Clipping Confusion


Today’s Question: I read the following in one of your recent answers, and am trying to get my mind around it:

<<The white slider lightens or darkens the lightest tones. This allows you to clip the whites.>>

What does it mean to ‘clip the whites’ and why would you do it? What function would that serve?

Tim’s Quick Answer: To “clip the whites” means to increase the brightness of the brightest pixels in an image to the point that detail is lost. In most cases you might want to brighten right to the point before clipping occurs, although in some specific cases you may want to create a small degree of clipping. The same basic concepts would apply in “reverse” when it comes to the darkest values in the image, where you could potentially choose to clip shadow detail.

More Detail: The “true” feature of the Whites slider is to set the white point. In other words, how bright should the brightest pixel in the image be? In theory, the brightest pixel in an image should be white, and the darkest pixel should be black. Obviously there are exceptions to this, but naturally this “rule” holds true much of the time.

So, in a typical workflow for a typical photo you might increase the value for Whites until the brightest pixel reaches true white (or thereabouts), and then reduce the value for the Blacks slider until the darkest pixel reaches true black (or thereabouts).

The challenge is to accurately find that position for each slider, and then to fine-tune as needed. This is where the “clipping preview” display comes in. Which brings us to your actual question here.

Clipping refers to a loss of detail when the brights get too bright or the darks get too dark. A simple example would be a gradient that goes from pure black to pure white, with a smooth gradation in between. Let’s assume initially that we are dealing with a simple linear gradient, where the bottom row is black and the top row is white. As you move up a row, the pixels get slightly brighter. As you move down a row the pixels get slightly darker. So we have a nice smooth gradation with detail at every step along the way.

If you were then to increase the value for the Whites slider for such an image, the brightest values would get brightened. Of course, that top row of white can’t be made any brighter, so it remains as it is. But the row below becomes pure white. That’s our first level of clipping, because there is no longer a difference between the top row and the second row. At this point that clipping is probably not especially problematic. But if you keep increasing the Whites value, more and more rows of bright gray pixels will become white, and there will be a larger area with no variation in tonal values. Eventually you’d have a big section of the top of the image that is pure white, with no detail at all.

The same basic concept could apply to darkening the shadows, causing a large area of pure black (with no detail) at the bottom of our gradient image.

Applying that concept to a photographic image, obviously we’re dealing with areas with more random shapes than with a gradient. But the same issues are at play. If, for example, we increase the value for Whites too much with a photo that includes a cloudy sky, those clouds could lose detail by virtue of being “clipped” to pure white.

So, again, the clipping refers to the loss of detail in an area that has gone to pure white or pure black. To a lesser extent it is also possible to have clipping on an individual color channel (red, green, or blue), so that detail represented by those channels is lose. For example, with a simple photo of a red rose, you could have no clipping for the white point, but still have clipping for the red channel. That could result in lost detail for the rose itself.