参考链接

FFmpeg源代码简单分析：libswscale的sws_scale()_雷霄骅的博客-CSDN博客_bad dst image pointers

libswscale的sws_scale()

FFmpeg的图像处理（缩放，YUV/RGB格式转换）类库libswsscale中的sws_scale()函数。
libswscale是一个主要用于处理图片像素数据的类库。可以完成图片像素格式的转换，图片的拉伸等工作。
该类库常用的函数数量很少，一般情况下就3个：
- sws_getContext()：初始化一个SwsContext。
- sws_scale()：处理图像数据。
- sws_freeContext()：释放一个SwsContext。

Libswscale处理数据流程

Libswscale处理像素数据的流程可以概括为下图

从图中可以看出，libswscale处理数据有两条最主要的方式：unscaled和scaled。
unscaled用于处理不需要拉伸的像素数据（属于比较特殊的情况），scaled用于处理需要拉伸的像素数据。
Unscaled只需要对图像像素格式进行转换；而Scaled则除了对像素格式进行转换之外，还需要对图像进行缩放。
Scaled方式可以分成以下几个步骤：
- XXX to YUV Converter：首先将数据像素数据转换为8bitYUV格式；
- Horizontal scaler：水平拉伸图像，并且转换为15bitYUV；
- Vertical scaler：垂直拉伸图像；
- Output converter：转换为输出像素格式。

sws_scale()

sws_scale()是用于转换像素的函数。它的声明位于libswscale\swscale.h，如下所示。

/*** Scale the image slice in srcSlice and put the resulting scaled* slice in the image in dst. A slice is a sequence of consecutive* rows in an image.** Slices have to be provided in sequential order, either in* top-bottom or bottom-top order. If slices are provided in* non-sequential order the behavior of the function is undefined.** @param c         the scaling context previously created with*                  sws_getContext()* @param srcSlice  the array containing the pointers to the planes of*                  the source slice* @param srcStride the array containing the strides for each plane of*                  the source image* @param srcSliceY the position in the source image of the slice to*                  process, that is the number (counted starting from*                  zero) in the image of the first row of the slice* @param srcSliceH the height of the source slice, that is the number*                  of rows in the slice* @param dst       the array containing the pointers to the planes of*                  the destination image* @param dstStride the array containing the strides for each plane of*                  the destination image* @return          the height of the output slice*/
int sws_scale(struct SwsContext *c, const uint8_t *const srcSlice[],const int srcStride[], int srcSliceY, int srcSliceH,uint8_t *const dst[], const int dstStride[]);

sws_scale()的定义位于libswscale\swscale.c，如下所示。

/*** swscale wrapper, so we don't need to export the SwsContext.* Assumes planar YUV to be in YUV order instead of YVU.*/
int attribute_align_arg sws_scale(struct SwsContext *c,const uint8_t * const srcSlice[],const int srcStride[], int srcSliceY,int srcSliceH, uint8_t *const dst[],const int dstStride[])
{if (c->nb_slice_ctx)c = c->slice_ctx[0];return scale_internal(c, srcSlice, srcStride, srcSliceY, srcSliceH,dst, dstStride, 0, c->dstH);
}

