clipping preview in levels query

Hi Paul,

I suspect there are as many ways to approach this as there are people out there using Photoshop, so don’t take this as a proclaimation of absolute truth, but here’s a generalized approach that works for me.

As you stated, start by expanding the histogram to use the entire dynamic range. This can be done by dragging the triangles in the RGB channel, but I get better results by dragging the triangles in each of the separate R, G, and B channels. Then when you look at the RGB channel, it too will have been expanded. An easier way to accomplish this is to simply use Auto Levels. In the levels dialog box, click Options, and select Enhance per Channed Contrast. That does exactly the adjustment you’re looking for. You can make an action button with all of this, and then when you open a file, a single click does it all. Now you’re usually in the ballpark.

Next, do the clipping preview thing your post is about. I use this to identify where in the image the blackest point(s) and whitest point(s) are. Then if I had to slide the triangle to find them, I return them to the ends. I then select the blackpoint eyedropper and go to the point on the image where the blackest point was. I move the cursor around a bit, watching the color numbers in the info palette, and when I find the darkest point, I click. This sets that as the black point. I have set up the black point default as 10,10,10. This ensures that all of my shadows don’t have a bluish, or greenish, or whateverish cast. Then I select the whitepoint eyedropper and do a similar thing for the whitest point. I’ve set my white to 245,245,245. This keeps me from ending up with, for example, blueish or yellowish snow or clouds. Then I OK the dialog box to save this as a history point, and open it right back up. Then I click on the greypoint eyedropper, and start hunting for a piece of grey to set the midtone color balance.

In general, I find that virtually all of my files have shadow where I can set a black point, even if it’s tiny, but not all have a white point that stands out. If there isn’t one, I just skip that step. Others may approach it differently. Likewise, I don’t always find a good grey point.

Next, depending on how successful I was with the greypoint eyedropper, I might adjust the midpoint triangles in each of the color channels, to get the correct color balance. I may also adjust the midpoint triangle in the RGB channel to increase overall brightness (or is it contrast?). Finally, if there is too much shadow or highlight, I might adjust the endpoint triangles in the RGB channel Output Levels box (bottom), but never too much. I do this more to open up shadow detail than to kill highlights.

Don’t know if this approach will work for you, as we all do things differently, but at least you now have an idea of how at least one person uses the clipping preview.

Mike

Paul Burdett wrote:

Hello,
I’ve been reading some info/tutorials on the "clipping preview" within the levels dialogue box…that is holding down the alt key when clicking the black/white points (triangles) on the histogram sliders. Now I know that a good levels adjustment can be achieved by dragging the black/white points to the points on the histogram where the curve starts, but I don’t fully understand what the clipping preview shows…only that you should (according to the tutorials) hold the alt key down and drag the sliders until the image just begins to show. Could someone explain in laymans terms what is happening here, or give a step by step explanation, or point to a tutorial that explains clearly (I have looked/searched google, but find the explanations too technical).
Many thanks in anticipation,
Paul

"Paul Burdett" wrote:

on the "clipping preview" within the levels dialogue box

Hi Paul, you get a color-coded view on how/where the clipping happens and on which of the channels it happens. As long as there is only colors visible you are only clipping the colors (gamut), once you get white or black then the tonal reproduction range (grayrange) will be clipped. Usually/often the colors can be clipped but never the grayrange

Note that blackpoint adjustment does not result good/correct result when the operation is done in a non-linear space such as the sadRGB or the AdobeRGB because it will affect rather strongly to the image gamma all over the tonal reproduction range making the image too dark and causing hue shifts. The only correct way to adjust the blackpoint is to do the operation in a linear RGB working-space.

Timo Autiokari http://www.aim-dtp.net

Thank you for your reply, but I’m still confused. What do you mean by clipping? If I hold the alt key and drag the left slider until I see the beginning of the image do I leave the slider there? if so, why? Cheers,
Paul

"Timo Autiokari" wrote in message

"Paul Burdett" wrote:

on the "clipping preview" within the levels dialogue box

Hi Paul, you get a color-coded view on how/where the clipping happens and on which of the channels it happens. As long as there is only colors visible you are only clipping the colors (gamut), once you get white or black then the tonal reproduction range (grayrange) will be clipped. Usually/often the colors can be clipped but never the grayrange

Note that blackpoint adjustment does not result good/correct result when the operation is done in a non-linear space such as the sadRGB or the AdobeRGB because it will affect rather strongly to the image gamma all over the tonal reproduction range making the image too dark and causing hue shifts. The only correct way to adjust the blackpoint is to do the operation in a linear RGB working-space.

