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1.0. INTRODUCTION
2.0. RGB TO YUV COLOR CONVERSION 
The Intel Architecture (IA) media extensions include singleinstruction, multidata (SIMD) instructions. This application note presents examples of code demonstrate how to convert RGB ColorSpace Pixels to YUV ColorSpace Pixels. Components of the YUV color space are linear combinations of the components of the RGB color space. Therefore, RGB to YUV color conversion is computed by multiplying a 3x3 coefficient matrix by a vector of RGB values.
The code presented here shows how to use the MMX instructions to significantly speed up RGB to YUV color conversion. The code includes the quadword shift instructions, PSLLQ and PSRLQ, which are used to position data in the 64bit MMX registers to facilitate single instruction multiple data (SIMD) operations. Once positioned, packedmultiplyaccumulate, PMADDWD, packedadd, PADDD, and packedrightshift, PSRAD, instructions perform the multiplications, additions, and shifts required to compute Y, U, and V values. The 32bit to 16bit conversion, PACKSSDW, and 16bit to 8bit conversion instructions reduce the data size and clamp YUV values.
Color spaces are threedimensional (3D) coordinate systems in which each color is represented by a single point. Colors appear as their primary components red, green and blue, in the RGB color space. RGB is the format generally used by monitors. Each color appears as a luminance component, Y, and two chrominance components, U and V, in the YUV space. Luminance, the intensity perceived, is decoupled from the chrominance components so the intensity can be varied without affecting the color. The YUV format is used by PAL, the European television transmission standard, and it is the defacto standard used for image and video compression.
The parameters of the color conversion routine presented here are the address of the RGB buffer, which stores the input data, the number of rows and columns, and the addresses of the separate Y, U, and V buffers, which store the output data. The R, G, and B values are interleaved, and the data size of each is one byte. The data size of the Y, U, and V results are one byte, also. Therefore, the size of the RGB buffer in units of bytes is three times the product of the number of rows and columns, and the sizes of the YUV buffers in units of bytes is the product of the number of rows and the number of columns.
Two sets of equations for RGB to YUV color conversion are given in Example 1. The first set is a floatingpoint version. The second set describes calculations made in the MMX code presented here. MMX registers execute integer operations. Coefficients in the second set are equal to the product of 32768, which equals 2^{15}, and the coefficients in the first set of equations rounded to the nearest integer and divided by 32768. The code adds 128 to the results for U and V to assure they are positive.
Y = 0.299R 0.587G + 0.114B Conventional floatingpoint equations U =0.146 R  0.288 G + 0.434 B V = 0.617 R  0.517 G  0.100 G Y = [(9798 R + 19235G + 3736 B) / 32768] Equations used by code. U = [(4784 R  9437 G + 4221 B) / 32768] + 128 V = [(20218R  16941G  3277 B) / 32768] + 128
The steps used to transform RGB to YUV are described in Example 2. A full loop processes 24 bytes. The arrangement of data shown in step 1 represents that for three loads. Effective use of MMX instructions requires that data be positioned in registers to take advantage of the SIMD capabilities of the MMX technology. A method for arranging data which permits efficient calculation of YUV values from interleaved RGB input is described in step 2. This facilitates the calculations in step 3. Steps 2 and 3 are described in Example 3. The first phase of step 2, represented by the shift instruction, varies depending on the arrangement of data loaded in step 1. Generally one instruction, and never more than three are required to in this phase. Step 2 positions data in the locations shown in the second two instructions shown in step 2 regardless of the locations when data is loaded in step 1. A first register is loaded, using the 8bit to 6bit unpack operation, with 16bit values arranged R_{B}B_{A}G_{A}R_{A} and a second register is similarly loaded with B_{B}G_{B}R_{B}B_{A }where an R, a G, and a B value in the first register are associated with pixel A and an R, a G, and a B value in the second register are associated with adjacent pixel B. Step 3 shows how the pmaddwd instruction takes advantage of this arrangement. The operand used with the register containing R_{B}B_{A}G_{A}R_{A} is a 64bit local variable containing four 16bit values in the form C_{R}0C_{B}C_{R}. The 32bit results of the PMADDWD instruction are C_{R}R_{B }and C_{G}G_{A}+C_{R}R_{A}. The operand with the register containing B_{B}G_{B}R_{B}B_{A} is the 64bit local variable containing the four 16bit values C_{B}C_{G}0C_{B}. The 32bit results of the PMADDWD instruction are C_{B}B_{B}+C_{G}G_{B} and B_{A}C_{B}. These results are combined with a 32bit add to give C_{B}B_{B}+C_{G}G_{B}+C_{R}R_{B} and C_{B}B_{A}+C_{G}G_{A}+C_{R}R_{A}. The 32bit results are shifted by 15 bits, the equivalent of dividing by 32768, and packed to reduce the data size to 8 bits. Values of the coefficients C_{R}, C_{G}, and C_{B} differ for the calculations of Y, U, and V.