sws_scale内部调用了scale_internal，scale_internal函数封装了sws_scale的大多数代码

static int scale_internal(SwsContext *c,const uint8_t * const srcSlice[], const int srcStride[],int srcSliceY, int srcSliceH,uint8_t *const dstSlice[], const int dstStride[],int dstSliceY, int dstSliceH)
{const int scale_dst = dstSliceY > 0 || dstSliceH < c->dstH;const int frame_start = scale_dst || !c->sliceDir;int i, ret;const uint8_t *src2[4];uint8_t *dst2[4];int macro_height_src = isBayer(c->srcFormat) ? 2 : (1 << c->chrSrcVSubSample);int macro_height_dst = isBayer(c->dstFormat) ? 2 : (1 << c->chrDstVSubSample);// copy strides, so they can safely be modifiedint srcStride2[4];int dstStride2[4];int srcSliceY_internal = srcSliceY;if (!srcStride || !dstStride || !dstSlice || !srcSlice) {av_log(c, AV_LOG_ERROR, "One of the input parameters to sws_scale() is NULL, please check the calling code\n");return AVERROR(EINVAL);}if ((srcSliceY  & (macro_height_src - 1)) ||((srcSliceH & (macro_height_src - 1)) && srcSliceY + srcSliceH != c->srcH) ||srcSliceY + srcSliceH > c->srcH) {av_log(c, AV_LOG_ERROR, "Slice parameters %d, %d are invalid\n", srcSliceY, srcSliceH);return AVERROR(EINVAL);}if ((dstSliceY  & (macro_height_dst - 1)) ||((dstSliceH & (macro_height_dst - 1)) && dstSliceY + dstSliceH != c->dstH) ||dstSliceY + dstSliceH > c->dstH) {av_log(c, AV_LOG_ERROR, "Slice parameters %d, %d are invalid\n", dstSliceY, dstSliceH);return AVERROR(EINVAL);}if (!check_image_pointers(srcSlice, c->srcFormat, srcStride)) {av_log(c, AV_LOG_ERROR, "bad src image pointers\n");return AVERROR(EINVAL);}if (!check_image_pointers((const uint8_t* const*)dstSlice, c->dstFormat, dstStride)) {av_log(c, AV_LOG_ERROR, "bad dst image pointers\n");return AVERROR(EINVAL);}// do not mess up sliceDir if we have a "trailing" 0-size sliceif (srcSliceH == 0)return 0;if (c->gamma_flag && c->cascaded_context[0])return scale_gamma(c, srcSlice, srcStride, srcSliceY, srcSliceH,dstSlice, dstStride, dstSliceY, dstSliceH);if (c->cascaded_context[0] && srcSliceY == 0 && srcSliceH == c->cascaded_context[0]->srcH)return scale_cascaded(c, srcSlice, srcStride, srcSliceY, srcSliceH,dstSlice, dstStride, dstSliceY, dstSliceH);if (!srcSliceY && (c->flags & SWS_BITEXACT) && c->dither == SWS_DITHER_ED && c->dither_error[0])for (i = 0; i < 4; i++)memset(c->dither_error[i], 0, sizeof(c->dither_error[0][0]) * (c->dstW+2));if (usePal(c->srcFormat))update_palette(c, (const uint32_t *)srcSlice[1]);memcpy(src2,       srcSlice,  sizeof(src2));memcpy(dst2,       dstSlice,  sizeof(dst2));memcpy(srcStride2, srcStride, sizeof(srcStride2));memcpy(dstStride2, dstStride, sizeof(dstStride2));if (frame_start && !scale_dst) {if (srcSliceY != 0 && srcSliceY + srcSliceH != c->srcH) {av_log(c, AV_LOG_ERROR, "Slices start in the middle!\n");return AVERROR(EINVAL);}c->sliceDir = (srcSliceY == 0) ? 1 : -1;} else if (scale_dst)c->sliceDir = 1;if (c->src0Alpha && !c->dst0Alpha && isALPHA(c->dstFormat)) {uint8_t *base;int x,y;av_fast_malloc(&c->rgb0_scratch, &c->rgb0_scratch_allocated,FFABS(srcStride[0]) * srcSliceH + 32);if (!c->rgb0_scratch)return AVERROR(ENOMEM);base = srcStride[0] < 0 ? c->rgb0_scratch - srcStride[0] * (srcSliceH-1) :c->rgb0_scratch;for (y=0; y<srcSliceH; y++){memcpy(base + srcStride[0]*y, src2[0] + srcStride[0]*y, 4*c->srcW);for (x=c->src0Alpha-1; x<4*c->srcW; x+=4) {base[ srcStride[0]*y + x] = 0xFF;}}src2[0] = base;}if (c->srcXYZ && !(c->dstXYZ && c->srcW==c->dstW && c->srcH==c->dstH)) {uint8_t *base;av_fast_malloc(&c->xyz_scratch, &c->xyz_scratch_allocated,FFABS(srcStride[0]) * srcSliceH + 32);if (!c->xyz_scratch)return AVERROR(ENOMEM);base = srcStride[0] < 0 ? c->xyz_scratch - srcStride[0] * (srcSliceH-1) :c->xyz_scratch;xyz12Torgb48(c, (uint16_t*)base, (const uint16_t*)src2[0], srcStride[0]/2, srcSliceH);src2[0] = base;}if (c->sliceDir != 1) {// slices go from bottom to top => we flip the image internallyfor (i=0; i<4; i++) {srcStride2[i] *= -1;dstStride2[i] *= -1;}src2[0] += (srcSliceH - 1) * srcStride[0];if (!usePal(c->srcFormat))src2[1] += ((srcSliceH >> c->chrSrcVSubSample) - 1) * srcStride[1];src2[2] += ((srcSliceH >> c->chrSrcVSubSample) - 1) * srcStride[2];src2[3] += (srcSliceH - 1) * srcStride[3];dst2[0] += ( c->dstH                         - 1) * dstStride[0];dst2[1] += ((c->dstH >> c->chrDstVSubSample) - 1) * dstStride[1];dst2[2] += ((c->dstH >> c->chrDstVSubSample) - 1) * dstStride[2];dst2[3] += ( c->dstH                         - 1) * dstStride[3];srcSliceY_internal = c->srcH-srcSliceY-srcSliceH;}reset_ptr(src2, c->srcFormat);reset_ptr((void*)dst2, c->dstFormat);if (c->convert_unscaled) {int offset  = srcSliceY_internal;int slice_h = srcSliceH;// for dst slice scaling, offset the pointers to match the unscaled APIif (scale_dst) {av_assert0(offset == 0);for (i = 0; i < 4 && src2[i]; i++) {if (!src2[i] || (i > 0 && usePal(c->srcFormat)))break;src2[i] += (dstSliceY >> ((i == 1 || i == 2) ? c->chrSrcVSubSample : 0)) * srcStride2[i];}for (i = 0; i < 4 && dst2[i]; i++) {if (!dst2[i] || (i > 0 && usePal(c->dstFormat)))break;dst2[i] -= (dstSliceY >> ((i == 1 || i == 2) ? c->chrDstVSubSample : 0)) * dstStride2[i];}offset  = dstSliceY;slice_h = dstSliceH;}ret = c->convert_unscaled(c, src2, srcStride2, offset, slice_h,dst2, dstStride2);if (scale_dst)dst2[0] += dstSliceY * dstStride2[0];} else {ret = swscale(c, src2, srcStride2, srcSliceY_internal, srcSliceH,dst2, dstStride2, dstSliceY, dstSliceH);}if (c->dstXYZ && !(c->srcXYZ && c->srcW==c->dstW && c->srcH==c->dstH)) {uint16_t *dst16;if (scale_dst) {dst16 = (uint16_t *)dst2[0];} else {int dstY = c->dstY ? c->dstY : srcSliceY + srcSliceH;av_assert0(dstY >= ret);av_assert0(ret >= 0);av_assert0(c->dstH >= dstY);dst16 = (uint16_t*)(dst2[0] + (dstY - ret) * dstStride2[0]);}/* replace on the same data */rgb48Toxyz12(c, dst16, dst16, dstStride2[0]/2, ret);}/* reset slice direction at end of frame */if ((srcSliceY_internal + srcSliceH == c->srcH) || scale_dst)c->sliceDir = 0;return ret;
}

从sws_scale()的定义可以看出，它封装了SwsContext中的swscale()（注意这个函数中间没有“_”）。函数最重要的一句代码就是“swscale()”。
除此之外，函数还做了一些增加“兼容性”的一些处理。
函数的主要步骤如下所示。

