1 基本概念
颜色空间（color space）是颜色集合的数学表示。三个最常用的颜色模型是：RGB（用于计算机图像学中）；YIQ、YUV或YCbCr（用于视频系统中）；CMYK(用于彩色打印)。
1.1 YCbCr颜色空间
YCbCr颜色空间是YUV颜色空间的缩放和偏移版本。Y定义为8bit,标称颜色范围为16-235；Cb和Cr标称颜色表示范围为16-240。YCbCr的采样格式一般有4:4:4、4:2:2、4:1:1、和4:2:0。
4:4:4 YCbCr格式：
图1表示4:4:4格式YCbCr采样点的定位。每个采样点有Y、Cb和Cr值，每个颜色值的颜色分量为8bit（典型），因此每个采样点24bit。
图1 4:4:4采样
1.2 RGB颜色空间
红、绿和蓝（RGB）颜色空间广泛用于计算机图像学和显示器。红绿蓝是三种基本的加性颜色，可以用三维的笛卡尔坐标系统来表示RGB颜色空间。
1.3 YCbCr到RGB颜色空间的转换：数学公式
为了将标称取值范围为16~235（Studio R’G’B’）的8位YCbCr数据转换为R’G’B’颜色公式可以简化为：
R’=Y+1.371(Cr-128)
G’=Y-0.689(Cr-128)-0.336(Cb-128)
B’=Y+1.732(Cb-128)
2 matlab实现ycbcr444转RGB
close all
clear all
clc
I=imread('1.bmp');
[H ,W ,D]=size(I);
R=double(I(:,:,1));
G=double(I(:,:,2));
B=double(I(:,:,3));
Y0= double(zeros(H,W));
Cb0 =double(zeros(H,W));
Cr0 = double(zeros(H,W));
R0= double(zeros(H,W));
G0 =double(zeros(H,W));
B0 = double(zeros(H,W));
%RGB转YCbCr444
for i = 1:H
for j = 1:W
Y0(i, j) = 0.299*R(i, j) + 0.587*G(i, j) + 0.114*B(i, j);
Cb0(i, j) = -0.172*R(i, j) - 0.339*G(i, j) + 0.511*B(i, j) + 128;
Cr0(i, j) = 0.511*R(i, j) - 0.428*G(i, j) - 0.083*B(i, j) + 128;
end
end
for i = 1:H
for j = 1:W
RGB(i, j,1) =Y0(i,j)+1.371*(Cr0(i,j)-128);
RGB(i, j,2) =Y0(i,j)-0.689*(Cr0(i,j)-128)-0.336*(Cb0(i,j)-128);
RGB(i, j,3) =Y0(i,j)+1.732*(Cb0(i,j)-128);
end
end
YCbCr(:,:,1)=Y0;
YCbCr(:,:,2)=Cb0;
YCbCr(:,:,3)=Cr0;
YCbCr=uint8(YCbCr);
RGB=uint8(RGB);
figure(1),
imshow(YCbCr),title('YCbCr');
figure(2),
imshow(RGB),title('RGB');
转之前（YCbCr444）
转之后（RGB）
3 fpga实现
/*
计算公式：
R = 1.164(Y - 16) + 1.793(CR - 128) = 1.164Y + 1.793CR - 248.128;
G = 1.164(Y - 16) - 0.534(CR - 128) - 0.213(CB - 128) = 1.164Y - 0.213CB - 0.534CR + 76.992;
B = 1.164(Y - 16) + 2.115(CB - 128) = 1.164Y + 2.115CB - 289.344;
其中，时序在计算过程中完全没有用到
输入到输出有三个clock的时延。
第一级流水线计算所有乘法；
第二级流水线计算所有加法，把正的和负的分开进行加法；
第三级流水线计算最终的和，若为负数取0；
*/
`timescale 1ns/1ps
module ycbcr_to_rgb(
input clk,
input wire[7 : 0] i_y_8b,
input wire[7 : 0] i_cb_8b,
input wire[7 : 0] i_cr_8b,
input i_h_sync,
input i_v_sync,
input i_data_en,
output wire[7 : 0] o_r_8b,
output wire[7 : 0] o_g_8b,
output wire[7 : 0] o_b_8b,
output reg o_h_sync,
output reg o_v_sync,
output reg o_data_en
);
/***************************************parameters*******************************************/
//multiply 256
parameter para_1164_10b = 10'd297; //1.160
parameter para_1793_10b = 10'd459; //1.793
parameter para_0534_10b = 10'd137; //0.535
parameter para_0213_10b = 10'd54; //0.211
parameter para_2115_10b = 10'd541; //2.113
parameter para_248128_18b = 18'd63521;//248.128
parameter para_76992_18b = 18'd19710; //76.992
parameter para_289344_18b = 18'd74072;//289.344
/********************************************************************************************/
/***************************************signals**********************************************/
wire sign_r;
wire sign_g;
wire sign_b;
reg[17 : 0] mult_y_for_r_18b;
reg[17 : 0] mult_y_for_g_18b;
reg[17 : 0] mult_y_for_b_18b;
reg[17 : 0] mult_cb_for_g_18b;
reg[17 : 0] mult_cb_for_b_18b;
reg[17 : 0] mult_cr_for_r_18b;
reg[17 : 0] mult_cr_for_g_18b;
reg[17 : 0] add_r_0_18b;
reg[17 : 0] add_g_0_18b;
reg[17 : 0] add_b_0_18b;
reg[17 : 0] add_r_1_18b;
reg[17 : 0] add_g_1_18b;
reg[17 : 0] add_b_1_18b;
reg[17 : 0] result_r_18b;
reg[17 : 0] result_g_18b;
reg[17 : 0] result_b_18b;
reg i_h_sync_delay_1;
reg i_v_sync_delay_1;