Step 1: Load 8bit data load mm0 with 1 byte data mm0 = G2R2B1G1R1B0G0R0 copy mm0 to mm1 mm1 = G2R2B1G1R1B0G0R0 Step 2: Position data and expand to 16bits giving R_{B}B_{A}G_{A}R_{A} and B_{B}G_{B}R_{B}B_{A }in MMX registers. shift mm1 right 16 mm1 = 00G2R2G1B1R1B0 unpack mm0 low bytes so data size is 2 bytes mm0 = R1B0G0R0 unpack mm2 low bytes so data size is 2 bytes mm2 = B1G1R1B0 Step 3: Convert RGB to 32bit YUV multiplyaccumulate mm0 using operand C_{R}0C_{B}C_{R } mm0 = C_{R}R1, C_{G}G0+C_{R}R0 multiplyaccumulate mm1 using operand C_{B}C_{G}0C_{B} mm1 = C_{B}B1+C_{G}G1, C_{R}R0 add mm0 and mm1 mm0 = C_{B}B1+C_{G}G1+C_{R}R1, C_{B}B0+C_{G}G0+C_{R}R0 shift 32bit results right 15 bits mm0 = (C_{B}B1+C_{G}G1+C_{R}R1)/2^{15}, {C_{B}B0+C_{G}G0+C_{R}R0)/2^{15 } Do step 3 for Y, U and V Repeat above steps so there are 4 values for each Y, U and V. Pack 4 values so each is 16bits. At this point 8 bytes have been processed. Repeat the steps above twice to process the remaining 16 bytes. Note the data arrangement in step 1 and instruction 1 in step 2 will vary. Step 4: Add offset, reduce results to 1 byte and store add an offset to 16bit U and V values pack and clamp 16bit results into 8 bits write 8 one byte Y, U and V results
The code presented here computes all U and V results and writes them into a buffer. In the cases of transmission and image and video compression U and V are generally subsampled because the eye is more sensitive to luminance represented by Y than chrominance represented by U and V. The code can be easily modified to subsample U and V. For example, subsampling with four Y values for each U and V value can be carried out by computing averages of U and V for 2x2 blocks. The averages of a two 2x2 blocks at a time are computed by first adding values in adjacent columns with two PMADDWD instructions, one instruction for each row of the 2x2 blocks. The PMADDWD operands are 16bit data along the rows and a constant equal to four 16bit ones. The sum of the two PMADDWD results yields sums of the values in the 2x2 blocks. Right shifts of these sums by two bits with a PSRAD instruction gives averages for U or V.
Sections of the loop which is the core of the color conversion code are listed in Example 4. Sections listed demonstrate how the Y component is obtained. Code which computes the U and V components is similar. The loop has 122 instructions, of which 116 are paired. A total of eight pixels are processed by the loop. Therefore, there are three 64bit loads of interleaved RGB data. The first load is on line 1, and the third load is on line 49. After data loaded it is shifted, and its size is increased to 16bits following a load. The first shift executed to position data is on line 4. Steps taken to position the data differ throughout the loop, but the resulting pattern is always R_{B}B_{A}G_{A}R_{A} and B_{B}G_{B}R_{B}B_{A}. Lines 5 and 7 increase the data size to 16bits. All of the multiplications and two of the additions required to compute two Y components are carried out with the pmaddwd instruction on lines 9 and 11. Similar operations to compute U and V components are carried out on lines 11, 13, 15, and 17. The PMADDWD instruction increases the size of the data to 32bits. The final two additions required to compute two Y components occur on line 18. Results of these additions are shifted by 15bits, corresponding to division by 32768, on line 36. These two 32bit values for Y are packed into two 16bit locations with two additional 32bit values for Y on line 46. These results are stored in a local variable to relieve register pressure on line 57. Line 107 reads the results back into a register where they, and for additional 16bit Y results, are packed as 8bit values on line 110. The PACKUSWB clamps the values between 255 and 0. The 8 Y results computed by the loop are store on line 115.