1.检查输入的图像参数的合理性。

这一步骤首先检查输入输出的参数是否为空，然后通过调用check_image_pointers()检查输入输出图像的内存是否正确分配。
check_image_pointers()的定义如下所示。

static int check_image_pointers(const uint8_t * const data[4], enum AVPixelFormat pix_fmt,const int linesizes[4])
{const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);int i;av_assert2(desc);for (i = 0; i < 4; i++) {int plane = desc->comp[i].plane;if (!data[plane] || !linesizes[plane])return 0;}return 1;
}

从check_image_pointers()的定义可以看出，在特定像素格式前提下，如果该像素格式应该包含像素的分量为空，就返回0，否则返回1。

2.如果输入像素数据中使用了“调色板”（palette），则进行一些相应的处理。

这一步通过函数usePal()来判定。
usePal()的定义如下。

static av_always_inline int usePal(enum AVPixelFormat pix_fmt)
{switch (pix_fmt) {case AV_PIX_FMT_PAL8:case AV_PIX_FMT_BGR4_BYTE:case AV_PIX_FMT_BGR8:case AV_PIX_FMT_GRAY8:case AV_PIX_FMT_RGB4_BYTE:case AV_PIX_FMT_RGB8:return 1;default:return 0;}
}

3.其它一些特殊格式的处理，比如说Alpha，XYZ等的处理（这方面没有研究过）。
4.如果输入的图像的扫描方式是从底部到顶部的（一般情况下是从顶部到底部），则将图像进行反转。
5.调用SwsContext中的swscale()。
SwsContext中的swscale()

swscale这个变量的类型是SwsFunc，实际上就是一个函数指针。它是整个类库的核心。当我们从外部调用swscale()函数的时候，实际上就是调用了SwsContext中的这个名称为swscale的变量（注意外部函数接口和这个内部函数指针的名字是一样的，但不是一回事）。
可以看一下SwsFunc这个类型的定义：

typedef int (*SwsFunc)(struct SwsContext *context, const uint8_t *src[],int srcStride[], int srcSliceY, int srcSliceH,uint8_t *dst[], int dstStride[]);

可以看出SwsFunc的定义的参数类型和libswscale类库外部接口函数swscale()的参数类型一模一样。
在libswscale中，该指针的指向可以分成2种情况：
- 1.图像没有伸缩的时候，指向专有的像素转换函数
- 2.图像有伸缩的时候，指向swscale()函数。
在调用sws_getContext()初始化SwsContext的时候，会在其子函数sws_init_context()中对swscale指针进行赋值。如果图像没有进行拉伸，则会调用ff_get_unscaled_swscale()对其进行赋值；如果图像进行了拉伸，则会调用ff_getSwsFunc()对其进行赋值。
下面分别看一下这2种情况。

没有拉伸--专有的像素转换函数

如果图像没有进行拉伸，则会调用ff_get_unscaled_swscale()对SwsContext的swscale进行赋值。
上篇文章中记录了这个函数，在这里回顾一下。

ff_get_unscaled_swscale()

