            red = y * y_scale + v * multiply(coef(73730), uv_scale);
+            green = y * y_scale + u * multiply(coef(-8228), uv_scale) + v * multiply(coef(-28568), uv_scale);
+            blue = y * y_scale + u * multiply(coef(94070), uv_scale);
+        }
+
+        red = clamp(red, 0, maximum_value * one);
+        green = clamp(green, 0, maximum_value * one);
+        blue = clamp(blue, 0, maximum_value * one);
+
+        // This compiles down to a bit shift if maximum_value == 255
+        red /= fraction(maximum_value, 255);
+        green /= fraction(maximum_value, 255);
+        blue /= fraction(maximum_value, 255);
+
+        return Gfx::Color(u8(red), u8(green), u8(blue));
+    }
+
 private:
     static constexpr size_t to_linear_size = 64;
     static constexpr size_t to_non_linear_size = 64;

Userland/Libraries/LibVideo/VideoFrame.cpp

@@ -57,8 +57,8 @@ ALWAYS_INLINE void interpolate_row(u32 const row, u32 const width, u16 const* pl
     }
 }
 
-template<u32 subsampling_horizontal, u32 subsampling_vertical>
-ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap(ColorConverter const& converter, u32 const width, u32 const height, FixedArray<u16> const& plane_y, FixedArray<u16> const& plane_u, FixedArray<u16> const& plane_v, Gfx::Bitmap& bitmap)
+template<u32 subsampling_horizontal, u32 subsampling_vertical, typename Convert>
+ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap_subsampled(Convert convert, u32 const width, u32 const height, FixedArray<u16> const& plane_y, FixedArray<u16> const& plane_u, FixedArray<u16> const& plane_v, Gfx::Bitmap& bitmap)
 {
     VERIFY(bitmap.width() >= 0 && static_cast<u32>(bitmap.width()) == width);
     VERIFY(bitmap.height() >= 0 && static_cast<u32>(bitmap.height()) == height);
@@ -99,13 +99,13 @@ ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap(ColorConverter const& conve
         auto* scan_line_a = bitmap.scanline(static_cast<int>(row));
 
         for (size_t column = 0; column < width; column++) {
-            scan_line_a[column] = converter.convert_yuv_to_full_range_rgb(y_row_a[column], u_row_a[column], v_row_a[column]).value();
+            scan_line_a[column] = convert(y_row_a[column], u_row_a[column], v_row_a[column]).value();
         }
         if constexpr (subsampling_vertical != 0) {
             auto const* y_row_b = &plane_y[static_cast<size_t>(row + 1) * width];
             auto* scan_line_b = bitmap.scanline(static_cast<int>(row + 1));
             for (size_t column = 0; column < width; column++) {
-                scan_line_b[column] = converter.convert_yuv_to_full_range_rgb(y_row_b[column], u_row_b[column], v_row_b[column]).value();
+                scan_line_b[column] = convert(y_row_b[column], u_row_b[column], v_row_b[column]).value();
             }
         }
 
@@ -119,7 +119,7 @@ ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap(ColorConverter const& conve
             auto const* y_row = &plane_y[static_cast<size_t>(height - 1) * width];
             auto* scan_line = bitmap.scanline(static_cast<int>(height - 1));
             for (size_t column = 0; column < width; column++) {
-                scan_line[column] = converter.convert_yuv_to_full_range_rgb(y_row[column], u_row_a[column], v_row_a[column]).value();
+                scan_line[column] = convert(y_row[column], u_row_a[column], v_row_a[column]).value();
             }
         }
     }
@@ -127,23 +127,49 @@ ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap(ColorConverter const& conve
     return {};
 }
 