RGBtoYUV: 1 movq mm1, [eax] ;load G2R2B1G1R1B0G0R0 2 pxor mm6, mm6 ;0 > mm6 3 movq mm0, mm1 ;G2R2B1G1R1B0G0R0 > mm0 4 psrlq mm1, 16 ;00G2R2B1G1R1B0 > mm1 5 punpcklbw mm0, ZEROS ;R1B0G0R0 > mm0 6 movq mm7, mm1 ;00G2R2B1G1R1B0 > mm7 7 punpcklbw mm1, ZEROS ;B1G1R1B0 > mm1 8 movq mm2, mm0 ;R1B0G0R0 > mm2 9 pmaddwd mm0, YR0GR ;yrR1,ygG0+yrR0 > mm0 10 movq mm3, mm1 ;B1G1R1B0 > mm3 11 pmaddwd mm1, YBG0B ;ybB1+ygG1,ybB0 > mm1 12 movq mm4, mm2 ;R1B0G0R0 > mm4 13 pmaddwd mm2, UR0GR ;urR1,ugG0+urR0 > mm2 14 movq mm5, mm3 ;B1G1R1B0 > mm5 15 pmaddwd mm3, UBG0B ;ubB1+ugG1,ubB0 > mm3 16 punpckhbw mm7, mm6 ;00G2R2 > mm7 17 pmaddwd mm4, VR0GR ;vrR1,vgG0+vrR0 > mm4 18 paddd mm0, mm1 ;Y1Y0 > mm0 36 psrad mm0, 15 ;32bit scaled Y1Y0 > mm0 37 movq TEMP0, mm6 ;R5B4G4R4 > TEMP0 38 movq mm6, mm3 ;R3B2G2R2 > mm6 39 pmaddwd mm6, UR0GR ;urR3,ugG2+urR2 > mm6 40 psrad mm2, 15 ;32bit scaled U1U0 > mm2 41 paddd mm1, mm5 ;Y3Y2 > mm1 42 movq mm5, mm7 ;B3G3R3B2 > mm5 43 pmaddwd mm7, UBG0B ;ubB3+ugG3,ubB2 > mm7 44 psrad mm1, 15 ;32bit scaled Y3Y2 > mm1 45 pmaddwd mm3, VR0GR vrR3,vgG2+vgR2 >mm3 46 packssdw mm0, mm1 ;Y3Y2Y1Y0 > mm0 47 pmaddwd mm5, VBG0B ;vbB3+vgG3,vbB2 > mm5 48 psrad mm6, mm7 ;U3U2 > mm6 51 movq mm7, mm1 ;B7G7R7B6G6R6B5G5 > mm1 52 psrad mm6, 15 ;32bit scaled U3U2 > mm6 53 paddd mm3, mm5 ;V3V2 > mm3 54 psllq mm7, 16 ;R7B6G6R6B5G500 > mm7 55 movq mm5, mm7 ;R7B6G6R6B5G500 > mm5 56 psrad mm3, 15 ;32bit scaled V3V2 > mm3 57 movq TEMPY, mm0 ;32bit scaled Y3Y2Y1Y0 > TEMPY 107 movq mm6, TEMPY ;32bit scaled Y3Y2Y1Y0 > mm6 108 packssdw mm0, mm7 ;32bit scaled U7U6U5U4 > mm0 109 movq mm4, TEMPU ;32bit scaled U3U2U1U0 > mm4 110 packuswb mm6, mm2 ;all 8 Y values > mm6 111 movq mm7, OFFSETB ;128,128,128,128 > mm7 112 paddd mm1, mm5 ;V7V6 > mm1 113 paddw mm4, mm7 ;add offset to U3U2U1U0/256 114 psrad mm1, 15 ;32bit scaled V7V6 > mm1 115 movq [ebx], mm6 ;store Y 127 dec edi ;decrement loop counter 128 jnz RGBtoYUV ;do 24 more bytes if not 0
Performance gains for color conversion from MMX instructions are difficult to specify because colors are generally converted with the use of tables. Although tables are less accurate than calculations, they are much more efficient. MMX technology color conversion performance is somewhat better than that of typical lookup table code and is gives more accurate results.
An example of IA color conversion code which uses lookup tables requires three instructions to read data, four instructions to increment read addresses, three instructions to read lookup tables, two instructions to combine table results, two shifts to get the correct YUV value to be stored, three instructions to write results, and three instructions to increment write addresses. If all instructions could be paired and all data were in the L1 cache the number of clocks per pixel using a lookup table would be 10.
