Hi,
i've only copied the avframe to copy the pointers to the data and
linesize arrays. You also could only copy the two arrays in it. It's the
same.
To get a quick overview about the linsizes see this example.
I have an memory block stored at the memory address 10000. The picture
is 120x60. So i need a block of 120*60 * 4bytes for an argb image.
linesize for this example is 120 * 4bytes.
1st line: 10000 ARGB ARGB ARGB(...)
2nd line: 10000 + 120*4 ARGB ARGB ARGB(...)
3rd line: 10000 + 120*4 * 2 ARGB ARGB ARGB(...)
4th line: 10000 + 120*4 * 3 ARGB ARGB ARGB(...)
The linepitch is added for every line. So you get a pointer on the start
of that line. This pitch can also get negative, then the startpointer
for thsi example is not 10000 anymore. Then its 38800 (10000 + 120*4 *
60). The nextline then is 38800 - 120*4, and so on.
if you now want to set a pixel in the firstline on the 6th position you
have to add 6 to the starting address of that row (10000 + 6) to get the
memory location of this actual pixel.
To get the location of the 6th pixel in the 2nd row you simply add the
linepitch to the 6th pixel of the firstrow. So you get the folowing:
10000+6 + 120*4 and so on. This is the same for your YUV picture!
So to get a shift in the x-direction, you simply need to add some to the
start pointers. so if you want to start at the half of your target image
simply add abs(linesize)/2 to it. Because linesize is the size of a full
row. For swscale this ist then an image with half width, nothing else.
You only have to make sure, that the swscale context will scale the
image to this smaller "virtual" image.
I missed the negative linesizes in my example before, you have to
caclulate the absolute values for this. This could be what lead to the
access violation in your application. You also have to check, how large
this array is. For RGB mostly the first index is set in the array. For
YUV data the first three should be set.
data[0] += abs(linesize[0]) / 2;
data[1] += abs(linesize[1]) / 2;
data[2] += abs(linesize[2]) / 2;
I hope this made it clearer for you, what I ment in this example
Best regards
Hendrik
Am 15.03.2010 05:24, schrieb Ke Yu:
Thanks, Hendrik, for your generous help. I still don't quite
understand it here. What's the reason to copy the target avframe data?
After the data copying, can I still scale the second image onto the
same target avframe? Also, I don't understand the algorithm to add
linesize/2 to the data pointers. After doing that, it seems like I got
a memory access error when calling sws_scale.
Your way should be the ideal one to accomplish what I want. I just
need to get it done right. :)
On 3/14/10, Hendrik Jürgens<[email protected]> wrote:
You simply have to adjust the data pointers of your target image, to scale
the second input on it.
Simply copy the data and linesize arrays.
Then add linesize/2 to the data pointers. With this you get a shift in the x
direction.
i.e.
avframecopy = avframe;
avframecopy.data[0] += avframecopy.linesize[0] / 2;
avframecopy.data[1] += avframecopy.linesize[1] / 2;
avframecopy.data[2] += avframecopy.linesize[2] / 2;
then you can scale your second image with a resolution of 320x480 on this
"new" target.
This should do the trick here.
Please note, that this needs more rethinking, if your target resolution
differs. But with yuv420 and 640x480 you shouldn't have a problem here.
________________________________________
Von: [email protected] [[email protected]] im
Auftrag von Ke Yu [[email protected]]
Gesendet: Freitag, 12. März 2010 21:00
An: Libav* user questions and discussions
Betreff: Re: [libav-user] stitch avframes
I think I figured out what you meant. Here's my understanding (my pixel_fmt
is PIX_FMT_YUV420P):
1). concatenate each corresponding field in avframe->data with data from
both src images (avframes);
2). double the value of each field in avframe->linesize;
3). sws_scale the avframe built with step 1) and 2) into the allocated
avframe with res. 640x480.
I'm trying it now. I'll let you know how it goes.
Best Regards.
On Fri, Mar 12, 2010 at 12:03 PM, Ke Yu<[email protected]> wrote:
Thanks for the great idea. Following your suggestion, I scaled one of the
source images to the allocated dest. frame (640x480). It is fast! However,
I
could only scale the first src image to the left portion of the dest.
frame.
How can I scale the second src image to the right portion of the dest.
frame? sws_scale/sws_getContext only give sizes no location coordinates.
Is there a way with which I can specify the coordinates as well?
Thanks again!
2010/3/12 Hendrik Jürgens<[email protected]>
Am 12.03.2010 01:14, schrieb Stefano Sabatini:
On date Thursday 2010-03-11 14:21:24 -0500, Ke Yu encoded:
Hi!
I've got avframes from two udp multicast video streams. I would like to
scale every avframe into half (only x coordinate, e.g.,
640x480->320x480) and stitch the two scaled avframes into one avframe
with
original size (e.g., 640x480). After that, I'll encode the generated
avframes into one output video stream. My input video streams have the
same
encoding context. The frame rate is 30 fps. So I have about 33ms to
finish
the scaling and stitching. The performance is critical. I plan to use
sws_scale to scale the avframe but don't know how to stitch two scaled
avframes together. I prefer not to use a third party image lib.
Can anyone help me to figure that out?
Try with libavfilter+overlay, integration into main SVN pending.
Regards.
Hi,
You can allocate your AVFrame with the target resolution (640x480) and
then let sws_scale the sourceimages directly onto this frame.
You simply have to modify the data pointers and linepitches of yout
AVFrame before giving them to sws_scale. If you don't need any
transparency
or some other fancy image stuff this should be the fastest way.
Best regards
Hendrik
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