ff_get_unscaled_swscale()的定义如下。

void ff_get_unscaled_swscale(SwsContext *c)
{const enum AVPixelFormat srcFormat = c->srcFormat;const enum AVPixelFormat dstFormat = c->dstFormat;const int flags = c->flags;const int dstH = c->dstH;const int dstW = c->dstW;int needsDither;needsDither = isAnyRGB(dstFormat) &&c->dstFormatBpp < 24 &&(c->dstFormatBpp < c->srcFormatBpp || (!isAnyRGB(srcFormat)));/* yv12_to_nv12 */if ((srcFormat == AV_PIX_FMT_YUV420P || srcFormat == AV_PIX_FMT_YUVA420P) &&(dstFormat == AV_PIX_FMT_NV12 || dstFormat == AV_PIX_FMT_NV21)) {c->convert_unscaled = planarToNv12Wrapper;}/* yv24_to_nv24 */if ((srcFormat == AV_PIX_FMT_YUV444P || srcFormat == AV_PIX_FMT_YUVA444P) &&(dstFormat == AV_PIX_FMT_NV24 || dstFormat == AV_PIX_FMT_NV42)) {c->convert_unscaled = planarToNv24Wrapper;}/* nv12_to_yv12 */if (dstFormat == AV_PIX_FMT_YUV420P &&(srcFormat == AV_PIX_FMT_NV12 || srcFormat == AV_PIX_FMT_NV21)) {c->convert_unscaled = nv12ToPlanarWrapper;}/* nv24_to_yv24 */if (dstFormat == AV_PIX_FMT_YUV444P &&(srcFormat == AV_PIX_FMT_NV24 || srcFormat == AV_PIX_FMT_NV42)) {c->convert_unscaled = nv24ToPlanarWrapper;}/* yuv2bgr */if ((srcFormat == AV_PIX_FMT_YUV420P || srcFormat == AV_PIX_FMT_YUV422P ||srcFormat == AV_PIX_FMT_YUVA420P) && isAnyRGB(dstFormat) &&!(flags & SWS_ACCURATE_RND) && (c->dither == SWS_DITHER_BAYER || c->dither == SWS_DITHER_AUTO) && !(dstH & 1)) {c->convert_unscaled = ff_yuv2rgb_get_func_ptr(c);c->dst_slice_align = 2;}/* yuv420p1x_to_p01x */if ((srcFormat == AV_PIX_FMT_YUV420P10 || srcFormat == AV_PIX_FMT_YUVA420P10 ||srcFormat == AV_PIX_FMT_YUV420P12 ||srcFormat == AV_PIX_FMT_YUV420P14 ||srcFormat == AV_PIX_FMT_YUV420P16 || srcFormat == AV_PIX_FMT_YUVA420P16) &&(dstFormat == AV_PIX_FMT_P010 || dstFormat == AV_PIX_FMT_P016)) {c->convert_unscaled = planarToP01xWrapper;}/* yuv420p_to_p01xle */if ((srcFormat == AV_PIX_FMT_YUV420P || srcFormat == AV_PIX_FMT_YUVA420P) &&(dstFormat == AV_PIX_FMT_P010LE || dstFormat == AV_PIX_FMT_P016LE)) {c->convert_unscaled = planar8ToP01xleWrapper;}if (srcFormat == AV_PIX_FMT_YUV410P && !(dstH & 3) &&(dstFormat == AV_PIX_FMT_YUV420P || dstFormat == AV_PIX_FMT_YUVA420P) &&!(flags & SWS_BITEXACT)) {c->convert_unscaled = yvu9ToYv12Wrapper;c->dst_slice_align = 4;}/* bgr24toYV12 */if (srcFormat == AV_PIX_FMT_BGR24 &&(dstFormat == AV_PIX_FMT_YUV420P || dstFormat == AV_PIX_FMT_YUVA420P) &&!(flags & SWS_ACCURATE_RND) && !(dstW&1))c->convert_unscaled = bgr24ToYv12Wrapper;/* RGB/BGR -> RGB/BGR (no dither needed forms) */if (isAnyRGB(srcFormat) && isAnyRGB(dstFormat) && findRgbConvFn(c)&& (!needsDither || (c->flags&(SWS_FAST_BILINEAR|SWS_POINT))))c->convert_unscaled = rgbToRgbWrapper;/* RGB to planar RGB */if ((srcFormat == AV_PIX_FMT_GBRP && dstFormat == AV_PIX_FMT_GBRAP) ||(srcFormat == AV_PIX_FMT_GBRAP && dstFormat == AV_PIX_FMT_GBRP))c->convert_unscaled = planarRgbToplanarRgbWrapper;#define isByteRGB(f) (             \f == AV_PIX_FMT_RGB32   || \f == AV_PIX_FMT_RGB32_1 || \f == AV_PIX_FMT_RGB24   || \f == AV_PIX_FMT_BGR32   || \f == AV_PIX_FMT_BGR32_1 || \f == AV_PIX_FMT_BGR24)if (srcFormat == AV_PIX_FMT_GBRP && isPlanar(srcFormat) && isByteRGB(dstFormat))c->convert_unscaled = planarRgbToRgbWrapper;if (srcFormat == AV_PIX_FMT_GBRAP && isByteRGB(dstFormat))c->convert_unscaled = planarRgbaToRgbWrapper;if ((srcFormat == AV_PIX_FMT_RGB48LE  || srcFormat == AV_PIX_FMT_RGB48BE  ||srcFormat == AV_PIX_FMT_BGR48LE  || srcFormat == AV_PIX_FMT_BGR48BE  ||srcFormat == AV_PIX_FMT_RGBA64LE || srcFormat == AV_PIX_FMT_RGBA64BE ||srcFormat == AV_PIX_FMT_BGRA64LE || srcFormat == AV_PIX_FMT_BGRA64BE) &&(dstFormat == AV_PIX_FMT_GBRP9LE  || dstFormat == AV_PIX_FMT_GBRP9BE  ||dstFormat == AV_PIX_FMT_GBRP10LE || dstFormat == AV_PIX_FMT_GBRP10BE ||dstFormat == AV_PIX_FMT_GBRP12LE || dstFormat == AV_PIX_FMT_GBRP12BE ||dstFormat == AV_PIX_FMT_GBRP14LE || dstFormat == AV_PIX_FMT_GBRP14BE ||dstFormat == AV_PIX_FMT_GBRP16LE || dstFormat == AV_PIX_FMT_GBRP16BE ||dstFormat == AV_PIX_FMT_GBRAP10LE || dstFormat == AV_PIX_FMT_GBRAP10BE ||dstFormat == AV_PIX_FMT_GBRAP12LE || dstFormat == AV_PIX_FMT_GBRAP12BE ||dstFormat == AV_PIX_FMT_GBRAP16LE || dstFormat == AV_PIX_FMT_GBRAP16BE ))c->convert_unscaled = Rgb16ToPlanarRgb16Wrapper;if ((srcFormat == AV_PIX_FMT_GBRP9LE  || srcFormat == AV_PIX_FMT_GBRP9BE  ||srcFormat == AV_PIX_FMT_GBRP16LE || srcFormat == AV_PIX_FMT_GBRP16BE ||srcFormat == AV_PIX_FMT_GBRP10LE || srcFormat == AV_PIX_FMT_GBRP10BE ||srcFormat == AV_PIX_FMT_GBRP12LE || srcFormat == AV_PIX_FMT_GBRP12BE ||srcFormat == AV_PIX_FMT_GBRP14LE || srcFormat == AV_PIX_FMT_GBRP14BE ||srcFormat == AV_PIX_FMT_GBRAP10LE || srcFormat == AV_PIX_FMT_GBRAP10BE ||srcFormat == AV_PIX_FMT_GBRAP12LE || srcFormat == AV_PIX_FMT_GBRAP12BE ||srcFormat == AV_PIX_FMT_GBRAP16LE || srcFormat == AV_PIX_FMT_GBRAP16BE) &&(dstFormat == AV_PIX_FMT_RGB48LE  || dstFormat == AV_PIX_FMT_RGB48BE  ||dstFormat == AV_PIX_FMT_BGR48LE  || dstFormat == AV_PIX_FMT_BGR48BE  ||dstFormat == AV_PIX_FMT_RGBA64LE || dstFormat == AV_PIX_FMT_RGBA64BE ||dstFormat == AV_PIX_FMT_BGRA64LE || dstFormat == AV_PIX_FMT_BGRA64BE))c->convert_unscaled = planarRgb16ToRgb16Wrapper;if (av_pix_fmt_desc_get(srcFormat)->comp[0].depth == 8 &&isPackedRGB(srcFormat) && dstFormat == AV_PIX_FMT_GBRP)c->convert_unscaled = rgbToPlanarRgbWrapper;if (isBayer(srcFormat)) {if (dstFormat == AV_PIX_FMT_RGB24)c->convert_unscaled = bayer_to_rgb24_wrapper;else if (dstFormat == AV_PIX_FMT_RGB48)c->convert_unscaled = bayer_to_rgb48_wrapper;else if (dstFormat == AV_PIX_FMT_YUV420P)c->convert_unscaled = bayer_to_yv12_wrapper;else if (!isBayer(dstFormat)) {av_log(c, AV_LOG_ERROR, "unsupported