A modified version of equations shown in Example 1 are given in Example 5. C code compiled with an optimizing compiler executes the first set of floatingpoint equations and clamps results in 108 clocks. C code executes the second set of integer equations in 125 clocks.
Y = 0.299 R + 0.587 G + 0.114 B Modified floatingpoint equations U = 0.492 (B  Y) V = 0.877 (R  Y) Y = [(9798 R + 19235G + 3736 B) >>15] Modified integer equations U = [(16122 (B  Y))>>15] V = [(25203 (R  Y))>>15]
The MMX code takes 64 clocks to convert eight pixels of interleaved 24bit RGB to 24bit YUV with 15bit accuracy. This result corresponds to conversion of one pixel in eight clocks. This result lower than the lookup table rate and it is more accurate. The speedup of MMX code compared with optimized C code for color space transformation calculations is more than a factor of 10. The high MMX code conversion rate and accuracy can be attributed to:
MMX code has a the fast multiply accumulate instruction, PMADDWD. The multiply accumulate operation requires three instructions and has significantly longer latency with conventional IA instructions.
;rgbtoyuv.asm ;The loop processes interleaved RGB values for 8 pixels. ;The notation in the comments which describe the data locate ;the first byte on the right. For example in a register containing ;G2R2B1G1R1B0G0R0, R0 is in the position of the lease significant ;byte and G2 is in the position of the most significant byte. ;The output is to separate Y, U, and V buffers. Both input and ;output data are bytes. TITLE rgbtoyuv .486P .model FLAT PUBLIC _rgbtoyuv _DATA SEGMENT ALIGN 8 ZEROSX dw 0,0,0,0 ZEROS dd ?,? OFFSETDX dw 0,64,0,64 ;offset used before shift OFFSETD dd ?,? OFFSETWX dw 128,0,128,0 ;offset used before pack 32 OFFSETW dd ?,? OFFSETBX dw 128,128,128,128 OFFSETB dd ?,? TEMP0 dd ?,? TEMPY dd ?,? TEMPU dd ?,? TEMPV dd ?,? YR0GRX dw 9798,19235,0,9798 YBG0BX dw 3736,0,19235,3736 YR0GR dd ?,? YBG0B dd ?,? UR0GRX dw 4784,9437,0,4784 UBG0BX dw 14221,0,9437,14221 UR0GR dd ?,? UBG0B dd ?,? VR0GRX dw 20218,16941,0,20218 VBG0BX dw 3277,0,16941,3277 VR0GR dd ?,? VBG0B dd ?,? _DATA ENDS _TEXT SEGMENT _inPtr$ = 8 _rows$ = 12 _columns$ = 16 _outyPtr$ = 20 _outuPtr$ = 24 _outvPtr$ = 28 _rgbtoyuv PROC NEAR push ebp mov ebp, esp push eax push ebx push ecx push edx push esi push edi lea eax, ZEROSX ;This section gets around a bug movq mm0, [eax] ;unlikely to persist movq ZEROS, mm0 lea eax, OFFSETDX movq mm0, [eax] movq OFFSETD, mm0 lea eax, OFFSETWX movq mm0, [eax] movq OFFSETW, mm0 