{ Conversion to and from YCbCr

Copyright (C) 2021 Bernd Kreuss prof7bit@gmail.com

This library is free software; you can redistribute it and/or modify it

under the terms of the GNU Library General Public License as published by

the Free Software Foundation; either version 2 of the License, or (at your

option) any later version with the following modification:

As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent modules,and

to copy and distribute the resulting executable under terms of your choice,

provided that you also meet, for each linked independent module, the terms

and conditions of the license of that module. An independent module is a

module which is not derived from or based on this library. If you modify

this library, you may extend this exception to your version of the library,

but you are not obligated to do so. If you do not wish to do so, delete this

exception statement from your version.

This program 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 Library General Public License

for more details.

You should have received a copy of the GNU Library General Public License

along with this library; if not, write to the Free Software Foundation,

Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1335, USA.

}

unit YCbCr;

{$mode ObjFPC}{$H+}

interface

uses

Graphics;

type

TItuRec = (

ituBT601, // ITU-R BT.601, full range

ituBT709, // ITU-R BT.709, full range

ituBT2020 // ITU-R BT.2020, full range

);

procedure ColorToYCbCr(C: TColor; out Y, Cb, Cr: Byte; Mode: TItuRec);

function YCbCrToColor(Y, Cb, Cr: Byte; Mode: TItuRec): TColor;

implementation

uses

Math;

type

TCoeff = array[0..10] of Double;

var

BT601: TCoeff;

BT709: TCoeff;

BT2020: TCoeff;

procedure ColorToYCbCr(C: TColor; out Y, Cb, Cr: Byte; constref K: TCoeff);

var

RGB: LongInt;

R, G, B: Byte;

begin

RGB := ColorToRGB(C);

R := Red(RGB);

G := Green(RGB);

B := Blue(RGB);

Y := EnsureRange(Round(K[0] * R + K[1] * G + K[2] * B), 0, 255);

Cb := EnsureRange(Round(K[3] * R + K[4] * G + 0.5 * B + 128), 0, 255);

Cr := EnsureRange(Round( 0.5 * R + K[5] * G + K[6] * B + 128), 0, 255);

end;

function YCbCrToColor(Y, Cb, Cr: Byte; constref K: TCoeff): TColor;

var

Cb0, Cr0: Integer;

R, G, B: Byte;

begin

Cb0 := Cb - 128;

Cr0 := Cr - 128;

R := EnsureRange(Round(Y + K[7] * Cr0 ), 0, 255);

G := EnsureRange(Round(Y + K[8] * Cr0 + K[9] * Cb0), 0, 255);

B := EnsureRange(Round(Y + K[10] * Cb0), 0, 255);

Result := RGBToColor(R, G, B);

end;

procedure ColorToYCbCr(C: TColor; out Y, Cb, Cr: Byte; Mode: TItuRec);

begin

case Mode of

ituBT601: ColorToYCbCr(C, Y, Cb, Cr, BT601);

ituBT709: ColorToYCbCr(C, Y, Cb, Cr, BT709);

ituBT2020: ColorToYCbCr(C, Y, Cb, Cr, BT2020);

end;

end;

function YCbCrToColor(Y, Cb, Cr: Byte; Mode: TItuRec): TColor;

begin

case Mode of

ituBT601: Result := YCbCrToColor(Y, Cb, Cr, BT601);

ituBT709: Result := YCbCrToColor(Y, Cb, Cr, BT709);

ituBT2020: Result := YCbCrToColor(Y, Cb, Cr, BT2020);

end;

end;

procedure InitYCbCrCoef(out K: TCoeff; Kr, Kg: Double);

begin // example values for BT601

K[0] := Kr; // 0.299

K[1] := Kg; // 0.587

K[2] := 1 - Kr - Kg; // Kb // 0.114

K[3] := -Kr / (2 * (Kr + Kg)); // -0.1687

K[4] := -Kg / (2 * (Kr + Kg)); // -0.3313

K[5] := -Kg / (2 * (Kg + K[2])); // -0.4187

K[6] := -K[2] / (2 * (Kg + K[2])); // -0.0813

K[7] := 2 * (Kg + K[2]); // 1.402

K[8] := -Kr * 2 * (Kg + K[2]) / Kg; // -0.7141

K[9] := -K[2] * 2 * (Kr + Kg) / Kg; // -0.3441

k[10] := 2 * (Kr + Kg); // 1.772

end;

initialization

InitYCbCrCoef(BT601, 0.299, 0.587);

InitYCbCrCoef(BT709, 0.2126, 0.7152);

InitYCbCrCoef(BT2020, 0.2627, 0.6780);

end.