By the Numbers

By the numbers is a phrase associated by many with Dan Margulis’s [] method of relying on numerical means, when possible, for determining the color correction adjustments made on an image. It means correcting an image based on numeric readouts instead of the visual appearance of the image. Many of the features of Curvemeister are intended to make the application of Dan’s techniques easier and faster.

In general, correction by the numbers means that first, shadow, highlight, and neutral values are set using data from the image, or visual cues based on that data. Then neutrals are set, again relying on numeric values, rather than the appearance on screen of the image.

That said, Curvemeister, in an attempt to please everyone, includes several features that do not contribute to its by the numbers roots, including the options to use a master channel, and the ability to display a histogram in the curve interface.

 Black Threshold

The black threshold is a global cutoff value that defines pure black colors. Adjust it using the curve interface’s threshold control. []

Related articles / links

[[Highlight and Shadow Thresholding ]] ;
[[Thresholding Using Highlight and Shadow Points ]]


In Photoshop and Curvemeister, a channel is a block of image data, in the form of 8 or 16 bit integers, that contains an image color or brightness value. All channels taken together describe the appearance of the image.

The meaning of the data values in a channel depends on the color space. For example, an RGB image is composed of three channels: red, green, and blue, and possibly a fourth channel containing transparency information. Other color spaces, such as Lab and CMYK

Each channel has its own curve in the curve window . In Curvemeister, a channel may be easily viewed as if it were a grayscale image, using the tabs along the lower edge of the image preview window , or by typing ctrl-1 to view channel 1, ctrl-2 to view channel 2, etc.

In this document, channel information for a single brightness value – normally corresponding to a single pixel – is represented as follows, for each color space:

 Color Space:

Example: Neutral
Meaning: Pixel or group of pixels with no distinct hue.  Each Color Space defines a neutral differently:


Red, green, and blue are equal to 128. This is normally a medium bright shade of gray.


Lab(50, 0, 0)
Lightness is 50 percent, and a and b are zero. This is a 50 percent gray.


Cyan, magenta, and yellow are equal to 50 percent, and there is no K (black) component.


HSB(0, 0, 50) Hue is red, saturation is zero (pure gray), and brightness is 50 percent. This is the same color as RGB(128,128,128)

Color Model, Color Space

Color Model is a widely used term for the numeric model used to represent the brightness and color of each point, or pixel, in an image.

Color Space, as used in with Photoshop, is a slightly more refined term, and means a color model together with a profile that characterizes the exact appearance of colors within the color model. Because Photoshop tends to hide this distinction, the terms Color Model and Color Space are used interchangeably in the Curvemeister manual. For some people, particularly for those whose work involves specialized use of profiles, the difference is important.

For example, the RGB color space, commonly used for computer monitors, represents each brightness value as a combination of red, green, and blue brightness values. Lab represents brightness and color information independently. HSB describes colors in yet another way.

Each color space can have strategic advantages that depend on the particular image you are working on.

In Curvemeister, you may quickly choose which color space you want using the color space radio buttons.

Color spaces are discussed in more detail in The Color Space Matrix.

Color Cast

A systematic undesirable color. A color cast degrades image quality by reducing the saturation of the image’s colors and giving it an overall drab look. Next to setting a good shadow and highlight value, removing the color cast is the most important way to improve your image’s appearance.
Most photographs begin life with a color cast, however slight it may be, and therefore you have an opportunity to improve an image simply by removing the cast. The easiest way to spot a color cast is to move the cursor over a known neutral object, and read the RGB value. If the values are different you have a color cast, and the image will probably be improved by adding a neutral point at or near that location.

Color Pin

An icon in the Pin Palette, normally displayed as a round pin, or the predefined color value defined for that pin. See the discussion of Color Pinning in the manual.

Color Pinning

The process of using a desired color value, called a Color Pin, together with an existing image color value, to create curve control points. This is normally accomplished by dragging a pin from the Pin Palette to an appropriately colored object in the Image Window. For example, a flesh colored pin may be dragged to a person’s face to correct the flesh tone.

Shadow, highlight, and neutral points may also be defined by dragging pins.

See also the discussion of Color Pinning in the manual.

Color Worms

Curvemeister’s Color Worms describe the dynamically moving broad line (worm!) on the curve preview.  Color Worms are composed of color appropriate segments that inform the user while mousing over the image, both on the composite curve (where they also show the interaction of channel values) and in the separate channels. See also: Color Worms tutorial linked in the tutorial section.

Constrained Sample

This an older term for Pinned Sample.

Contrast Pinning

This process allows you to create two control points at once on any color channel, by pinning and selecting the two ends of a color worm.  The selected points may then be rotated or moved together for quick and easy manipulation of the local tonal or color contrast.

Control Point

These are the draggable “handles” that determine the shape of each curve. Each curve is guaranteed to have at least two control points, one at each end. You may add points to the middle of the curve by clicking on an empty area of the curve, and you may change the shape of the curve by dragging the control points. A control point is deleted by dragging it off the curve.


CurveAlert causes such any “illegal” curve moves to highlight the curve in red. “illegal” curves usually have a downward slope; any point in a curve with a downward slope results in an undesirable inverted effect in an image.


CurveGuard is a Curvemeister option that prevents you from dragging a control point to create a region anywhere in the curve with a negative slope. Due to the elastic nature of the curve, the negative slope may occur some distance away from the point you are dragging.

Eyedropper Sample Point

An area in the image window for which a measuring location has been defined, usually by alt-clicking on the Image Window. Eyedropper sample points are used for monitoring the image values at a particular point in the image, or for constraining image values by adding one or more control points to the curves. An eyedropper sample may be converted to a pinned sample using the menu in the sample palette.. Constraint points reside in the image window, and are similar to Photoshop’s eyedropper sample points.


