ETTR Live View and the RGB Histogram

[nma]

The Canon 5dii offers a real time RGB histogram in Live View. I have not seen a discussion of how this histogram is formed, directly from RAW or via some sort of jpeg rendering? On the other hand, I have noticed that exposure based on the RGB histogram in Live View in is more reliable, making my ETTR more controllable. Does anyone have information on how the LV histogram is computed?

[asf]

If you change the picture style does it affect the histogram?

[nma]

If you change the picture style does it affect the histogram?

That is a good question. I have set all my picture style and jpg settings so as to (sort of) optimize the jpg histogram. I will do an experiment and report back.

[nma]

If you change the picture style does it affect the histogram?

Yes, that was a good question. Changing the picture style from neutral to landscape made an immediate change in the histogram. Important information but it does not definitively answer the original question about what process is used to compute the histogram?

Anyone know?

[madmanchan]

The histogram is based on the JPEG rendering.  Any in-camera controls that affect the color or tone of the JPEG rendering (e.g., white balance, contrast, picture style, etc.) will all affect the appearance of the histogram.  The histogram is not based on raw data.  It is not at all accurate for ETTR.  With some work, it is possible to make its RGB histogram a reasonable indicator/predictor of ETTR, but it is still fairly far removed.

[nma]

The histogram is based on the JPEG rendering.  Any in-camera controls that affect the color or tone of the JPEG rendering (e.g., white balance, contrast, picture style, etc.) will all affect the appearance of the histogram.  The histogram is not based on raw data.  It is not at all accurate for ETTR.  With some work, it is possible to make its RGB histogram a reasonable indicator/predictor of ETTR, but it is still fairly far removed.

I appreciate your explanation.

Thanks

[asf]

Yes, that's what I meant by asking the question - the picture style is for the jpeg, if changing the style affects the live view histogram then it is based off the jpeg.

[nma]

Yes, that's what I meant by asking the question - the picture style is for the jpeg, if changing the style affects the live view histogram than it is based off the jpeg.

Sorry ASF,

I did take your point. However, I did not think that it necessarily followed that the jpg image had to reconstructed in real time, in order to compute the histogram. In fact, I am still sort of in disbelief, except that I have faith that Eric Chan really knows his stuff. Consider this for a moment: Make believe you were a designing a live view system for raw shooters: You have the RGB information all separated out directly from the sensor. All that is required is to compute the histogram. You don't need to do actually form the image. You just normalize each histogram before display. Just thinking.

[RFPhotography]

I also find it interesting that the histogram is based on some sort of JPEG in Live View when no image has been created.  I can see how this would be the case after the fact.  And before the madding crowd jumps on my back; YES, I know the LCD image is a JPEG even when shooting RAW and that the histo is based on that JPEG.  But beforehand it would be interesting to know what's going on.  Is the in camera processor effectively processing the Live View image as a JPEG in real time?  It would make sense, since the mirror is up, the shutter optn and the image in the Live View is coming off the sensor.

[torger]

Is the in camera processor effectively processing the Live View image as a JPEG in real time?

I would guess that the Live View histogram thing is based on the video function rather than still image function, but I don't know for sure. Live View came about the same time as video function came. Anyway, the hardware, Canon's Digic 4 in this case, is specialized on making JPEGs from RAW data (and video stream) so it is not a tough task to go via JPEGs / video stream rather than RAW data to make the histogram, since the chip contains special hardware to do this type of processing realtime anyway.

Of course, all serious still life photographers would want RAW histograms instead, but the Japanese camera manufacturers are conservative and still design their cameras mainly for a JPEG workflow, and sure if you shoot JPEG you'd want JPEG histograms.

[asf]

I agree it would be preferable to have the histogram based on the raw data. I don't know that it's based on the jpeg per se, but I do know that since the picture style affects the histogram it cannot be based off the raw. BobFisher is probably right that it's based on the video, as the picture styles affect that too. The video is not raw as far as I know.

The live view and jpeg histograms may differ from each other as well. If you can set up a stable continuous light scene you can test this out and compare the histograms from the live view feed and a shot jpeg.

[madmanchan]

As an analogy: If you consider opening a raw file in your raw conversion software, you see a preview image on your monitor, right?  That preview itself is not really a jpeg, but it does show you essentially how your image would look if you did choose to save it out as a jpeg.

Back to the camera:  The "live view" image that you see on the camera LCD is a preview image that is generated on-the-fly from the raw data.  It's not strictly a jpeg in the sense that they haven't written out a jpeg file to the memory card, but rather they've gone through many of the steps that raw converters normally do when processing a raw file (e.g., white balance, color profiles, tone curve, noise reduction, sharpening, lens corrections, etc.).  So it's a processed result, and the histogram reflects that.  Change the in-camera white balance (e.g., Daylight to Tungsten) and you change the on-screen preview, and the histogram correspondingly.  Same with any other in-camera color & tone setting.