bayer conversion\n");av_assert0(0);}}/* bswap 16 bits per pixel/component packed formats */if (IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BAYER_BGGR16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BAYER_RGGB16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BAYER_GBRG16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BAYER_GRBG16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BGR444) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BGR48)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BGR555) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BGR565) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_BGRA64) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GRAY9)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GRAY10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GRAY12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GRAY14) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GRAY16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YA16)   ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_AYUV64) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRP9)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRP10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRP12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRP14) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRP16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRAP10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRAP12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRAP16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_RGB444) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_RGB48)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_RGB555) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_RGB565) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_RGBA64) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_XYZ12)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV420P9)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV420P10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV420P12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV420P14) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV420P16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV422P9)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV422P10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV422P12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV422P14) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV422P16) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV440P10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV440P12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV444P9)  ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV444P10) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV444P12) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV444P14) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_YUV444P16))c->convert_unscaled = bswap_16bpc;/* bswap 32 bits per pixel/component formats */if (IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRPF32) ||IS_DIFFERENT_ENDIANESS(srcFormat, dstFormat, AV_PIX_FMT_GBRAPF32))c->convert_unscaled = bswap_32bpc;if (usePal(srcFormat) && isByteRGB(dstFormat))c->convert_unscaled = palToRgbWrapper;if (srcFormat == AV_PIX_FMT_YUV422P) {if (dstFormat == AV_PIX_FMT_YUYV422)c->convert_unscaled = yuv422pToYuy2Wrapper;else if (dstFormat == AV_PIX_FMT_UYVY422)c->convert_unscaled = yuv422pToUyvyWrapper;}/* uint Y to float Y */if (srcFormat == AV_PIX_FMT_GRAY8 && dstFormat == AV_PIX_FMT_GRAYF32){c->convert_unscaled = uint_y_to_float_y_wrapper;}/* float Y to uint Y */if (srcFormat == AV_PIX_FMT_GRAYF32 && dstFormat == AV_PIX_FMT_GRAY8){c->convert_unscaled = float_y_to_uint_y_wrapper;}/* LQ converters if -sws 0 or -sws 4*/if (c->flags&(SWS_FAST_BILINEAR|SWS_POINT)) {/* yv12_to_yuy2 */if (srcFormat == AV_PIX_FMT_YUV420P || srcFormat == AV_PIX_FMT_YUVA420P) {if (dstFormat == AV_PIX_FMT_YUYV422)c->convert_unscaled = planarToYuy2Wrapper;else if (dstFormat == AV_PIX_FMT_UYVY422)c->convert_unscaled = planarToUyvyWrapper;}}if (srcFormat == AV_PIX_FMT_YUYV422 &&(dstFormat == AV_PIX_FMT_YUV420P || dstFormat == AV_PIX_FMT_YUVA420P))c->convert_unscaled = yuyvToYuv420Wrapper;if (srcFormat == AV_PIX_FMT_UYVY422 &&(dstFormat == AV_PIX_FMT_YUV420P || dstFormat == AV_PIX_FMT_YUVA420P))c->convert_unscaled = uyvyToYuv420Wrapper;if (srcFormat == AV_PIX_FMT_YUYV422 && dstFormat == AV_PIX_FMT_YUV422P)c->convert_unscaled = yuyvToYuv422Wrapper;if (srcFormat == AV_PIX_FMT_UYVY422 && dstFormat == AV_PIX_FMT_YUV422P)c->convert_unscaled = uyvyToYuv422Wrapper;#define isPlanarGray(x) (isGray(x) && (x) != AV_PIX_FMT_YA8 && (x) != AV_PIX_FMT_YA16LE && (x) != AV_PIX_FMT_YA16BE)/* simple copy */if ( srcFormat == dstFormat ||(srcFormat == AV_PIX_FMT_YUVA420P && dstFormat == AV_PIX_FMT_YUV420P) ||(srcFormat == AV_PIX_FMT_YUV420P && dstFormat == AV_PIX_FMT_YUVA420P) ||(isFloat(srcFormat) == isFloat(dstFormat)) && ((isPlanarYUV(srcFormat) && isPlanarGray(dstFormat)) ||(isPlanarYUV(dstFormat) && isPlanarGray(srcFormat)) ||(isPlanarGray(dstFormat) && isPlanarGray(srcFormat)) ||(isPlanarYUV(srcFormat) && isPlanarYUV(dstFormat) &&c->chrDstHSubSample == c->chrSrcHSubSample &&c->chrDstVSubSample == c->chrSrcVSubSample &&!isSemiPlanarYUV(srcFormat) && !isSemiPlanarYUV(dstFormat)))){if (isPacked(c->srcFormat))c->convert_unscaled = packedCopyWrapper;else /* Planar YUV or gray */c->convert_unscaled = planarCopyWrapper;}if (ARCH_PPC)ff_get_unscaled_swscale_ppc(c);if (ARCH_ARM)ff_get_unscaled_swscale_arm(c);if (ARCH_AARCH64)ff_get_unscaled_swscale_aarch64(c);
}

