public static JpegQuantizationTable ScaleByQuality(JpegQuantizationTable quantizationTable, int quality)
{
if (quantizationTable.IsEmpty)
{
throw new ArgumentException("Quantization table is not initialized.", nameof(quantizationTable));
}
if ((uint)quality > 100)
{
throw new ArgumentOutOfRangeException(nameof(quality));
}
int scale = quality < 50 ? 5000 / quality : 200 - (quality * 2);
ReadOnlySpan <ushort> source = quantizationTable.Elements;
ushort[] elements = new ushort[64];
for (int i = 0; i < elements.Length; i++)
{
int x = source[i];
x = ((x * scale) + 50) / 100;
elements[i] = (ushort)JpegMathHelper.Clamp(x, 1, 255);
}
return(new JpegQuantizationTable(quantizationTable.ElementPrecision, quantizationTable.Identifier, elements));
}
public bool TryEstimateQuality(out float quality)
{
if (_quantizationTables is null)
{
quality = 0;
return(false);
}
// Luminance
JpegQuantizationTable quantizationTable = GetQuantizationTable(0);
if (quantizationTable.IsEmpty)
{
quality = 0;
return(false);
}
quality = EstimateQuality(quantizationTable, JpegStandardQuantizationTable.GetLuminanceTable(0, 0), out _);
// Chrominance
quantizationTable = GetQuantizationTable(1);
if (!quantizationTable.IsEmpty)
{
float quality2 = EstimateQuality(quantizationTable, JpegStandardQuantizationTable.GetChrominanceTable(0, 0), out _);
quality = Math.Min(quality, quality2);
}
quality = JpegMathHelper.Clamp(quality, 0f, 100f);
return(true);
}