I am able to read frames from av_open_input_file and decode and reencode it with another codec and save to file.
Now, I want the same thing, except i don't want to decode or reencode. I want to do av_interleaved_write_frame on the just read frame to file. To do this, i need to set AVOutputFormat
AVOutputFormat *fmt;
fmt = av_guess_format(NULL, filename, NULL);
However, i can not supply a filename, because it is a remote stream. So, is there any way to generate AVOutputFormat from the current AVFormatContext which already reads input frames? So i can write the same frame to the output file from any part of it (not necessarily from the beginning) in the correct way.
To summarize: I need to write the just read frame to the file without decoding and reencoding. (I will open that file another time to be able to play that file)
Any ideas?
Related
I've written a simple http server in C and am now trying to implement HTML files.
For this I need send a response, containing the content of the HTML file.
How do I do that best?
Do I read the file line by line, and if so how do I store them in a single string?
Thanks already!
Here is an example of reading a text file by chunks which, if the file is big, would be faster than reading the file line by line.
As #tadman said in his comment, text files aren't generally big so reading them in chunks doesn't make any real difference in speed but web servers serve other files too - like perhaps photos or movies (which are big). So if you are only going to read text files then reading line by line might be simpler (you could use fgets instead of fread) but if you are going to read other kinds of files then reading all of them in chunks means you can do it the same way for all of them.
However, as #chux said in his comment, there is another difference between reading text files and binary files. The difference is that text files are opened in text mode: fopen(filename,"r"); and binary files must be opened in binary mode: fopen(filename,"rb"); A web server could probably open all files in binary mode because web browsers ignore whitespace anyway but other kinds of programs need to know what the line endings will be so it can make a difference.
https://onlinegdb.com/HkM---r2X
#include <stdio.h>
int main()
{
// we will make the buffer 200 bytes in size
// this is big enough for the whole file
// in reality you would probably stat the file
// to find it's size and then malloc the memory
// or you could read the file twice:
// - first time counting the bytes
// - second time reading the bytes
char buffer[200]="", *current=buffer;
// we will read 20 bytes at a time to show that the loop works
// in reality you would pick something approaching the page size
// perhaps 4096? Benchmarking might help choose a good size
int bytes, chunk=20, size=sizeof(buffer)/sizeof(char);
// open the text file in text mode
// if it was a binary file you would need "rb" instead of "r"
FILE *file=fopen("test.html","r");
if(file)
{
// loop through reading the bytes
do {
bytes=fread(current,sizeof(char),chunk,file);
current+=bytes;
} while (bytes==chunk);
// close the file
fclose(file);
// terminate the buffer so that string function will work
*current='\0';
// print the buffer
printf("%s",buffer);
}
return 0;
}
Here's my issue: I have a file with the highscore written in it (just the first line, no nicknames, just the highscore), I need to read that line and compare it with the actual score obtained in the game session, if the score is higher, overwrite the file with the new value, but if I try to read it I get a null value... Seems like i'm not reading it the right way. What's wrong with my code?
Thanks for the help!
local path = system.pathForFile( "data.sav", system.DocumentsDirectory )
local file = io.open( path, "w+" )
highscore_letta = file:read("*n")
print(highscore_letta)
if (_G.player_score > tonumber(highscore_letta)) then
file:write(_G.player_score)
end
io.close( file )
I had this problem myself. I found out that if you open a file in "w+" mode, the current contents are deleted, so that you can write new contents. So to read and write you have to open the file twice. First, you open the file in "rb" mode and get the file contents, then close it. Then you reopen it in "wb" mode, write the new number, and close it.
In Windows, you need "b" in the file mode. Otherwise, the strings that you are reading and writing may be modified in unexpected ways: for instance, a newline ("\n") may be replaced with carriage return–newline ("\r\n").
The file modes that Lua supports are borrowed from the C language. (I found a description on page 305 of what I guess is a draft of the C specification.) I think the Lua manual sort of assumes that you will know what these modes mean, as an experienced C programmer would, but to me it wasn't at all obvious.
