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LibGfx/JPEG2000: Pull dequantization step size computation out of loop

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step_size is identical for all coefficients. No behavior change.

Takes

    hyperfine \
        'Build/lagom/bin/image --no-output ~/Downloads/balloon.jpf'

from 741.7 ms ± 8.0 ms to 648.0 ms ± 1.9 ms on my system,
a 12.5% reduction.
This commit is contained in:
Nico Weber committed 2026-07-22 08:53:37 -04:00
1 parent ada0a3c5fc
commit f261084476
1 file changed
+30 -32
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Userland/Libraries/LibGfx/ImageFormats/JPEG2000Loader.cpp
+30 -32
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@@ -2061,40 +2061,38 @@ static ErrorOr<void> decode_bitplanes_to_coefficients(JPEG2000LoadingContext& co
int h = output.size.height();
VERIFY(w * h == static_cast<int>(input.size()));
// E.1 Inverse quantization procedure
// The coefficients store qbar_b.
float step_size = 1.0f;
if (quantization_parameters.quantization_style != QuantizationDefault::QuantizationStyle::NoQuantization) {
// E.1.1 Irreversible transformation
auto R_I = context.siz.components[component_index].bit_depth();
// Table E.1 – Sub-band gains
auto log_2_gain_b = sub_band_type == JPEG2000::SubBand::HorizontalLowpassVerticalLowpass ? 0 : (sub_band_type == JPEG2000::SubBand::HorizontalHighpassVerticalLowpass || sub_band_type == JPEG2000::SubBand::HorizontalLowpassVerticalHighpass ? 1 : 2);
auto R_b = R_I + log_2_gain_b; // (E-4)
u16 mantissa;
if (quantization_parameters.quantization_style == QuantizationDefault::QuantizationStyle::ScalarDerived) {
// (E-5)
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[0].mantissa;
} else {
if (r == 0)
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[0].mantissa;
else
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[3 * (r - 1) + (int)sub_band_type].mantissa;
}
// (E-3)
auto exponent = get_exponent(quantization_parameters, sub_band_type, r, N_L);
step_size = powf(2.0f, R_b - exponent) * (1.0f + mantissa / powf(2.0f, 11.0f));
}
// (E-6), with r chosen as 0 (see NOTE below (E-6)).
for (int y = 0; y < h; ++y) {
for (int x = 0; x < w; ++x) {
float value = input[y * w + x];
// E.1 Inverse quantization procedure
// The coefficients store qbar_b.
if (quantization_parameters.quantization_style != QuantizationDefault::QuantizationStyle::NoQuantization) {
// E.1.1 Irreversible transformation
auto R_I = context.siz.components[component_index].bit_depth();
// Table E.1 – Sub-band gains
auto log_2_gain_b = sub_band_type == JPEG2000::SubBand::HorizontalLowpassVerticalLowpass ? 0 : (sub_band_type == JPEG2000::SubBand::HorizontalHighpassVerticalLowpass || sub_band_type == JPEG2000::SubBand::HorizontalLowpassVerticalHighpass ? 1 : 2);
auto R_b = R_I + log_2_gain_b; // (E-4)
u16 mantissa;
if (quantization_parameters.quantization_style == QuantizationDefault::QuantizationStyle::ScalarDerived) {
// (E-5)
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[0].mantissa;
} else {
if (r == 0)
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[0].mantissa;
else
mantissa = quantization_parameters.step_sizes.get<Vector<QuantizationDefault::IrreversibleStepSize>>()[3 * (r - 1) + (int)sub_band_type].mantissa;
}
// (E-3)
auto exponent = get_exponent(quantization_parameters, sub_band_type, r, N_L);
float step_size = powf(2.0f, R_b - exponent) * (1.0f + mantissa / powf(2.0f, 11.0f));
// (E-6), with r chosen as 0 (see NOTE below (E-6)).
value *= step_size;
}
output.data[y * output.pitch + x] = value;
output.data[y * output.pitch + x] = input[y * w + x] * step_size;
}
}
};