Y'CbCr is often confused with the YUV color space, and typically the terms YCbCr and YUV are used interchangeably, leading to some confusion; when referring to signals in video or digital form, the term "YUV" mostly means "Y'CbCr".
--源自wikipedia—YUV
Y'UV is often used as the term for YCbCr. However, they are different formats. Y'UV is an analog system with scale factors different from the digital Y'CbCr system.
In digital video/image systems, Y'CbCr is the most common way to express color in a way suitable for compression/transmission. The confusion stems from computer implementations and text-books erroneously using the term YUV where Y'CbCr would be correct.
--源自wikipedia—YcbCr
实际上，数字视频编码中所说的YUV就是YCbCr。
Ø YCbCr与RGB格式的相互转换
RGB to YUV Conversion
Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 16 Cr = V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 128 Cb = U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128
如果是rgb是12bit的话
Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 256 Cr = V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 2048 Cb = U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 2048
YUV to RGB Conversion
B = 1.164(Y - 16) + 2.018(U - 128) G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128) R = 1.164(Y - 16) + 1.596(V - 128)
In both these cases, you have to clamp the output values
Ø 系数矩阵整型化
在RGB2YCbCr的公式中左右各乘以1024即左移10位
Y << 10 = (263 * R) + (516 * G) + (100 * B) + 262144
Cr << 10 = (450 * R) - (377 * G) - ( 73 * B) + 2097152
Cb << 10 = -(152 * R) - (298 * G) + (450 * B) + 2097152
负数取补，保守一点，系数取12位
Y << 10 = (12'h107 * R) + (12'h204 * G) + (12'h064 * B) + 24'h040000
Cr << 10 = (12'h1C2 * R) + (12'hE87 * G) + (12'hFB7 * B) + 24'h200000
Cb << 10 = (12'hF68 * R) + (12'hED6 * G) + (12'h1C2 * B) + 24'h200000
友晶的D5M中采样出来的RGB格式是各占12bit，最终YCrCb要截取成8位的，还是在转换之前保持精度，在转换之后再截取吧。以下设计方法参考友晶DE2系列中YCbCr2RGB的参考例程。
系数12位有符号，输入的RGB数据是12位无符号，乘加之后输出数据为26位。在MegaWizard Plug-In Manager工具中配置如下。
不好的一点，没有找到ALTMUT_ADD核中datab为常数的配置，应该是没有的，可以手动用其它乘法器核（设置一个相乘的系数为常数）和加法器核来搭。
Ø RGB2YCrCb核的设计
采用了3个乘加器，之后又各自用了3个加法器和移位器，来完成RGB到YUV的转换工作。rgb2yuv.v的代码如下
// Author(s): // - Huailu Ren, [email protected], http://lunix.cnblogs.com // // Revision 1.0 21:58 2011-7-30 hlren // created // // synopsys translate_off `timescale 1 ps / 1 ps // synopsys translate_on module RGB2YCrCb ( iCLK, iRESET, iRed, iGreen, iBlue, iDVAL, oY, oCb, oCr, oDVAL ); // Input input iCLK,iRESET,iDVAL; input [11:0] iRed,iGreen,iBlue; // Output output reg [11:0] oY,oCb,oCr; output reg oDVAL; // Internal Registers/Wires reg [3:0] oDVAL_d; reg [15:0] tY_r,tU_r,tV_r; wire [25:0] tY,tU,tV; always@(posedge iCLK) begin if(iRESET) begin oDVAL<=0; oDVAL_d<=0; oY <=0; oCr<=0; oCb<=0; end else begin // Red if(tY_r[15]) oY<=0; else if(tY_r[14:0]>4095) oY<=4095; else oY<=tY_r[11:0]; // Green if(tU_r[15]) oCr<=0; else if(tU_r[14:0]>4095) oCr<=4095; else oCr<=tU_r[11:0]; // Blue if(tV_r[15]) oCb<=0; else if(tV_r[14:0]>4095) oCb<=4095; else oCb<=tV_r[11:0]; // Control {oDVAL,oDVAL_d}<={oDVAL_d,iDVAL}; end end always@(posedge iCLK) begin if(iRESET) begin tY_r <= 0; tU_r <= 0; tV_r <= 0; end else begin tY_r <= ( tY + 262144 ) >> 10; tU_r <= ( tU + 2097152 ) >> 10; tV_r <= ( tV + 2097152 ) >> 10; end end // Y << 10 = (12'h107 * R) + (12'h204 * G) + (12'h064 * B) + 20'h04000 MAC_3 u0( .aclr0 ( iRESET ), .clock0 ( iCLK ), .dataa_0 ( iRed ), .dataa_1 ( iGreen ), .dataa_2 ( iBlue ), .datab_0 ( 12'h107 ), .datab_1 ( 12'h204 ), .datab_2 ( 12'h064 ), .result ( tY ) ); // Cr << 10 = (12'h1C2 * R) + (12'hE87 * G) + (12'hFB7 * B) + 20'h20000 MAC_3 u1( .aclr0 ( iRESET ), .clock0 ( iCLK ), .dataa_0 ( iRed ), .dataa_1 ( iGreen ), .dataa_2 ( iBlue ), .datab_0 ( 12'h1C2 ), .datab_1 ( 12'hE87 ), .datab_2 ( 12'hFB7 ), .result ( tU ) ); // Cb << 10 = (12'hF68 * R) + (12'hED6 * G) + (12'h1C2 * B) + 20'h20000 MAC_3 u2( .aclr0 ( iRESET ), .clock0 ( iCLK ), .dataa_0 ( iRed ), .dataa_1 ( iGreen ), .dataa_2 ( iBlue ), .datab_0 ( 12'hF68 ), .datab_1 ( 12'hED6 ), .datab_2 ( 12'h1C2 ), .result ( tV ) ); endmodule
Ø RGB2YCrCb核的验证
这是最关键的一步。首先要保证所做的“YCbCr与RGB格式的相互转换”和“系数整型化”这两节的操作是没有错误的。先保证有浮点数操作转化成整型数操作时没有错误，再验证整型数运算与用ALTMULT_ADD核运算时数据没有错误。本文中，第一步用C语言来进行验证，第二步在testbench中验证。
所写的C语言的验证代码如下：
// Author(s): // - Huailu Ren, [email protected], http://lunix.cnblogs.com // // Revision 1.0 9:56 2011-7-31 hlren // created // #include "stdio.h" #include "math.h" int main(){ FILE * f_r2b_v;//rgb2yuv_verification log file int r, g, b; int i; float y_f, u_f, v_f; // floating point calculation int y_h, u_h, v_h; // fixed point for hardware implementation if(NULL==(f_r2b_v = fopen("rgb2yuv.log","w"))){ printf("open file rgb2yuv.log error!\n"); }; fprintf(f_r2b_v, "rgb2yuv testcase:\n"); for(i=0;i<10;i=i+1) { r = (i+1)*(i+2)*(i+3)*19*23*29*41%4096; g = (i+1)*(i+2)*(i+3)*17*13*31*37%4096; b = (i+1)*(i+2)*(i+3)*13*11*37*41%4096; y_f = (0.257 * r) + (0.504 * g) + (0.098 * b) + 256 ; u_f = (0.439 * r) - (0.368 * g) - (0.071 * b) + 2048; v_f = -(0.148 * r) - (0.291 * g) + (0.439 * b) + 2048; y_h = ( (263 * r) + (516 * g) + (100 * b) + 262144 )>>10; u_h = ( (450 * r) - (377 * g) - ( 73 * b) + 2097152)>>10; v_h = (-(152 * r) - (298 * g) + (450 * b) + 2097152)>>10; fprintf(f_r2b_v, "%2d, rgb: %5d, %5d, %5d", i, r, g, b); fprintf(f_r2b_v, "\tyuv_f: %9.3f, %9.3f, %9.3f", y_f, u_f, v_f); fprintf(f_r2b_v, "\tyuv_h: %5d, %5d, %5d", y_h, u_h, v_h); if( (((fabsf(y_f - y_h))/y_f)<0.1) && (((fabsf(u_f - u_h))/u_f)<0.1) && (((fabsf(v_f - v_h))/v_f)<0.1) ){ fprintf(f_r2b_v, "\tpass\n"); } else { //for debug fprintf(f_r2b_v, "\tdiff: \t%f,\t%f,\t%f",fabsf(y_f - y_h), fabsf(u_f - u_h), fabsf(v_f - v_h)); fprintf(f_r2b_v, "\tRatio: \t%f,\t%f,\t%f", ((fabsf(y_f - y_h))/y_f), ((fabsf(u_f - u_h))/u_f), ((fabsf(v_f - v_h))/v_f)); fprintf(f_r2b_v, "\tnot pass\n"); } } }
输出结果如下：
