什么是“线条对齐”的意思？ [英] what is 'linesize alignment' meaning?

问题描述

我正在 http://dranger.com/ffmpeg/tutorial01.html。

我刚刚发现 avpicture_get_size 函数已被弃用。

I have just found that avpicture_get_size function is deprecated.

所以我检查了ffmpeg的文档（ https：// www。 ffmpeg.org/doxygen/3.0/group__lavu__picture.html#ga24a67963c3ae0054a2a4bab35930e694 ），并找到替换 av_image_get_buffer_size 。

So I have checked ffmpeg's document(https://www.ffmpeg.org/doxygen/3.0/group__lavu__picture.html#ga24a67963c3ae0054a2a4bab35930e694) and found substitute av_image_get_buffer_size.

但是我不明白 align 参数含义'linesize对齐'......

But I can't understand align parameter meaning 'linesize alignment'......

什么是这意味着？

推荐答案

FFmpeg的一些部分，特别是libavcodec，需要对齐的linesizes []，这意味着它需要： p>

Some parts of FFmpeg, notably libavcodec, require aligned linesizes[], which means that it requires:

assert(linesize[0] % 32 == 0);
assert(linesize[1] % 32 == 0);
assert(linesize[2] % 32 == 0);

这允许它使用快速/对齐的SIMD例程（例如SSE2 / AVX2 movdqa 或 vmovdqa 指令）用于数据访问，而不是较慢的未对齐的对应。

This allows it to use fast/aligned SIMD routines (for example SSE2/AVX2 movdqa or vmovdqa instructions) for data access instead of their slower unaligned counterparts.

align 参数到这个 av_image_get_buffer_size 函数是这个行对齐，你需要它，因为缓冲区的大小是受它影响。例如，YUV缓冲区中的Y平面的大小实际上不是width * height，它是linesize [0] * height。你会看到（特别是图像尺寸不是16或32的倍数），因为你将 align 增加到2的更高的权力，返回值缓慢增加。

The align parameter to this av_image_get_buffer_size function is this line alignment, and you need it because the size of the buffer is affected by it. E.g., the size of a Y plane in a YUV buffer isn't actually width * height, it's linesize[0] * height. You'll see that (especially for image sizes that are not a multiple of 16 or 32), as you increase align to higher powers of 2, the return value slowly increases.

实际上，如果要将此图片用作调用的输出缓冲区，例如 avcodec_decode_video2 ，这应该是32.对于swscale / avfilter，我相信没有绝对的要求，但是你建议仍然要32。

Practically speaking, if you're going to use this picture as output buffer for calls to e.g. avcodec_decode_video2, this should be 32. For swscale/avfilter, I believe there is no absolute requirement, but you're recommended to still make it 32.

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