FFMPEG 内部YUV转RGB过程

以ffmpeg 4.2 为例

 1. 建立YUV-RGB映射表

/* Color space conversion coefficients for YCbCr -> RGB mapping.
*
* Entries are {crv, cbu, cgu, cgv}
*
* crv = (255 / 224) * 65536 * (1 - cr) / 0.5
* cbu = (255 / 224) * 65536 * (1 - cb) / 0.5
* cgu = (255 / 224) * 65536 * (cb / cg) * (1 - cb) / 0.5
* cgv = (255 / 224) * 65536 * (cr / cg) * (1 - cr) / 0.5
*
* where Y = cr * R + cg * G + cb * B and cr + cg + cb = 1.
*/
const int32_t ff_yuv2rgb_coeffs[11][4] = {
{ 117489, 138438, 13975, 34925 }
};

    
const int bpp = c->dstFormatBpp;
    uint8_t *y_table;
    uint16_t *y_table16;
    uint32_t *y_table32;
    int i, base, rbase, gbase, bbase, av_uninit(abase), needAlpha;
    const int yoffs = (fullRange ? 384 : 326) + YUVRGB_TABLE_LUMA_HEADROOM;
    const int table_plane_size = 1024 + 2*YUVRGB_TABLE_LUMA_HEADROOM;

    int64_t crv =  inv_table[0];
    int64_t cbu =  inv_table[1];
    int64_t cgu = -inv_table[2];
    int64_t cgv = -inv_table[3];
    int64_t cy  = 1 << 16;
    int64_t oy  = 0;
    int64_t yb  = 0;

    if (!fullRange) {
        cy = (cy * 255) / 219;
        oy = 16 << 16;
    } else {
        crv = (crv * 224) / 255;
        cbu = (cbu * 224) / 255;
        cgu = (cgu * 224) / 255;
        cgv = (cgv * 224) / 255;
    }

    cy   = (cy  * contrast)              >> 16;
    crv  = (crv * contrast * saturation) >> 32;
    cbu  = (cbu * contrast * saturation) >> 32;
    cgu  = (cgu * contrast * saturation) >> 32;
    cgv  = (cgv * contrast * saturation) >> 32;
    oy  -= 256 * brightness;

    c->uOffset = 0x0400040004000400LL;
    c->vOffset = 0x0400040004000400LL;
    c->yCoeff  = roundToInt16(cy  * (1 << 13)) * 0x0001000100010001ULL;
    c->vrCoeff = roundToInt16(crv * (1 << 13)) * 0x0001000100010001ULL;
    c->ubCoeff = roundToInt16(cbu * (1 << 13)) * 0x0001000100010001ULL;
    c->vgCoeff = roundToInt16(cgv * (1 << 13)) * 0x0001000100010001ULL;
    c->ugCoeff = roundToInt16(cgu * (1 << 13)) * 0x0001000100010001ULL;
    c->yOffset = roundToInt16(oy  * (1 <<  3)) * 0x0001000100010001ULL;

    c->yuv2rgb_y_coeff   = (int16_t)roundToInt16(cy  * (1 << 13));
    c->yuv2rgb_y_offset  = (int16_t)roundToInt16(oy  * (1 <<  9));
    c->yuv2rgb_v2r_coeff = (int16_t)roundToInt16(crv * (1 << 13));
    c->yuv2rgb_v2g_coeff = (int16_t)roundToInt16(cgv * (1 << 13));
    c->yuv2rgb_u2g_coeff = (int16_t)roundToInt16(cgu * (1 << 13));
    c->yuv2rgb_u2b_coeff = (int16_t)roundToInt16(cbu * (1 << 13));

    //scale coefficients by cy
    crv = ((crv * (1 << 16)) + 0x8000) / FFMAX(cy, 1);
    cbu = ((cbu * (1 << 16)) + 0x8000) / FFMAX(cy, 1);
    cgu = ((cgu * (1 << 16)) + 0x8000) / FFMAX(cy, 1);
    cgv = ((cgv * (1 << 16)) + 0x8000) / FFMAX(cy, 1);

    av_freep(&c->yuvTable);

2. 实际转换过程

    ret = c->swscale(c, src2, srcStride2, srcSliceY_internal, srcSliceH, dst2, dstStride2);  //FFmpeg-master/libswscale/swscale.c:969

　

   YUV2RGBFUNC(yuv2rgb_c_24_bgr, uint8_t, 0) : yuv2rgb.c:378

// only trivial mods from yuv2rgb_c_24_rgb
YUV2RGBFUNC(yuv2rgb_c_24_bgr, uint8_t, 0)
    LOADCHROMA(0);
    PUTBGR24(dst_1, py_1, 0);计算第一行2个像素的RGB
    PUTBGR24(dst_2, py_2, 0);计算第二行2个像素RGB

    LOADCHROMA(1);
    PUTBGR24(dst_2, py_2, 1);
    PUTBGR24(dst_1, py_1, 1);

    LOADCHROMA(2);
    PUTBGR24(dst_1, py_1, 2);
    PUTBGR24(dst_2, py_2, 2);

    LOADCHROMA(3);
    PUTBGR24(dst_2, py_2, 3);
    PUTBGR24(dst_1, py_1, 3);

#define LOADCHROMA(i)                               
    U = pu[i];                                      
    V = pv[i];                                      
    r = (void *)c->table_rV[V+YUVRGB_TABLE_HEADROOM];                     
    g = (void *)(c->table_gU[U+YUVRGB_TABLE_HEADROOM] + c->table_gV[V+YUVRGB_TABLE_HEADROOM]);  
    b = (void *)c->table_bU[U+YUVRGB_TABLE_HEADROOM];

　　

#define PUTRGB(dst, src, i)                         
    Y              = src[2 * i];                    
    dst[2 * i]     = r[Y] + g[Y] + b[Y];            
    Y              = src[2 * i + 1];                
    dst[2 * i + 1] = r[Y] + g[Y] + b[Y];