如何解决在 x264 中正确使用 `nalu_process` 回调
我希望利用 libx264 的低延迟编码机制,一旦单个 NAL 单元可用,就会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 );
这似乎很直接。但是,当我运行以下虚拟代码时,在标记的行上出现段错误。我试图向 x264_nal_encode
本身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? (让我们忽略使用 assert
可能使 cb
不可重入的事实——它只是向读者表明我的工作区缓冲区足够大。)
#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,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
时。如果我切换到不同的预设,在第一个回调发生之前编码更多帧,然后在第一个回调之前成功调用 x264_encoder_encode
,因此会发生段错误。
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