lea eax, OFFSETBX movq mm0, [eax] movq OFFSETB, mm0 lea eax, YR0GRX movq mm0, [eax] movq YR0GR, mm0 lea eax, YBG0BX movq mm0, [eax] movq YBG0B, mm0 lea eax, UR0GRX movq mm0, [eax] movq UR0GR, mm0 lea eax, UBG0BX movq mm0, [eax] movq UBG0B, mm0 lea eax, VR0GRX movq mm0, [eax] movq VR0GR, mm0 lea eax, VBG0BX movq mm0, [eax] movq VBG0B, mm0 mov eax, _rows$[ebp] mov ebx, _columns$[ebp] mul ebx ;number pixels shr eax, 3 ;number of loops mov edi, eax ;loop counter in edi mov eax, _inPtr$[ebp] mov ebx, _outyPtr$[ebp] mov ecx, _outuPtr$[ebp] mov edx, _outvPtr$[ebp] sub edx, 8 ;incremented before write RGBtoYUV: movq mm1, [eax] ;load G2R2B1G1R1B0G0R0 pxor mm6, mm6 ;0 > mm6 movq mm0, mm1 ;G2R2B1G1R1B0G0R0 > mm0 psrlq mm1, 16 ;00G2R2B1G1R1B0> mm1 punpcklbw mm0, ZEROS ;R1B0G0R0 > mm0 movq mm7, mm1 ;00G2R2B1G1R1B0> mm7 punpcklbw mm1, ZEROS ;B1G1R1B0 > mm1 movq mm2, mm0 ;R1B0G0R0 > mm2 pmaddwd mm0, YR0GR ;yrR1,ygG0+yrR0 > mm0 movq mm3, mm1 ;B1G1R1B0 > mm3 pmaddwd mm1, YBG0B ;ybB1+ygG1,ybB0 > mm1 movq mm4, mm2 ;R1B0G0R0 > mm4 pmaddwd mm2, UR0GR ;urR1,ugG0+urR0 > mm2 movq mm5, mm3 ;B1G1R1B0 > mm5 pmaddwd mm3, UBG0B ;ubB1+ugG1,ubB0 > mm3 punpckhbw mm7, mm6; 00G2R2 > mm7 pmaddwd mm4, VR0GR ;vrR1,vgG0+vrR0 > mm4 paddd mm0, mm1 ;Y1Y0 > mm0 pmaddwd mm5, VBG0B ;vbB1+vgG1,vbB0 > mm5 movq mm1, 8[eax] ;R5B4G4R4B3G3R3B2 > mm1 paddd mm2, mm3 ;U1U0 > mm2 movq mm6, mm1 ;R5B4G4R4B3G3R3B2 > mm6 punpcklbw mm1, ZEROS ;B3G3R3B2 > mm1 paddd mm4, mm5 ;V1V0 > mm4 movq mm5, mm1 ;B3G3R3B2 > mm5 psllq mm1, 32 ;R3B200 > mm1 paddd mm1, mm7 ;R3B200+00G2R2=R3B2G2R2>mm1 punpckhbw mm6, ZEROS ;R5B4G4R3 > mm6 movq mm3, mm1 ;R3B2G2R2 > mm3 pmaddwd mm1, YR0GR ;yrR3,ygG2+yrR2 > mm1 movq mm7, mm5 ;B3G3R3B2 > mm7 pmaddwd mm5, YBG0B ;ybB3+ygG3,ybB2 > mm5 psrad mm0, 15 ;32bit scaled Y1Y0 > mm0 movq TEMP0, mm6 ;R5B4G4R4 > TEMP0 movq mm6, mm3 ;R3B2G2R2 > mm6 pmaddwd mm6, UR0GR ;urR3,ugG2+urR2 > mm6 psrad mm2, 15 ;32bit scaled U1U0 > mm2 paddd mm1, mm5 ;Y3Y2 > mm1 movq mm5, mm7 ;B3G3R3B2 > mm5 pmaddwd mm7, UBG0B ;ubB3+ugG3,ubB2 psrad mm1, 15 ;32bit scaled Y3Y2 > mm1 pmaddwd mm3, VR0GR ;vrR3,vgG2+vgR2 packssdw mm0, mm1 ;Y3Y2Y1Y0 > mm0 pmaddwd mm5, VBG0B ;vbB3+vgG3,vbB2 > mm5 psrad mm4, 15 ;32bit scaled V1V0 > mm4 movq mm1, 16[eax] ;B7G7R7B6G6R6B5G5 > mm7 paddd mm6, mm7 ;U3U2 > mm6 movq mm7, mm1 ;B7G7R7B6G6R6B5G5 > mm1 psrad mm6, 15 ;32bit scaled U3U2 > mm6 paddd mm3, mm5 ;V3V2 > mm3 psllq mm7, 16 ;R7B6G6R6B5G500 > mm7 movq mm5, mm7 ;R7B6G6R6B5G500 > mm5 psrad mm3, 15 ;32bit scaled V3V2 > mm3 movq TEMPY, mm0 ;32bit scaled Y3Y2Y1Y0 > TEMPY packssdw mm2, mm6 ;32bit scaled U3U2U1U0 > mm2 movq mm0, TEMP0 ;R5B4G4R4 > mm0 punpcklbw mm7, ZEROS ;B5G500 > mm7 movq mm6, mm0 ;R5B4G4R4 > mm6 movq TEMPU, mm2 ;32bit scaled U3U2U1U0 > TEMPU psrlq mm0, 32 ;00R5B4 > mm0 paddw mm7, mm0 ;B5G5R5B4 > mm7 movq mm2, mm6 ;B5B4G4R4 > mm2 pmaddwd mm2, YR0GR ;yrR5,ygG4+yrR4 > mm2 movq mm0, mm7 ;B5G5R5B4 > mm0 pmaddwd mm7, YBG0B ;ybB5+ygG5,ybB4 > mm7 packssdw mm4, mm3 ;32bit scaled V3V2V1V0 > mm4 add eax, 24 ;increment RGB count add edx, 8 ;increment V count movq TEMPV, mm4 ;(V3V2V1V0)/256 > mm4 movq mm4, mm6 ;B5B4G4R4 > mm4 pmaddwd mm6, UR0GR ;urR5,ugG4+urR4 movq mm3, mm0 ;B5G5R5B4 > mm0 pmaddwd mm0, UBG0B ;ubB5+ugG5,ubB4 paddd mm2, mm7 ;Y5Y4 > mm2 pmaddwd mm4, VR0GR ;vrR5,vgG4+vrR4 > mm4 pxor mm7, mm7 ;0 > mm7 pmaddwd mm3, VBG0B ;vbB5+vgG5,vbB4 > mm3 punpckhbw mm1, mm7 ;B7G7R7B6 > mm1 paddd mm0, mm6 ;U5U4 > mm0 movq mm6, mm1 ;B7G7R7B6 > mm6 pmaddwd mm6, YBG0B ;ybB7+ygG7,ybB6 > mm6 punpckhbw mm5, mm7 ;R7B6G6R6 > mm5 movq mm7, mm5 ;R7B6G6R6 > mm7 paddd mm3, mm4 ;V5V4 > mm3 pmaddwd mm5, YR0GR ;yrR7,ygG6+yrR6 > mm5 movq mm4, mm1 ;B7G7R7B6 > mm4 pmaddwd mm4, UBG0B ;ubB7+ugG7,ubB6 > mm4 psrad mm0, 15 ;32bit scaled U5U4 > mm0 paddd mm0, OFFSETW ;add offset to U5U4 > mm0 psrad mm2, 15 ;32bit scaled Y5Y4 > mm2 paddd mm6, mm5 ;Y7Y6 > mm6 movq mm5, mm7 ;R7B6G6R6 > mm5 pmaddwd mm7, UR0GR ;urR7,ugG6+ugR6 > mm7 psrad mm3, 15 ;32bit scaled V5V4 > mm3 pmaddwd mm1, VBG0B ;vbB7+vgG7,vbB6 > mm1 psrad mm6, 15 ;32bit scaled Y7Y6 > mm6 paddd mm4, OFFSETD ;add offset to U7U6 packssdw mm2, mm6 ;Y7Y6Y5Y4 > mm2 pmaddwd mm5, VR0GR ;vrR7,vgG6+vrR6 > mm5 paddd mm7, mm4 ;U7U6 > mm7 psrad mm7, 15 ;32bit scaled U7U6 > mm7 movq mm6, TEMPY ;32bit scaled Y3Y2Y1Y0 > mm6 packssdw mm0, mm7 ;32bit scaled U7U6U5U4 > mm0 movq mm4, TEMPU ;32bit scaled U3U2U1U0 > mm4 packuswb mm6, mm2 ;all 8 Y values > mm6 movq mm7, OFFSETB ;128,128,128,128 > mm7 paddd mm1, mm5 ;V7V6 > mm1 paddw mm4, mm7 ;add offset to U3U2U1U0/256 psrad mm1, 15 ;32bit scaled V7V6 > mm1 movq [ebx], mm6 ;store Y packuswb mm4, mm0 ;all 8 U values > mm4 movq mm5, TEMPV ;32bit scaled V3V2V1V0 > mm5 packssdw mm3, mm1 ;V7V6V5V4 > mm3 paddw mm5, mm7 ;add offset to V3V2V1V0 paddw mm3, mm7 ;add offset to V7V6V5V4 movq [ecx], mm4 ;store U packuswb mm5, mm3 ;ALL 8 V values > mm5 add ebx, 8 ;increment Y count add ecx, 8 ;increment U count movq [edx], mm5 ;store V dec edi ;decrement loop counter jnz RGBtoYUV ;do 24 more bytes if not 0 pop edi pop esi pop edx pop ecx pop ebx pop eax pop ebp ret 0 _rgbtoyuv ENDP _TEXT ENDS END