On Mon, 09 Jun 2008 13:57:45 -0400, Konstantin Gorskov
<[EMAIL PROTECTED]> wrote:
> Hi! I can not understand this part of code:
> for(int y=0; y<240; y++)
> //fwrite(pFrame->data[0]+y*pFrameRGB->linesize[0], 1, 320*3, bufptr);
>
> I know, it outputs image file with frame. But, for sample, what
> linesize[0] is there for?
Imagine this: you have an image which is 11 pixels wide and 5 pixels tall,
1 byte per pixel. In order to copy the frame using SSE2 instructions, the
row length must be a multiple of 16. So for this example, libraries like
ffmpeg would allocate 16x5 bytes.
PPPPPPPPPPP.....
PPPPPPPPPPP.....
PPPPPPPPPPP.....
PPPPPPPPPPP.....
PPPPPPPPPPP.....
So:
frame->w == 11
frame->h == 5
frame->linesize[0] == 16
But, when saving to disk, you don't want to waste space, so you get rid of
the padding:
PPPPPPPPPPP
PPPPPPPPPPP
PPPPPPPPPPP
PPPPPPPPPPP
PPPPPPPPPPP
> Can I, for sample, access pixel number 32 in a row?
So, to access pixel (column,row) in a packed image which was saved to
disk, you use
pixel= data[ row * width + column ];
but for image frames with padding, you need
pixel= frame->data[0][ row * frame->linesize[0] + column ];
> And why there is only 240 output steps? Not 240*320?
fwrite copies a block of data, not just one pixel. Also, the *only*
reason fwrite was called more than once was to skip the padding. **If
there was no padding**, the entire frame could be copied with
fwrite(frame->data[0], 1, 320*240*3, file)
> If I understand correct, all pixel data are stored here, at data[0]?
Yes. For RGB, all data is in plane 0. In some formats, however, the
colors are in separate "planes" ("planar RGB", "planar YUV", etc). For
these, plane N is data[N] with a width of linesize[N].
Hope that helps ;-)
-Mike
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