bmp2avi app, how do i calculate frame duration in microseconds .. [solved]

[vhelp]

i'm trying to calculate the duration in microseconds (µs) the same way that virtualdub (information screen) calculates it but am having trouble getting the algo to properly show 33667µs.

29.970 fps = 33667µs
23.976 fps = 41708µs
25.000 fps = 40000µs
..

given dwScale=1001 (for video/interlace) and dwRate=30000 written to the avi header comes out wrong:

1001 * 30.000 = 33.3667
1000 * 29.970 = 33.3667
1001 * 29970 = 0.0334
1000 * 29970 = 0.0334

i know i can just hold the values in a list internally but i need it to calculate in case the framerate is different and wouldn't be in a list.

this is for a raw bmp2avi app i'm working on and want to show this value in the gui when the user sets the parameters up. thank you.

[jagabo]

Why don't you just look at the VirtualDub source? And µs is microseconds (1/1000000), not milliseconds (1/1000). It looks to me like it's simply scale/rate * 1000000. I think "29.970 fps = 33667µs" is a typo on your part:



That was from a file with rate=30000, scale=1001. 1001 / 30000 * 1000000 = 33367 rounded to the nearest microsecond.

[vhelp]

[solved]

thank you for the tip, jagabo. i knew i would have to trial and error until i finally figured it out.

procedure TForm1.cbxFrameRateChange(Sender: TObject);
begin
  case cbxFrameRate.ItemIndex of
    2: sb1.Panels[0].Text := inttostr(round( (100100/24000000)*10000000 ))+'µs'; //film,  41708µs
    5: sb1.Panels[0].Text := inttostr(round( (100101/30000000)*10000000 ))+'µs'; //video, 33367µs
  end;
end;