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Multiple flash shots while taking a long exposure photo

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Offline SSaHnAdDrWis

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Multiple flash shots while taking a long exposure photo
« on: 23 / October / 2013, 12:23:49 »
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I wonder if it's even possible to make this. I have an idea I'd like to make, but it involves flashing the flash multiple times while shutter is open for ~5secs. I'm no programmer, no idea how to even start, so maybe there are good people who could write this? I'm sure other people would find this interesting too. :)
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Offline jmac698

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Re: Multiple flash shots while taking a long exposure photo
« Reply #1 on: 26 / December / 2013, 21:20:44 »
I second that!  It would be very useful.  If you could make a strobe effect, you could catch waterdrops in multiple positions in one frame, and even cut them out to make a slow motion video (faster than 240fps like some cameras do, and higher quality).

Another way to do it is to open the shutter multiple times without saving the file aka double exposure.
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Offline waterwingz

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Re: Multiple flash shots while taking a long exposure photo
« Reply #2 on: 28 / December / 2013, 14:43:40 »
Quote from: jmac698 on 26 / December / 2013, 21:20:44
I second that!  It would be very useful.  If you could make a strobe effect, you could catch waterdrops in multiple positions in one frame, and even cut them out to make a slow motion video (faster than 240fps like some cameras do, and higher quality).
AFIAK,  there is no way to actually control flash firing with CHDK.  There have no doubt been some experiments but nothing has been released.  Exploring the Canon firmware to find ways to do that would be a big job for somebody who really knows what they are doing.

Quote from: jmac698 on 26 / December / 2013, 21:20:44
Another way to do it is to open the shutter multiple times without saving the file aka double exposure.
Opening and closing the shutter multiple times over the same exposure is actually pretty easy.  Most of the code necessary to simply script something like that can be found here : http://chdk.wikia.com/wiki/Meteor_Intervalometer_with_Dark_Frame_Management in the functions openshutter() & closeshutter().
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Offline reyalp

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Re: Multiple flash shots while taking a long exposure photo
« Reply #3 on: 28 / December / 2013, 15:23:06 »
Quote from: waterwingz on 28 / December / 2013, 14:43:40
AFIAK,  there is no way to actually control flash firing with CHDK.  There have no doubt been some experiments but nothing has been released.  Exploring the Canon firmware to find ways to do that would be a big job for somebody who really knows what they are doing.
If you are feeling adventurous, there's usually some MMIOs you can write that trigger the flash to fire, usually somewhere close the the LED addresses.

See also http://chdk.setepontos.com/index.php?topic=5366.0

Disclaimer: writing random MMIOs and playing with the flash hardware scores pretty high on the void your warranty scale ;)
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Offline waterwingz

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Re: Multiple flash shots while taking a long exposure photo
« Reply #4 on: 28 / December / 2013, 15:39:36 »
Quote from: reyalp on 28 / December / 2013, 15:23:06
If you are feeling adventurous, there's usually some MMIOs you can write that trigger the flash to fire, usually somewhere close the the LED addresses.  See also http://chdk.setepontos.com/index.php?topic=5366.0  Disclaimer: writing random MMIOs and playing with the flash hardware scores pretty high on the void your warranty scale ;)
The tricky bit is probably actually controlling the strobe flash duration.  Somehow you need to let it fire and then cut-off the flash (so that you don't overexpose or don't totally drain the flash capacitor).  I'm not sure the cheap P&S cams actually control flash duration - the low/med/high setting might just be a hardware trick that limits the current to the flash unit rather than limit the flash duration?
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Offline srsa_4c

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Re: Multiple flash shots while taking a long exposure photo
« Reply #5 on: 28 / December / 2013, 15:49:59 »
Quote from: waterwingz on 28 / December / 2013, 15:39:36
The tricky bit is probably actually controlling the strobe flash duration.  Somehow you need to let it fire and then cut-off the flash (so that you don't overexpose or don't totally drain the flash capacitor).  I'm not sure the cheap P&S cams actually control flash duration - the low/med/high setting might just be a hardware trick that limits the current to the flash unit rather than limit the flash duration?
Nope. The so-called HP timers are involved which poke GPIOs. I think these may have usec accuracy. I don't know more details, but I remember seeing flash-related stuff when searching for HP timer functions.
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Offline waterwingz

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Re: Multiple flash shots while taking a long exposure photo
« Reply #6 on: 28 / December / 2013, 15:53:36 »
Quote from: srsa_4c on 28 / December / 2013, 15:49:59
The so-called HP timers are involved which poke GPIOs. I think these may have usec accuracy. I don't know more details, but I remember seeing flash-related stuff when searching for HP timer functions.
So we just have to hope that whatever switches the current on to the flash tube has the necessary power to also cut the current and switch it off.
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Offline srsa_4c

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Re: Multiple flash shots while taking a long exposure photo
« Reply #7 on: 28 / December / 2013, 16:16:24 »

As you see, the flash circuit is not intelligent (this is an old low-end A430), everything is controlled by the GPIOs (and the ARM core). Poking these with 10msec resolution is hopeless, so one either finds firmware functions or does it with those high resolution timers. In addition, there may even be an interrupt involved (for STRB_DONE).
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Offline reyalp

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Re: Multiple flash shots while taking a long exposure photo
« Reply #8 on: 28 / December / 2013, 16:24:29 »
FWIW, "timer" is a dryos object.

The following strings (from elph130) appear to be related to the drysh timer command

ID   STATE     ENTRY(ARGUMENT)

REMAINDER(sec:nsec)
EXPIRE(sec:nsec)
CYCLE(sec:nsec)
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Offline srsa_4c

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Re: Multiple flash shots while taking a long exposure photo
« Reply #9 on: 28 / December / 2013, 16:33:02 »
Quote from: reyalp on 28 / December / 2013, 16:24:29
FWIW, "timer" is a dryos object.
Yes. DryOS on the a470 has 0 of those (as "maximum"), I have no idea how they are related to the successors of the various timer and hp timer functions found in VxWorks cameras. These functions are located by the sigfinders, but are not documented yet.
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