+template<u32 subsampling_horizontal, u32 subsampling_vertical>
+static ALWAYS_INLINE DecoderErrorOr<void> convert_to_bitmap_selecting_converter(CodingIndependentCodePoints cicp, u8 bit_depth, u32 const width, u32 const height, FixedArray<u16> const& plane_y, FixedArray<u16> const& plane_u, FixedArray<u16> const& plane_v, Gfx::Bitmap& bitmap)
+{
+    constexpr auto output_cicp = CodingIndependentCodePoints(ColorPrimaries::BT709, TransferCharacteristics::SRGB, MatrixCoefficients::BT709, VideoFullRangeFlag::Full);
+
+    if (bit_depth == 8 && cicp.transfer_characteristics() == output_cicp.transfer_characteristics() && cicp.color_primaries() == output_cicp.color_primaries() && cicp.video_full_range_flag() == VideoFullRangeFlag::Studio) {
+        switch (cicp.matrix_coefficients()) {
+        case MatrixCoefficients::BT709:
+            return convert_to_bitmap_subsampled<subsampling_horizontal, subsampling_vertical>([](u16 y, u16 u, u16 v) { return ColorConverter::convert_simple_yuv_to_rgb<MatrixCoefficients::BT709, VideoFullRangeFlag::Studio>(y, u, v); }, width, height, plane_y, plane_u, plane_v, bitmap);
+        case MatrixCoefficients::BT601:
+            return convert_to_bitmap_subsampled<subsampling_horizontal, subsampling_vertical>([](u16 y, u16 u, u16 v) { return ColorConverter::convert_simple_yuv_to_rgb<MatrixCoefficients::BT601, VideoFullRangeFlag::Studio>(y, u, v); }, width, height, plane_y, plane_u, plane_v, bitmap);
+        case MatrixCoefficients::BT2020ConstantLuminance:
+        case MatrixCoefficients::BT2020NonConstantLuminance:
+            return convert_to_bitmap_subsampled<subsampling_horizontal, subsampling_vertical>([](u16 y, u16 u, u16 v) { return ColorConverter::convert_simple_yuv_to_rgb<MatrixCoefficients::BT2020ConstantLuminance, VideoFullRangeFlag::Studio>(y, u, v); }, width, height, plane_y, plane_u, plane_v, bitmap);
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    auto converter = TRY(ColorConverter::create(bit_depth, cicp, output_cicp));
+    return convert_to_bitmap_subsampled<subsampling_horizontal, subsampling_vertical>([&](u16 y, u16 u, u16 v) { return converter.convert_yuv(y, u, v); }, width, height, plane_y, plane_u, plane_v, bitmap);
+}
+
+static DecoderErrorOr<void> convert_to_bitmap_selecting_subsampling(bool subsampling_horizontal, bool subsampling_vertical, CodingIndependentCodePoints cicp, u8 bit_depth, u32 const width, u32 const height, FixedArray<u16> const& plane_y, FixedArray<u16> const& plane_u, FixedArray<u16> const& plane_v, Gfx::Bitmap& bitmap)
+{
+    if (subsampling_horizontal && subsampling_vertical) {
+        return convert_to_bitmap_selecting_converter<true, true>(cicp, bit_depth, width, height, plane_y, plane_u, plane_v, bitmap);
+    }
+
+    if (subsampling_horizontal && !subsampling_vertical) {
+        return convert_to_bitmap_selecting_converter<true, false>(cicp, bit_depth, width, height, plane_y, plane_u, plane_v, bitmap);
+    }
+
+    if (!subsampling_horizontal && subsampling_vertical) {
+        return convert_to_bitmap_selecting_converter<false, true>(cicp, bit_depth, width, height, plane_y, plane_u, plane_v, bitmap);
+    }
+
+    return convert_to_bitmap_selecting_converter<false, false>(cicp, bit_depth, width, height, plane_y, plane_u, plane_v, bitmap);
+}
+
 DecoderErrorOr<void> SubsampledYUVFrame::output_to_bitmap(Gfx::Bitmap& bitmap)
 {
-    auto converter = TRY(ColorConverter::create(bit_depth(), cicp()));
-
-    if (m_subsampling_horizontal && m_subsampling_vertical) {
-        return convert_to_bitmap<true, true>(converter, width(), height(), m_plane_y, m_plane_u, m_plane_v, bitmap);
-    }
-
-    if (m_subsampling_horizontal && !m_subsampling_vertical) {
-        return convert_to_bitmap<true, false>(converter, width(), height(), m_plane_y, m_plane_u, m_plane_v, bitmap);
-    }
-
-    if (!m_subsampling_horizontal && m_subsampling_vertical) {
-        return convert_to_bitmap<false, true>(converter, width(), height(), m_plane_y, m_plane_u, m_plane_v, bitmap);
-    }
-
-    return convert_to_bitmap<false, false>(converter, width(), height(), m_plane_y, m_plane_u, m_plane_v, bitmap);
+    return convert_to_bitmap_selecting_subsampling(m_subsampling_horizontal, m_subsampling_vertical, cicp(), bit_depth(), width(), height(), m_plane_y, m_plane_u, m_plane_v, bitmap);
 }
 
 }