Timo Autiokari http://www.aim-dtp.net

"Mike G." wrote in message

Hi Paul,

I suspect there are as many ways to approach this as there are people out there using Photoshop, so don’t take this as a proclaimation of absolute truth, but here’s a generalized approach that works for me.
As you stated, start by expanding the histogram to use the entire dynamic range. This can be done by dragging the triangles in the RGB channel, but I get better results by dragging the triangles in each of the separate R, G, and B channels. Then when you look at the RGB channel, it too will have been expanded. An easier way to accomplish this is to simply use Auto Levels. In the levels dialog box, click Options, and select Enhance per Channed Contrast. That does exactly the adjustment you’re looking for. You can make an action button with all of this, and then when you open a file, a single click does it all. Now you’re usually in the ballpark.

Next, do the clipping preview thing your post is about. I use this to identify where in the image the blackest point(s) and whitest point(s) are. Then if I had to slide the triangle to find them, I return them to the ends. I then select the blackpoint eyedropper and go to the point on the image where the blackest point was. I move the cursor around a bit, watching the color numbers in the info palette, and when I find the darkest point, I click. This sets that as the black point. I have set up the black point default as 10,10,10. This ensures that all of my shadows don’t have a bluish, or greenish, or whateverish cast. Then I select the whitepoint eyedropper and do a similar thing for the whitest point. I’ve set my white to 245,245,245. This keeps me from ending up with, for example, blueish or yellowish snow or clouds. Then I OK the dialog box to save this as a history point, and open it right back up. Then I click on the greypoint eyedropper, and start hunting for a piece of grey to set the midtone color balance.

In general, I find that virtually all of my files have shadow where I can set a black point, even if it’s tiny, but not all have a white point that stands out. If there isn’t one, I just skip that step. Others may approach it differently. Likewise, I don’t always find a good grey point.
Next, depending on how successful I was with the greypoint eyedropper, I might adjust the midpoint triangles in each of the color channels, to get the correct color balance. I may also adjust the midpoint triangle in the RGB channel to increase overall brightness (or is it contrast?). Finally, if there is too much shadow or highlight, I might adjust the endpoint triangles in the RGB channel Output Levels box (bottom), but never too much. I do this more to open up shadow detail than to kill highlights.

Don’t know if this approach will work for you, as we all do things differently, but at least you now have an idea of how at least one person uses the clipping preview.

Mike

Mike: Thank you so much for your detailed explanation…it has helped a

lot.
Cheers,
Paul

"Paul Burdett" wrote:

What do you mean by clipping?

The input sliders will:

stretch (scale) the tonal reproduction range that the image has in case there is some code range that does not hold image detail.

More strong adjustment will clip the image data.

If I hold the alt key and drag the left slider until I see the beginning of the image do I leave the slider there? if so, why?

How to scale or clip depends on the image. A safe way is to only scale, so that you are in verge of "seeing the image" in the threshold mode. But it may be that the image does not look the best it could after that.

Generally, the whitepoint (right input) slider can be adjusted so that specular reflections and/or direct light rays from lamps are clipped. And generally the blackpoint (left input) slider can be adjusted upwards until you see black in the threshold mode of the Levels dialog, this way (generally) no major image detail is lost, only the gamut is clipped.

Timo Autiokari http://www.aim-dtp.net

Thank you for your reply. I now undrstand "clipping" a little more. Cheers,
Paul

"Timo Autiokari" wrote in message

"Paul Burdett" wrote:

What do you mean by clipping?

The input sliders will:

stretch (scale) the tonal reproduction range that the image has in case there is some code range that does not hold image detail.
More strong adjustment will clip the image data.

If I hold the alt key and drag the left slider until I see the beginning of the image do I leave the slider there? if so, why?

How to scale or clip depends on the image. A safe way is to only scale, so that you are in verge of "seeing the image" in the threshold mode. But it may be that the image does not look the best it could after that.

Generally, the whitepoint (right input) slider can be adjusted so that specular reflections and/or direct light rays from lamps are clipped. And generally the blackpoint (left input) slider can be adjusted upwards until you see black in the threshold mode of the Levels dialog, this way (generally) no major image detail is lost, only the gamut is clipped.

Timo Autiokari http://www.aim-dtp.net