Thus to read a number and then write a new one:
local filepath = "path/to/file"
-- Create a file handle that will allow you to read the current contents.
local read_file = io.open(filepath, "rb")
number = read_file:read "*n" -- Read one number. In Lua 5.3, use "n"; the asterisk is not needed.
read_file:close() -- Close the file handle.
local new_number = 0 -- Replace this with the number you actually want to write.
-- Create a file handle that allows you to write new contents to the file,
-- while deleting the current contents.
write_file = io.open(filepath, "wb")
write_file:write(new_number) -- Overwrite the entire contents of the file.
write_file:flush() -- Make sure the new contents are actually saved.
write_file:close() -- Close the file handle.
I created a script to do these operations automatically, as they're somewhat annoying to type every time.
The mode "r+" or "r+b" is supposed to allow you to read and write, but I couldn't get it to work when the original contents are longer than the new contents. If the original contents are "abcd", four bytes, and the new contents are "efg", three bytes, and you write at offset 0 in the file, the file will now have "efgd": the last byte of the original contents is not deleted.
I'm working on a project that needs to open .mp4 file format, read it's frames 1 by 1, decode them and encode them with better type of lossless compression and save them into a file.
Please correct me if i'm wrong with order of doing things, because i'm not 100% sure how this particular thing should be done. From my understanding it should go like this:
1. Open input .mp4 file
2. Find stream info -> find video stream index
3. Copy codec pointer of found video stream index into AVCodecContext type pointer
4. Find decoder -> allocate codec context -> open codec
5. Read frame by frame -> decode the frame -> encode the frame -> save it into a file
So far i encountered couple of problems. For example, if i want to save a frame using av_interleaved_write_frame() function, i can't open input .mp4 file using avformat_open_input() since it's gonna populate filename part of the AVFormatContext structure with input file name and therefore i can't "write" into that file. I've tried different solution using av_guess_format() but when i dump format using dump_format() i get nothing so i can't find stream information about which codec is it using.
So if anyone have any suggestions, i would really appreciate them. Thank you in advance.
See the "detailed description" in the muxing docs. You:
set ctx->oformat using av_guess_format
set ctx->pb using avio_open2
call avformat_new_stream for each stream in the output file. If you're re-encoding, this is by adding each stream of the input file into the output file.
call avformat_write_header
call av_interleaved_write_frame in a loop
call av_write_trailer
close the file (avio_close) and clear up all allocated memory
You can convert a video to a sequence of losses images with:
ffmpeg -i video.mp4 image-%05d.png
and then from a series of images back to a video with:
ffmpeg -i image-%05d.png video.mp4
The functionality is also available via wrappers.
You can see a similar question at: Extracting frames from MP4/FLV?
I am developing an application in vb6.In my application i am trying to copy various files in a single file.The problem is i am trying to read 1000 bytes from the source file and write it to the target file in reverse order.Then another 1000 bytes and so on until i reach the last of the source file.I did similar type of work in java using file pointer.But here i am not finding the solution.Please help.
This tutorial covers how to read and write from binary files, there is a section about reading blocks of data from a file.
You could create a buffer for this purpose. Here is some code to get you started. (I don't have vb6 at this moment so the code is not verified)
Example Code:
Dim Buffer As String * 1000
Open "C:\Windows\FileName.txt" For Binary As #1
Get #1, 1, Data
Close #1
Moreover in your case you will need to keep track of the position in the file
Get #file handle, position, Buffer
Also use Put to write the read buffer to another file.
Put #file handle, position, Buffer
I am able to encode video frames using libavcodec, by calling avcodec_encode_video function. How do I save these encoded frames into an AVI file?
Check this out:
http://forum.doom9.org/archive/index.php/t-112286.html
You must open file for binary write, write header in it, and simultaniosly put binary frames in it, I think.