从代码中可以看出，它根据输入输出像素格式的不同，选择了不同的转换函数。
例如YUV420P转换NV12的时候，就会将planarToNv12Wrapper()赋值给SwsContext的swscale指针。

有拉伸--swscale()

如果图像进行了拉伸，则会调用ff_getSwsFunc()对SwsContext的swscale进行赋值。
ff_getSwsFunc 函数被弃用
参考链接：FFmpeg源代码简单分析-其他-libswscale的sws_getContext()_MY CUP OF TEA的博客-CSDN博客
ff_sws_init_scale函数的内部执行逻辑和 ff_getSwsFunc 类似

void ff_sws_init_scale(SwsContext *c)
{sws_init_swscale(c);if (ARCH_PPC)ff_sws_init_swscale_ppc(c);if (ARCH_X86)ff_sws_init_swscale_x86(c);if (ARCH_AARCH64)ff_sws_init_swscale_aarch64(c);if (ARCH_ARM)ff_sws_init_swscale_arm(c);
}

static av_cold void sws_init_swscale(SwsContext *c)
{enum AVPixelFormat srcFormat = c->srcFormat;ff_sws_init_output_funcs(c, &c->yuv2plane1, &c->yuv2planeX,&c->yuv2nv12cX, &c->yuv2packed1,&c->yuv2packed2, &c->yuv2packedX, &c->yuv2anyX);ff_sws_init_input_funcs(c);if (c->srcBpc == 8) {if (c->dstBpc <= 14) {c->hyScale = c->hcScale = hScale8To15_c;if (c->flags & SWS_FAST_BILINEAR) {c->hyscale_fast = ff_hyscale_fast_c;c->hcscale_fast = ff_hcscale_fast_c;}} else {c->hyScale = c->hcScale = hScale8To19_c;}} else {c->hyScale = c->hcScale = c->dstBpc > 14 ? hScale16To19_c: hScale16To15_c;}ff_sws_init_range_convert(c);if (!(isGray(srcFormat) || isGray(c->dstFormat) ||srcFormat == AV_PIX_FMT_MONOBLACK || srcFormat == AV_PIX_FMT_MONOWHITE))c->needs_hcscale = 1;
}

未找到代码依据

注意，sws_init_context()对SwsContext的swscale进行赋值的语句是：
c->swscale = ff_getSwsFunc(c);
即把ff_getSwsFunc()的返回值赋值给SwsContext的swscale指针；而ff_getSwsFunc()的返回值是一个静态函数，名称就叫做“swscale”。

