c - 在 x264 中正确使用 `nalu_process` 回调
问题描述
我希望利用 libx264 的低延迟编码机制,从而在单个 NAL 单元可用时立即调用用户提供的回调,而不必在开始处理之前等待整个帧被编码。
x264 文档说明了有关该设施的以下内容:
/* Optional low-level callback for low-latency encoding. Called for each output NAL unit
* immediately after the NAL unit is finished encoding. This allows the calling application
* to begin processing video data (e.g. by sending packets over a network) before the frame
* is done encoding.
*
* This callback MUST do the following in order to work correctly:
* 1) Have available an output buffer of at least size nal->i_payload*3/2 + 5 + 64.
* 2) Call x264_nal_encode( h, dst, nal ), where dst is the output buffer.
* After these steps, the content of nal is valid and can be used in the same way as if
* the NAL unit were output by x264_encoder_encode.
*
* This does not need to be synchronous with the encoding process: the data pointed to
* by nal (both before and after x264_nal_encode) will remain valid until the next
* x264_encoder_encode call. The callback must be re-entrant.
*
* This callback does not work with frame-based threads; threads must be disabled
* or sliced-threads enabled. This callback also does not work as one would expect
* with HRD -- since the buffering period SEI cannot be calculated until the frame
* is finished encoding, it will not be sent via this callback.
*
* Note also that the NALs are not necessarily returned in order when sliced threads is
* enabled. Accordingly, the variable i_first_mb and i_last_mb are available in
* x264_nal_t to help the calling application reorder the slices if necessary.
*
* When this callback is enabled, x264_encoder_encode does not return valid NALs;
* the calling application is expected to acquire all output NALs through the callback.
*
* It is generally sensible to combine this callback with a use of slice-max-mbs or
* slice-max-size.
*
* The opaque pointer is the opaque pointer from the input frame associated with this
* NAL unit. This helps distinguish between nalu_process calls from different sources,
* e.g. if doing multiple encodes in one process.
*/
void (*nalu_process)( x264_t *h, x264_nal_t *nal, void *opaque );
这似乎很简单。但是,当我运行以下虚拟代码时,我在标记的行上得到一个段错误。我试图为x264_nal_encode
自己添加一些调试以了解它出错的地方,但似乎是函数调用本身导致了段错误。我在这里错过了什么吗?(让我们忽略这样一个事实,即使用assert
可能是cb
不可重入的——它只是向读者表明我的工作区缓冲区已经足够大了。)
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <x264.h>
#define WS_SIZE 10000000
uint8_t * workspace;
void cb(x264_t * h, x264_nal_t * nal, void * opaque)
{
assert((nal->i_payload*3)/2 + 5 + 64 < WS_SIZE);
x264_nal_encode(h, workspace, nal); // Segfault here.
// Removed: Process nal.
}
int main(int argc, char ** argv)
{
uint8_t * fake_frame = malloc(1280*720*3);
memset(fake_frame, 0, 1280*720*3);
workspace = malloc(WS_SIZE);
x264_param_t param;
int status = x264_param_default_preset(¶m, "ultrafast", "zerolatency");
assert(status == 0);
param.i_csp = X264_CSP_RGB;
param.i_width = 1280;
param.i_height = 720;
param.i_threads = 1;
param.i_lookahead_threads = 1;
param.i_frame_total = 0;
param.i_fps_num = 30;
param.i_fps_den = 1;
param.i_slice_max_size = 1024;
param.b_annexb = 1;
param.nalu_process = cb;
status = x264_param_apply_profile(¶m, "high444");
assert(status == 0);
x264_t * h = x264_encoder_open(¶m);
assert(h);
x264_picture_t pic;
status = x264_picture_alloc(&pic, param.i_csp, param.i_width, param.i_height);
assert(pic.img.i_plane == 1);
x264_picture_t pic_out;
x264_nal_t * nal; // Not used. We process NALs in cb.
int i_nal;
for (int i = 0; i < 100; ++i)
{
pic.i_pts = i;
pic.img.plane[0] = fake_frame;
status = x264_encoder_encode(h, &nal, &i_nal, &pic, &pic_out);
}
x264_encoder_close(h);
x264_picture_clean(&pic);
free(workspace);
free(fake_frame);
return 0;
}
编辑:段错误发生在第一次cb
调用x264_nal_encode
。如果我切换到不同的预设,在第一次回调发生之前编码更多帧,那么在第一次回调之前会进行几次成功调用x264_encoder_encode
,因此会发生段错误。
解决方案
在 IRC 上与 x264 开发人员讨论后,我看到的行为似乎实际上是 x264 中的一个错误。x264_t * h
传递给回调的参数不正确。如果一个人用好的人(从 获得的人)覆盖了那个处理x264_encoder_open
,事情就很好了。
我将 x264 git commit 71ed44c7312438fac7c5c5301e45522e57127db4 确定为第一个坏的。该错误记录为此 x264 问题。
未来读者更新:我相信这个问题已在提交 544c61f082194728d0391fb280a6e138ba320a96 中得到修复。
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