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get_histo_range strangeness

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Offline pigeonhill

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Re: get_histo_range strangeness
« Reply #40 on: 20 / May / 2022, 12:43:49 »
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@c_joerg

As with many things, I use the ML feedback to inform my exposure setting, not control it.

For this reason, the CHDK gamma based histo feedback is more than sufficient to set/control auto bracketing, ie set the exposure for the shadows and bracket until the ETTR point.

I personally, in my script, give the user the option to define the ETTR/saturation point, ie when the top 1 to 5, user selectable, bins are ‘just’ non zero, from there I can adjust by a 1/3 of a stop, for additional ETTR insurance.

My gut feeling is, the RAW feedback won’t change things that much, if at all, but I still welcome us having more ‘accurate’ feedback.

Finally, I think Lua CHDK has the edge over ML for photography. ML is now totally a video user platform, ie the ML blog is ‘just’ video stuff. CHDK on the M3 = a great photographers platform IMHO.

It’s sad we are going to lose the M platform. Fingers crossed those trying to get ML running on the R platform succeed and that the project doesn’t get too hijacked by the video world ;-)
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Offline Caefix

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Re: get_histo_range strangeness
« Reply #41 on: 20 / May / 2022, 12:47:49 »
Probably success depends on scenery, as so often...  :haha
I think, we try here to optimize the last steps of pre-postprocessing finetuning,
so how to perform the best presets?  :-*
Maybe a good view from Scene Luminance...  ???
https://chdk.setepontos.com/index.php?topic=9537.msg97763#msg97763

Edit: For experiments exposed to luascript.c
Code: [Select]
// from shooting_get_luminance()
static int luaCB_get_luminance( lua_State* L ) // http://en.wikipedia.org/wiki/APEX_system
{//    static const double K = 12.5;  // K is the reflected-light meter calibration constant (*100)
    short bv = shooting_get_bv96();
    int b=(int)(1250*pow(2.0,((double)(bv-168)/96.0)));
  lua_pushnumber(L, b);
  return 1;
}
//    FUNC(get_luminance)
« Last Edit: 20 / May / 2022, 14:27:45 by Caefix »
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Offline philmoz

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Re: get_histo_range strangeness
« Reply #42 on: 21 / May / 2022, 02:58:21 »
Thanks for the tips & links. I will take a closer look at the ML code to see what I can find.

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Offline pigeonhill

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Re: get_histo_range strangeness
« Reply #43 on: 21 / May / 2022, 03:09:17 »
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Offline philmoz

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Re: get_histo_range strangeness
« Reply #44 on: 22 / May / 2022, 00:48:34 »
Looking at the ML code the ETTR hint for an over exposed image appears to be a simple value calculated from the % of over exposed pixels (relative to the total pixel count).


     correction = -log2(1 + over_exposed_count * over_exposed_count)


1% -> -1 EV
2% -> -2 EV
5% -> -4 EV
10% -> -6 EV


etc.
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CHDK ports:
  sx30is (1.00c, 1.00h, 1.00l, 1.00n & 1.00p)
  g12 (1.00c, 1.00e, 1.00f & 1.00g)
  sx130is (1.01d & 1.01f)
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  sx40hs (1.00d, 1.00g & 1.00i)
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Offline pigeonhill

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Re: get_histo_range strangeness
« Reply #45 on: 22 / May / 2022, 01:07:19 »
@philmoz

That doesn’t look mathematically/scientifically ‘robust’ to me, it looks more like a heuristic or empirical type of approach.

May still be helpful however ;-)
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Offline c_joerg

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Re: get_histo_range strangeness
« Reply #46 on: 22 / May / 2022, 12:58:51 »
Quote from: pigeonhill on 22 / May / 2022, 01:07:19
That doesn’t look mathematically/scientifically ‘robust’ to me, it looks more like a heuristic or empirical type of approach.

That is exactly my experience with the ML ETTR.

You just need an underexposed picture for an exact calculation.
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Offline pigeonhill

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Re: get_histo_range strangeness
« Reply #47 on: 22 / May / 2022, 13:18:50 »
@c_joerg

I’m finding that I can achieve all my needs, on the M3, by either setting exposure for the shadows, bracketing to the ETTRing point; or setting an ETTR exposure, using the ETTR hint bars, and ISO  bracketing by a single exposure, eg ISO 800, say.

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Offline philmoz

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Re: get_histo_range strangeness
« Reply #48 on: 06 / June / 2022, 05:38:49 »
The Ximr builds have an updated version of the expiremental raw EV histogram.


The histogram bars are now at 1/9 EV instead of 1/3 to be able to calculate an ETTR hint.


The ETTR hint is only calculated for under exposed images. I have tried a few ways of calculating for an over exposed image; but I can't find anything that I think is usable.


The attached image shows what it looks like. The grey lines are spaced at 1 stop intervals - they are clustered closer at the left end because of the decreased resolution at the shadow/black end.

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CHDK ports:
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  g12 (1.00c, 1.00e, 1.00f & 1.00g)
  sx130is (1.01d & 1.01f)
  ixus310hs (1.00a & 1.01a)
  sx40hs (1.00d, 1.00g & 1.00i)
  g1x (1.00e, 1.00f & 1.00g)
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  g7x2 (1.01a, 1.01b, 1.10b)
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Offline Caefix

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Re: get_histo_range strangeness
« Reply #49 on: 06 / June / 2022, 10:16:11 »
Quote from: philmoz on 06 / June / 2022, 05:38:49
I have tried a few ways of calculating for an over exposed image; but I can't find anything that I think is usable.

How to implement the eqiuvalent of an audio-compressor/limiter...  :o
Probably need to analyze sceen first. (far overexposed spot (sun)  :-[ or bright winter plane?)
Maybe need for additional edge-detection?  :-*
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