下面我们看一下这个swscale()静态函数的定义。

static int swscale(SwsContext *c, const uint8_t *src[],int srcStride[], int srcSliceY, int srcSliceH,uint8_t *dst[], int dstStride[],int dstSliceY, int dstSliceH)
{const int scale_dst = dstSliceY > 0 || dstSliceH < c->dstH;/* load a few things into local vars to make the code more readable?* and faster */const int dstW                   = c->dstW;int dstH                         = c->dstH;const enum AVPixelFormat dstFormat = c->dstFormat;const int flags                  = c->flags;int32_t *vLumFilterPos           = c->vLumFilterPos;int32_t *vChrFilterPos           = c->vChrFilterPos;const int vLumFilterSize         = c->vLumFilterSize;const int vChrFilterSize         = c->vChrFilterSize;yuv2planar1_fn yuv2plane1        = c->yuv2plane1;yuv2planarX_fn yuv2planeX        = c->yuv2planeX;yuv2interleavedX_fn yuv2nv12cX   = c->yuv2nv12cX;yuv2packed1_fn yuv2packed1       = c->yuv2packed1;yuv2packed2_fn yuv2packed2       = c->yuv2packed2;yuv2packedX_fn yuv2packedX       = c->yuv2packedX;yuv2anyX_fn yuv2anyX             = c->yuv2anyX;const int chrSrcSliceY           =                srcSliceY >> c->chrSrcVSubSample;const int chrSrcSliceH           = AV_CEIL_RSHIFT(srcSliceH,   c->chrSrcVSubSample);int should_dither                = isNBPS(c->srcFormat) ||is16BPS(c->srcFormat);int lastDstY;/* vars which will change and which we need to store back in the context */int dstY         = c->dstY;int lastInLumBuf = c->lastInLumBuf;int lastInChrBuf = c->lastInChrBuf;int lumStart = 0;int lumEnd = c->descIndex[0];int chrStart = lumEnd;int chrEnd = c->descIndex[1];int vStart = chrEnd;int vEnd = c->numDesc;SwsSlice *src_slice = &c->slice[lumStart];SwsSlice *hout_slice = &c->slice[c->numSlice-2];SwsSlice *vout_slice = &c->slice[c->numSlice-1];SwsFilterDescriptor *desc = c->desc;int needAlpha = c->needAlpha;int hasLumHoles = 1;int hasChrHoles = 1;if (isPacked(c->srcFormat)) {src[1] =src[2] =src[3] = src[0];srcStride[1] =srcStride[2] =srcStride[3] = srcStride[0];}srcStride[1] *= 1 << c->vChrDrop;srcStride[2] *= 1 << c->vChrDrop;DEBUG_BUFFERS("swscale() %p[%d] %p[%d] %p[%d] %p[%d] -> %p[%d] %p[%d] %p[%d] %p[%d]\n",src[0], srcStride[0], src[1], srcStride[1],src[2], srcStride[2], src[3], srcStride[3],dst[0], dstStride[0], dst[1], dstStride[1],dst[2], dstStride[2], dst[3], dstStride[3]);DEBUG_BUFFERS("srcSliceY: %d srcSliceH: %d dstY: %d dstH: %d\n",srcSliceY, srcSliceH, dstY, dstH);DEBUG_BUFFERS("vLumFilterSize: %d vChrFilterSize: %d\n",vLumFilterSize, vChrFilterSize);if (dstStride[0]&15 || dstStride[1]&15 ||dstStride[2]&15 || dstStride[3]&15) {SwsContext *const ctx = c->parent ? c->parent : c;if (flags & SWS_PRINT_INFO &&!atomic_exchange_explicit(&ctx->stride_unaligned_warned, 1, memory_order_relaxed)) {av_log(c, AV_LOG_WARNING,"Warning: dstStride is not aligned!\n""         ->cannot do aligned memory accesses anymore\n");}}#if ARCH_X86if (   (uintptr_t)dst[0]&15 || (uintptr_t)dst[1]&15 || (uintptr_t)dst[2]&15|| (uintptr_t)src[0]&15 || (uintptr_t)src[1]&15 || (uintptr_t)src[2]&15|| dstStride[0]&15 || dstStride[1]&15 || dstStride[2]&15 || dstStride[3]&15|| srcStride[0]&15 || srcStride[1]&15 || srcStride[2]&15 || srcStride[3]&15) {SwsContext *const ctx = c->parent ? c->parent : c;int cpu_flags = av_get_cpu_flags();if (flags & SWS_PRINT_INFO && HAVE_MMXEXT && (cpu_flags & AV_CPU_FLAG_SSE2) &&!atomic_exchange_explicit(&ctx->stride_unaligned_warned,1, memory_order_relaxed)) {av_log(c, AV_LOG_WARNING, "Warning: data is not aligned! This can lead to a speed loss\n");}}
#endifif (scale_dst) {dstY         = dstSliceY;dstH         = dstY + dstSliceH;lastInLumBuf = -1;lastInChrBuf = -1;} else if (srcSliceY == 0) {/* Note the user might start scaling the picture in the middle so this* will not get executed. This is not really intended but works* currently, so people might do it. */dstY         = 0;lastInLumBuf = -1;lastInChrBuf = -1;}if (!should_dither) {c->chrDither8 = c->lumDither8 = sws_pb_64;}lastDstY = dstY;ff_init_vscale_pfn(c, yuv2plane1, yuv2planeX, yuv2nv12cX,yuv2packed1, yuv2packed2, yuv2packedX, yuv2anyX, c->use_mmx_vfilter);ff_init_slice_from_src(src_slice, (uint8_t**)src, srcStride, c->srcW,srcSliceY, srcSliceH, chrSrcSliceY, chrSrcSliceH, 1);ff_init_slice_from_src(vout_slice, (uint8_t**)dst, dstStride, c->dstW,dstY, dstSliceH, dstY >> c->chrDstVSubSample,AV_CEIL_RSHIFT(dstSliceH, c->chrDstVSubSample), scale_dst);if (srcSliceY == 0) {hout_slice->plane[0].sliceY = lastInLumBuf + 1;hout_slice->plane[1].sliceY = lastInChrBuf + 1;hout_slice->plane[2].sliceY = lastInChrBuf + 1;hout_slice->plane[3].sliceY = lastInLumBuf + 1;hout_slice->plane[0].sliceH =hout_slice->plane[1].sliceH =hout_slice->plane[2].sliceH =hout_slice->plane[3].sliceH = 0;hout_slice->width = dstW;}for (; dstY < dstH; dstY++) {const int chrDstY = dstY >> c->chrDstVSubSample;int use_mmx_vfilter= c->use_mmx_vfilter;// First line needed as inputconst int firstLumSrcY  = FFMAX(1 - vLumFilterSize, vLumFilterPos[dstY]);const int firstLumSrcY2 = FFMAX(1 - vLumFilterSize, vLumFilterPos[FFMIN(dstY | ((1 << c->chrDstVSubSample) - 1), c->dstH - 1)]);// First line needed as inputconst int firstChrSrcY  = FFMAX(1 - vChrFilterSize, vChrFilterPos[chrDstY]);// Last line needed as inputint lastLumSrcY  = FFMIN(c->srcH,    firstLumSrcY  + vLumFilterSize) - 1;int lastLumSrcY2 = FFMIN(c->srcH,    firstLumSrcY2 + vLumFilterSize) - 1;int lastChrSrcY  = FFMIN(c->chrSrcH, firstChrSrcY  + vChrFilterSize) - 1;int enough_lines;int i;int posY, cPosY, firstPosY, lastPosY, firstCPosY, lastCPosY;// handle holes (FAST_BILINEAR & weird filters)if (firstLumSrcY > lastInLumBuf) {hasLumHoles = lastInLumBuf != firstLumSrcY - 1;if (hasLumHoles) {hout_slice->plane[0].sliceY = firstLumSrcY;hout_slice->plane[3].sliceY = firstLumSrcY;hout_slice->plane[0].sliceH =hout_slice->plane[3].sliceH = 0;}lastInLumBuf = firstLumSrcY - 1;}if (firstChrSrcY > lastInChrBuf) {hasChrHoles = lastInChrBuf != firstChrSrcY - 1;if (hasChrHoles) {hout_slice->plane[1].sliceY = firstChrSrcY;hout_slice->plane[2].sliceY = firstChrSrcY;hout_slice->plane[1].sliceH =hout_slice->plane[2].sliceH = 0;}lastInChrBuf = firstChrSrcY - 1;}DEBUG_BUFFERS("dstY: %d\n", dstY);DEBUG_BUFFERS("\tfirstLumSrcY: %d lastLumSrcY: %d lastInLumBuf: %d\n",firstLumSrcY, lastLumSrcY, lastInLumBuf);DEBUG_BUFFERS("\tfirstChrSrcY: %d lastChrSrcY: %d lastInChrBuf: %d\n",firstChrSrcY, lastChrSrcY, lastInChrBuf);// Do we have enough lines in this slice to output the dstY lineenough_lines = lastLumSrcY2 < srcSliceY + srcSliceH &&lastChrSrcY < AV_CEIL_RSHIFT(srcSliceY + srcSliceH, c->chrSrcVSubSample);if (!enough_lines) {lastLumSrcY = srcSliceY + srcSliceH - 1;lastChrSrcY = chrSrcSliceY + chrSrcSliceH - 1;DEBUG_BUFFERS("buffering slice: lastLumSrcY %d lastChrSrcY %d\n",lastLumSrcY, lastChrSrcY);}av_assert0((lastLumSrcY - firstLumSrcY + 1) <= hout_slice->plane[0].available_lines);av_assert0((lastChrSrcY - firstChrSrcY + 1) <= hout_slice->plane[1].available_lines);posY = hout_slice->plane[0].sliceY + hout_slice->plane[0].sliceH;if (posY <= lastLumSrcY && !hasLumHoles) {firstPosY = FFMAX(firstLumSrcY, posY);lastPosY = FFMIN(firstLumSrcY + hout_slice->plane[0].available_lines - 1, srcSliceY + srcSliceH - 1);} else {firstPosY = posY;lastPosY = lastLumSrcY;}cPosY = hout_slice->plane[1].sliceY + hout_slice->plane[1].sliceH;if (cPosY <= lastChrSrcY && !hasChrHoles) {firstCPosY = FFMAX(firstChrSrcY, cPosY);lastCPosY = FFMIN(firstChrSrcY + hout_slice->plane[1].available_lines - 1, AV_CEIL_RSHIFT(srcSliceY + srcSliceH, c->chrSrcVSubSample) - 1);} else {firstCPosY = cPosY;lastCPosY = lastChrSrcY;}ff_rotate_slice(hout_slice, lastPosY, lastCPosY);if (posY < lastLumSrcY + 1) {for (i = lumStart; i < lumEnd; ++i)desc[i].process(c, &desc[i], firstPosY, lastPosY - firstPosY + 1);}lastInLumBuf = lastLumSrcY;if (cPosY < lastChrSrcY + 1) {for (i = chrStart; i < chrEnd; ++i)desc[i].process(c, &desc[i], firstCPosY, lastCPosY - firstCPosY + 1);}lastInChrBuf = lastChrSrcY;if (!enough_lines)break;  // we can't output a dstY line so let's try with the next slice#if HAVE_MMX_INLINEff_updateMMXDitherTables(c, dstY);
#endifif (should_dither) {c->chrDither8 = ff_dither_8x8_128[chrDstY & 7];c->lumDither8 = ff_dither_8x8_128[dstY    & 7];}if (dstY >= c->dstH - 2) {/* hmm looks like we can't use MMX here without overwriting* this array's tail */ff_sws_init_output_funcs(c, &yuv2plane1, &yuv2planeX, &yuv2nv12cX,&yuv2packed1, &yuv2packed2, &yuv2packedX, &yuv2anyX);use_mmx_vfilter= 0;ff_init_vscale_pfn(c, yuv2plane1, yuv2planeX, yuv2nv12cX,yuv2packed1, yuv2packed2, yuv2packedX, yuv2anyX, use_mmx_vfilter);}for (i = vStart; i < vEnd; ++i)desc[i].process(c, &desc[i], dstY, 1);}if (isPlanar(dstFormat) && isALPHA(dstFormat) && !needAlpha) {int offset = lastDstY - dstSliceY;int length = dstW;int height = dstY - lastDstY;if (is16BPS(dstFormat) || isNBPS(dstFormat)) {const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(dstFormat);fillPlane16(dst[3], dstStride[3], length, height, offset,1, desc->comp[3].depth,isBE(dstFormat));} else if (is32BPS(dstFormat)) {const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(dstFormat);fillPlane32(dst[3], dstStride[3], length, height, offset,1, desc->comp[3].depth,isBE(dstFormat), desc->flags & AV_PIX_FMT_FLAG_FLOAT);} elsefillPlane(dst[3], dstStride[3], length, height, offset, 255);}#if HAVE_MMXEXT_INLINEif (av_get_cpu_flags() & AV_CPU_FLAG_MMXEXT)__asm__ volatile ("sfence" ::: "memory");
#endifemms_c();/* store changed local vars back in the context */c->dstY         = dstY;c->lastInLumBuf = lastInLumBuf;c->lastInChrBuf = lastInChrBuf;return dstY - lastDstY;
}

