FFmpeg源代码简单分析 libswscale的sws scale

本文继续上一篇文章《FFmpeg源代码分析：sws_getContext()》的内容，简单分析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。有关libswscale的使用可以参考文章：
《最简单的基于FFmpeg的libswscale的示例（YUV转RGB）》

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。

在分析sws_scale()的源代码之前，先简单回顾一下上篇文章中分析得到的两张图。

函数调用结构图

分析得到的libswscale的函数调用关系如下图所示。

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：首相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 sws_scale(struct SwsContext *c,                                  const uint8_t * const srcSlice[],                                  const int srcStride[], int srcSliceY,                                  int srcSliceH, uint8_t *const dst[],                                  const int dstStride[]){    int i, ret;    const uint8_t *src2[4];    uint8_t *dst2[4];    uint8_t *rgb0_tmp = NULL;    //检查输入参数    if (!srcStride || !dstStride || !dst || !srcSlice) {        av_log(c, AV_LOG_ERROR, "One of the input parameters to sws_scale() is NULL, please check the calling code\n");        return 0;    }    if (c->cascaded_context[0] && srcSliceY == 0 && srcSliceH == c->cascaded_context[0]->srcH) {        ret = sws_scale(c->cascaded_context[0],                        srcSlice, srcStride, srcSliceY, srcSliceH,                        c->cascaded_tmp, c->cascaded_tmpStride);        if (ret < 0)            return ret;        ret = sws_scale(c->cascaded_context[1],                        (const uint8_t * const * )c->cascaded_tmp, c->cascaded_tmpStride, 0, c->cascaded_context[0]->dstH,                        dst, dstStride);        return ret;    }    memcpy(src2, srcSlice, sizeof(src2));    memcpy(dst2, dst, sizeof(dst2));    // do not mess up sliceDir if we have a "trailing" 0-size slice    if (srcSliceH == 0)        return 0;    //检查    if (!check_image_pointers(srcSlice, c->srcFormat, srcStride)) {        av_log(c, AV_LOG_ERROR, "bad src image pointers\n");        return 0;    }    if (!check_image_pointers((const uint8_t* const*)dst, c->dstFormat, dstStride)) {        av_log(c, AV_LOG_ERROR, "bad dst image pointers\n");        return 0;    }    if (c->sliceDir == 0 && srcSliceY != 0 && srcSliceY + srcSliceH != c->srcH) {        av_log(c, AV_LOG_ERROR, "Slices start in the middle!\n");        return 0;    }    if (c->sliceDir == 0) {        if (srcSliceY == 0) c->sliceDir = 1; else c->sliceDir = -1;    }    //使用调色板palette的特殊处理？应该不常见    if (usePal(c->srcFormat)) {        for (i = 0; i < 256; i++) {            int r, g, b, y, u, v, a = 0xff;            if (c->srcFormat == AV_PIX_FMT_PAL8) {                uint32_t p = ((const uint32_t *)(srcSlice[1]))[i];                a = (p >> 24) & 0xFF;                r = (p >> 16) & 0xFF;                g = (p >>  8) & 0xFF;                b =  p        & 0xFF;            } else if (c->srcFormat == AV_PIX_FMT_RGB8) {                r = ( i >> 5     ) * 36;                g = ((i >> 2) & 7) * 36;                b = ( i       & 3) * 85;            } else if (c->srcFormat == AV_PIX_FMT_BGR8) {                b = ( i >> 6     ) * 85;                g = ((i >> 3) & 7) * 36;                r = ( i       & 7) * 36;            } else if (c->srcFormat == AV_PIX_FMT_RGB4_BYTE) {                r = ( i >> 3     ) * 255;                g = ((i >> 1) & 3) * 85;                b = ( i       & 1) * 255;            } else if (c->srcFormat == AV_PIX_FMT_GRAY8 || c->srcFormat == AV_PIX_FMT_GRAY8A) {                r = g = b = i;            } else {                av_assert1(c->srcFormat == AV_PIX_FMT_BGR4_BYTE);                b = ( i >> 3     ) * 255;                g = ((i >> 1) & 3) * 85;                r = ( i       & 1) * 255;            }#define RGB2YUV_SHIFT 15#define BY ( (int) (0.114 * 219 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define BV (-(int) (0.081 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define BU ( (int) (0.500 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define GY ( (int) (0.587 * 219 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define GV (-(int) (0.419 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define GU (-(int) (0.331 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define RY ( (int) (0.299 * 219 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define RV ( (int) (0.500 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))#define RU (-(int) (0.169 * 224 / 255 * (1 << RGB2YUV_SHIFT) + 0.5))            y = av_clip_uint8((RY * r + GY * g + BY * b + ( 33 << (RGB2YUV_SHIFT - 1))) >> RGB2YUV_SHIFT);            u = av_clip_uint8((RU * r + GU * g + BU * b + (257 << (RGB2YUV_SHIFT - 1))) >> RGB2YUV_SHIFT);            v = av_clip_uint8((RV * r + GV * g + BV * b + (257 << (RGB2YUV_SHIFT - 1))) >> RGB2YUV_SHIFT);            c->pal_yuv[i]= y + (u<<8) + (v<<16) + ((unsigned)a<<24);            switch (c->dstFormat) {            case AV_PIX_FMT_BGR32:#if !HAVE_BIGENDIAN            case AV_PIX_FMT_RGB24:#endif                c->pal_rgb[i]=  r + (g<<8) + (b<<16) + ((unsigned)a<<24);                break;            case AV_PIX_FMT_BGR32_1:#if HAVE_BIGENDIAN            case AV_PIX_FMT_BGR24:#endif                c->pal_rgb[i]= a + (r<<8) + (g<<16) + ((unsigned)b<<24);                break;            case AV_PIX_FMT_RGB32_1:#if HAVE_BIGENDIAN