The Hot Spot

by Steve Simon

When making adjustments in Aperture, like most image-editing software, there is usually more than one way to arrive at the desired result.

To make sure your images are within the tolerances of the final print, it's good to check yours histogram to see that information is not being "clipped".

1.Original Pie.jpg
Pie eating without utensils. Copyright Steve Simon

If a histogram represents all the pixels in your digital capture, you can see when the shadows or highlights get pushed to the edge of the graph. This tells you that some areas of the scene in shadow on the left side, or highlights on the right, won't be reproduced with any detail.

The wonder of raw is that you can often bring back this lost information, particularly in the highlights that would otherwise reproduce as pure white; but it helps to know where these extreme highlights or hot spots are in your image.

In Aperture, the shortcut for "Highlight Hot Areas" is Option>Shift>H. Once activated, you will see a red mask over areas that are off the map so to speak, clipped on the histogram and won't reproduce with any detail.

2.Highlight Hot Areas.jpg
Image masked with red indicating "Hot Spots" where there is highlight clipping.

With minor clipping, the Highlights Slider alone can bring back detail to the hot areas, and you will see the red masked areas disappear as you move the slider to the right.

For more severe clipping, a combination of reducing the exposure with the exposure slider and highlight recovery slider can do magic. You may have to play with brightness and contrast to get the image looking like you want, but in the end, like the superhero you are, you can save the highlights and save the day.

3.After adjustments.jpg
After Adjustments Pie Boy. Copyright Steve Simon

4.No clipping Histogram.jpg
There is no such thing as an ideal histogram since every image is different and high Key (mostly light toned) and Low Key (mostly dark toned) images will have histograms weighted to one side, but if you can avoid clipping to retain detail, it's often a good thing. This histogram shows no clipping.


2007-05-24 05:57:43
What do you recommend as a good Hot Area Display Threshold (available in preferences)? I ask for two reasons. First, I mucked with this before realizing the default levels aren't recorded anywhere and two, if it is adjustable, doesn't that mean there could be reasons to choose a different level?


2007-05-24 11:23:05
One thing that irritates me the most about Aperture is, I cannot set the threshold to just show me where there is actual clipping. If I set it to 99%, then pixels getting very close to, but not quite clipped, are highlighted. If I set it to 100%, then it's just like turning off this feature altogether. For this reason, I have to make too many unnecessary trips to PhotoShop, just to fine tune my high key pictures, because unlike Aperture, the "Show Clipping" function actually shows clipping. Any suggestions?


Ellen Anon
2007-05-25 07:08:19
Al, I think one reason that it seems like at 99% Aperture's Hot Spot shows you pixels that are close to being clipped but not actually clipped is because the Hot Spot appears if even a single channel is clipped. There are a lot of colors that use a value or 255 in one or more channels, and any pixel that is one of those colors will show up as a Hot Spot. It appears that the Hot Spot is based on the RGB histogram rather than a luminosity histogram, which would be more accurate to tell you when an area will be pure white or black without detail.
2007-05-25 13:43:33
Thanks, Ellen, for your reply!

In front of me is a closeup of a pink tulip. The 99% "hot" zone starts at around R:222, G:148, B:173, L:185. Given 99% of 255 equals 252.45, clearly the "hot" algorithm is a little too eager. But that's not even what I want. I want to be able to set the Hot Area Threshold to 100% and have only the pixels that have a 255 value in R, G, B or L highlighted, so that I know what is actually clipped, not close to clipping.

Is that too much to ask (the Apple engineers)?


Ellen Anon
2007-05-25 16:41:31
Al, I just checked with a purple calla lilly (I wanted something that would clip in a channel other than red so I could see the hot spot.) I pulled the Levels white point until the first clipping appeared in the blue channel and small areas of red show up on the preview. I then used the loupe tool to check the values of the pixels that are highlighted red versus those nearby that are not highlighted. The red ones have a blue channel value of 252 or higher - which is precisely as you caalculate. When the blue channel is below that, the area is not highlighted.

So I'm not sure why your hot zone appears to be so much larger - unless it's just hard to tell since it's a pink tulip and the overlay is red (another spearate issue.)

But all that aside, I agree, I'd like to have a setting that only shows when something is 255, but in practical terms 99% works well. I suppose some might want to consider something clipped at lower values if their output media is less sensitive to highlight information.

Steve Simon
2007-05-26 14:18:22
Hi Al and Ellen, was offline for a few days and came back to read your interesting comments on this feature. I didn't realize that the hot zone reads 99 per cent and not 100 per cent of when clipping starts. Frankly, the 99 per cent reading is well within my needs for printing but it's important to kI'm glad you pointed out the reality of this function.
2007-05-28 04:36:42
According to my MBP monitor (calibrated with a Huey) makes it almost impossible to see differences of roughly less than 5% (ie, 12 on the 0-255 scale) in the highlights. Therefore I might as well set things to 95%.
BTW, where do you set that 99 vs 100%?
2007-05-28 05:13:12
Oups, I have found the threshold setting myself.
2007-10-28 14:28:21
this is sick but cool

2007-10-31 08:24:05
hi im a noob im goin to be sick all over you
2007-11-07 13:30:16
that pic is gross
2007-11-28 14:55:36
i think the pic is disgusting
Billy Bob
2007-11-30 18:41:13
This is so gross it makes me rather be dead.