可以看出swscale()是一行一行的进行图像缩放工作的。其中每行数据的处理按照“先水平拉伸，然后垂直拉伸”的方式进行处理。
具体的实现函数如下所示：
1. 水平拉伸
- a) 亮度水平拉伸：hyscale()
- b) 色度水平拉伸：hcscale()
2. 垂直拉伸
a) Planar
- i. 亮度垂直拉伸-不拉伸：yuv2plane1()
- ii. 亮度垂直拉伸-拉伸：yuv2planeX()
- iii. 色度垂直拉伸-不拉伸：yuv2plane1()
- iv. 色度垂直拉伸-拉伸：yuv2planeX()
b) Packed
- i. 垂直拉伸-不拉伸：yuv2packed1()
- ii. 垂直拉伸-拉伸：yuv2packedX()
下面具体看看这几个函数的定义。

hyscale()

水平亮度拉伸函数hyscale()的定义位于libswscale\swscale.c，如下所示。并不存在

    /*** Scale one horizontal line of input data using a filter over the input* lines, to produce one (differently sized) line of output data.** @param dst        pointer to destination buffer for horizontally scaled*                   data. If the number of bits per component of one*                   destination pixel (SwsContext->dstBpc) is <= 10, data*                   will be 15 bpc in 16 bits (int16_t) width. Else (i.e.*                   SwsContext->dstBpc == 16), data will be 19bpc in*                   32 bits (int32_t) width.* @param dstW       width of destination image* @param src        pointer to source data to be scaled. If the number of*                   bits per component of a source pixel (SwsContext->srcBpc)*                   is 8, this is 8bpc in 8 bits (uint8_t) width. Else*                   (i.e. SwsContext->dstBpc > 8), this is native depth*                   in 16 bits (uint16_t) width. In other words, for 9-bit*                   YUV input, this is 9bpc, for 10-bit YUV input, this is*                   10bpc, and for 16-bit RGB or YUV, this is 16bpc.* @param filter     filter coefficients to be used per output pixel for*                   scaling. This contains 14bpp filtering coefficients.*                   Guaranteed to contain dstW * filterSize entries.* @param filterPos  position of the first input pixel to be used for*                   each output pixel during scaling. Guaranteed to*                   contain dstW entries.* @param filterSize the number of input coefficients to be used (and*                   thus the number of input pixels to be used) for*                   creating a single output pixel. Is aligned to 4*                   (and input coefficients thus padded with zeroes)*                   to simplify creating SIMD code.*//** @{ */void (*hyScale)(struct SwsContext *c, int16_t *dst, int dstW,const uint8_t *src, const int16_t *filter,const int32_t *filterPos, int filterSize);void (*hcScale)(struct SwsContext *c, int16_t *dst, int dstW,const uint8_t *src, const int16_t *filter,const int32_t *filterPos, int filterSize);