reg i_data_en_delay_1;
reg i_h_sync_delay_2;
reg i_v_sync_delay_2;
reg i_data_en_delay_2;
/********************************************************************************************/
/***************************************initial**********************************************/
initial
begin
mult_y_for_r_18b <=<> 18'd0;
mult_y_for_g_18b <=<> 18'd0;
mult_y_for_b_18b <=<> 18'd0;
mult_cb_for_g_18b <=<> 18'd0;
mult_cb_for_b_18b <=<> 18'd0;
mult_cr_for_r_18b <=<> 18'd0;
mult_cr_for_g_18b <=<> 18'd0;
add_r_0_18b <=<> 18'd0;
add_g_0_18b <=<> 18'd0;
add_b_0_18b <=<> 18'd0;
add_r_1_18b <=<> 18'd0;
add_g_1_18b <=<> 18'd0;
add_b_1_18b <=<> 18'd0;
result_r_18b <=<> 18'd0;
result_g_18b <=<> 18'd0;
result_b_18b <=<> 18'd0;
i_h_sync_delay_1 <=<> 1'd0;
i_v_sync_delay_1 <=<> 1'd0;
i_data_en_delay_1 <=<> 1'd0;
i_h_sync_delay_2 <=<> 1'd0;
i_v_sync_delay_2 <=<> 1'd0;
i_data_en_delay_2 <=<> 1'd0;
o_h_sync <=<> 1'd0;
o_v_sync <=<> 1'd0;
o_data_en <=<> 1'd0;
end
/********************************************************************************************/
/***************************************arithmetic*******************************************/
//LV1 pipeline : mult
always @ (posedge clk)
begin
mult_y_for_r_18b <=<> i_y_8b * para_1164_10b;
mult_y_for_g_18b <=<> i_y_8b * para_1164_10b;
mult_y_for_b_18b <=<> i_y_8b * para_1164_10b;
end
always @ (posedge clk)
begin
mult_cb_for_g_18b <=<> i_cb_8b * para_0213_10b;
mult_cb_for_b_18b <=<> i_cb_8b * para_2115_10b;
end
always @ (posedge clk)
begin
mult_cr_for_r_18b <=<> i_cr_8b * para_1793_10b;
mult_cr_for_g_18b <=<> i_cr_8b * para_0534_10b;
end
//LV2 pipeline : add
always @ (posedge clk)
begin
add_r_0_18b <=<> mult_y_for_r_18b + mult_cr_for_r_18b;
add_r_1_18b <=<> para_248128_18b;
add_g_0_18b <=<> mult_y_for_g_18b + para_76992_18b;
add_g_1_18b <=<> mult_cb_for_g_18b + mult_cr_for_g_18b;
add_b_0_18b <=<> mult_y_for_b_18b + mult_cb_for_b_18b;
add_b_1_18b <=<> para_289344_18b;
end
//LV3 pipeline : y + cb + cr
assign sign_r = (add_r_0_18b >= add_r_1_18b);
assign sign_g = (add_g_0_18b >= add_g_1_18b);
assign sign_b = (add_b_0_18b >= add_b_1_18b);
always @ (posedge clk)
begin
result_r_18b = sign_r ? (add_r_0_18b - add_r_1_18b) : 18'd0;
result_g_18b = sign_g ? (add_g_0_18b - add_g_1_18b) : 18'd0;
result_b_18b = sign_b ? (add_b_0_18b - add_b_1_18b) : 18'd0;
end
//output 溢出处理
assign o_r_8b = (result_r_18b[17:16] == 2'b00) ? result_r_18b[15 : 8] : 8'hff;
assign o_g_8b = (result_g_18b[17:16] == 2'b00) ? result_g_18b[15 : 8] : 8'hff;
assign o_b_8b = (result_b_18b[17:16] == 2'b00) ? result_b_18b[15 : 8] : 8'hff;
/********************************************************************************************/
/***************************************timing***********************************************/
always @ (posedge clk)
begin
i_h_sync_delay_1 <=<> i_h_sync;
i_v_sync_delay_1 <=<> i_v_sync;
i_data_en_delay_1 <=<> i_data_en;
i_h_sync_delay_2 <=<> i_h_sync_delay_1;
i_v_sync_delay_2 <=<> i_v_sync_delay_1;
i_data_en_delay_2 <=<> i_data_en_delay_1;
o_h_sync <=<> i_h_sync_delay_2;
o_v_sync <=<> i_v_sync_delay_2;
o_data_en <=<> i_data_en_delay_2;
end
/********************************************************************************************/
Endmodule
fpga实现YCbCr444转RGB效果和matlab一致。
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