rgb2yuv testcase: 0, rgb: 502, 1306, 3154 yuv_f: 1352.330, 1563.836, 2978.264 yuv_h: 1351, 1562, 2979 pass 1, rgb: 2008, 1128, 328 yuv_f: 1372.712, 2491.120, 1566.560 yuv_h: 1372, 2491, 1565 pass 2, rgb: 924, 772, 2868 yuv_f: 1163.620, 1965.912, 2945.648 yuv_h: 1162, 1965, 2946 pass 3, rgb: 1848, 1544, 1640 yuv_f: 1669.832, 2174.640, 2045.152 yuv_h: 1668, 2174, 2045 pass 4, rgb: 1186, 654, 3894 yuv_f: 1272.030, 2051.508, 3391.624 yuv_h: 1270, 2050, 3392 pass 5, rgb: 3536, 3504, 496 yuv_f: 2979.376, 2275.616, 722.752 yuv_h: 2978, 2276, 721 pass 6, rgb: 1208, 3208, 2792 yuv_f: 2456.904, 1199.536, 2161.376 yuv_h: 2455, 1198, 2162 pass 7, rgb: 2896, 1072, 1648 yuv_f: 1702.064, 2807.840, 2030.912 yuv_h: 1700, 2808, 2030 pass 8, rgb: 910, 2498, 218 yuv_f: 1770.226, 1512.748, 1282.104 yuv_h: 1769, 1512, 1281 pass 9, rgb: 3944, 600, 1656 yuv_f: 1734.296, 3441.040, 2016.672 yuv_h: 1733, 3442, 2015 pass
结果表明，在整型化转换过程中，是没有错误的。
下面开始写testbench，验证采用altera的ALTMULT_ADD核后是否有错误。
Testbench的代码如下
// Author(s): // - Huailu Ren, [email protected], http://lunix.cnblogs.com // // Revision 1.0 9:56 2011-7-31 hlren // created // // synopsys translate_off `include "timescale.v" // synopsys translate_on module tb_rgb2yuv; reg clk, reset, idval; reg [11:0] r, g, b; wire [11:0] y, u, v; wire odval; RGB2YCrCb DUT_rgb2yuv( .iCLK (clk), .iRESET (reset), .iRed (r), .iGreen (g), .iBlue (b), .iDVAL (idval), .oY (y), .oCr (u), .oCb (v), .oDVAL (odval) ); initial clk = 0; always #5 clk = ~clk; initial begin reset = 1; repeat (2) @ (posedge clk); reset = 0; end reg [11:0] veri_y[9:0]; reg [11:0] veri_u[9:0]; reg [11:0] veri_v[9:0]; integer i; initial begin idval = 0; repeat (3) @ (posedge clk); idval = 1; for(i=0;i<10;i=i+1) begin r = (i+1)*(i+2)*(i+3)*19*23*29*41%4096; g = (i+1)*(i+2)*(i+3)*17*13*31*37%4096; b = (i+1)*(i+2)*(i+3)*13*11*37*41%4096; veri_y[i] = ( (263 * r) + (516 * g) + (100 * b) + 262144 )>>10; veri_u[i] = ( (450 * r) - (377 * g) - ( 73 * b) + 2097152)>>10; veri_v[i] = (-(152 * r) - (298 * g) + (450 * b) + 2097152)>>10; //veri_y[i] = (0.257 * r) + (0.504 * g) + (0.098 * b) + 256 ; //veri_u[i] = (0.439 * r) - (0.368 * g) - (0.071 * b) + 2048; //veri_v[i] = -(0.148 * r) - (0.291 * g) + (0.439 * b) + 2048; repeat (1) @ (posedge clk); end idval = 0; repeat (100) @ (posedge clk); $finish; end integer j; always @ (posedge clk) if(reset) j <= 0; else if (odval) j <= j + 1; always @ (posedge clk) if (odval) begin $display("veri: \t%d,\t%d,\t%d, yuv: \t%d,\t%d,\t%d", veri_y[j], veri_u[j], veri_v[j], y, u, v); end `ifdef FSDBDUMP initial begin $fsdbDumpfile("test.fsdb"); $fsdbDumpvars; end `endif endmodule
验证结果如下
在modelsim中
在debussy中
其它的一些脚本文件见附件。
Ø 总结
讨论了YCrCb和YUV格式的区别，YCrCb来自于YUV，在数字图像处理领域所说的YUV也就是YCrCb；对RGB到YCrCb转换的算法作了处理转换成整型数操作；用Verilog结合Altera的ALT的库设计了RGB2YCrCb的核；用C语言和Verilog语言对所设计的核进行了验证。
源码在这里下载
rgb2yuv.7z
设计文件在顶层目录中，仿真所需文件在bench目录下，仿真环境也在bench目录下。
vsim –do tb_rgb2yuv.do –c
即可。Bench目录下还包括仿真脚本文件，debussy的信号文件，C语言验证的文件，altera的库文件。
To Do
- 把用到的乘加器ALTMULT_ADD用三个常熟系数的乘法器和一个并行加法器来代替。
- 把文中的两个验证步骤，用Verilog Pli的方式合并成一个。
- 以此设计为基础，试着用数字IC设计的方法设计，探索和学习数字IC设计的整个流程
(原創) 如何使用Verilog將YCbCr轉RGB? (SOC) (Verilog) (DE2-70)
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