GCR is an offset printing trade term that stands for Gray Component Removal, and refers to the amount of Magent, Cyan, and Yellow that will be converted into the K channel when converting to the Wide Gamut CMYK color space. GCR thus controls how heavy the (K) black channel will be. If the original or native image is Lab or HSB, an RGB conversion is performed first, and GCR is calculated from the resulting RGB values.

In Curvemeister the GCR value indicates the maximum percentage value that may appear in the K channel when the image is converted to wgCMYK. GCR may be varied from 0% to 100%, in 25% increments. There is also an option to use the black generation curve that Photoshop uses for its normal CMYK conversion, as specified in Photoshop’s Color Settings dialog.

A value of 0% effectively eliminates the K channel from the calculation, with the result that wgCMYK is operationally equivalent to RGB. A value of 100% maximizes the value of the K channel, and therefore guarantees that at least one of the CMY channels is zero. A lighter GCR value is generally more useful for correcting an image, since it provides more control of shadow values, makes it easier to eliminate color casts, and provides more overall control of color. A heavier GCR value makes it easier to make large changes to the CMY color channels without causing an overall color cast.

Highlight and Shadow

Highlight and shadow are the darkest and lightest parts of the image that should contain important detail, but contain no color information. For example, in a portrait white clothing usually contains the highlight , and a deep shadow under the eaves of a house would be a good example of a shadow.  The key to a highlight or shadow is the presence of important detail. A bright glinting reflection from a metal object is not a suitable as a highlight point, because although it may appear white, it does not contain any detail. Similarly, a pure black unlit window may not be a valid shadow point, again because it contains no detail. Most images have both a highlight and shadow, but not all. For best results, it is important for you to distinguish these two situations, and several of the tutorials are devoted to this subject.

Related articles / links

[[Highlight and Shadow Thresholding ]] ;
[[Thresholding Using Highlight and Shadow Points ]]

High Key

A photographic term describing an image whose average brightness is significantly greater than 50% gray. For example, the proverbial polar bear in a snowstorm, eating an ice cream cone is a well-known example of a high key image.  The darkest shadow in a high key image is middle gray.  Often measured as a gray card or 18% grey.


Histogram is a general term for a graph or diagram showing a graphic tally of pixel values in an image. It is often used to assess the overall spread of data in an image. For example, an underexposed image will show a histogram whose data is, generally speaking, crowded on the dark side of the graph. Histograms can be misleading, however, and it is always better to look at the image itself, rather than relying on the histogram.  Another way of thinking about histograms is “how much; not where.”

In Curvemeister the histogram is displayed inside of the curve area, sharing the horizontal axis with the graph portion of the curve interface, and showing the relative number of pixels at each value. Because most scenes consist of three dimensional illuminated objects, interpreting a histogram is usually less reliable than looking at, and measuring, the image itself. As an example of how a histogram can be deceiving, consider two images, one of a a red disk and the other a red sphere, both against black backgrounds. A front lit red disk against a black background will show a peak in the red histogram, centered around the red value at the center of the disk. The sphere, if lit from the front so that the edges fall off, will show an inverted shape with two peaks.

On balance, histograms are like vice grips. Many people use them and like them. They just might not be the best tool for the job.  The moral is to always be careful of the relationship of the histogram to the image, and rely more on the image itself than the histogram, particular for selecting the highlight and shadow points.

Hue Clock

A feature of Curvemeister used to show the exact color sample selected on an Image.  To bring a Hue clock onto your image you use the “Alt-Click” command / mouse combination.  The hue clock is configurable, you need only click on the small icon in the upper right hand corner of the hue clock to open the configuration menu.  You can select sample size, display type and color space displayed.  this powerful tool is most often used in “by the numbers” color correction within the Curvemeister program.

A stand alone version of the hue clock is also included with the Curvemeister program and can be found on the start menu under the Curvemeister group.

Low Key

Describes an image whose average brightness is significantly less than 50%. For example, a black cat sitting on the hood of a black car. Such an image often has no meaningful highlight point. The brightest highlight in a low key image is middle gray.  Often measured as a gray card or 18% grey.

Master Curve

An optional curve that, as its name implies, controls image data data from all the channels. For example, the master curve of RGB specifies a multiplier for each of the R, G, and B channels. The display of the Master Curve is controlled using the View>Curves>Master Curve menu item.

The master curve is available for RGB and wgCMYK modes, and functions as a global data modifier that affects data from all the channels equally. It provides the quickest way to alter the overall brightness while in RGB or wgCMYK, particularly if the changes are reasonably subtle. The master curve does present the risk of causing a color shifts in your image as a result of overuse. For this reason, Curves provides you with the option of working without the master curve. Yet the master curve remains the quickest way to alter the overall brightness of the image, particularly if the changes are reasonable.

The L channel of Lab, and the B channel of HSB function as a master channel, but without the problem of a potential color cast.

Mid tone

A brightness value near 50% gray, or a portion of the image with such a brightness value. There are five general terms used to describe image brightness values: shadow, quartertone, midtone, three-quartertone, and highlight. The midtone corresponds to the middle value of a well-exposed, average image. Foliage, dark blue sky, and the well-know 18% gray cards are normally represented as midtones.

Native Image or Color Space

This term denotes the original Photoshop image or the color space of that image. For example if your image is a CMYK image, the native color space is CMYK. The opposite of native is working.

Neutral Point

A type of Constraint Point that is intended to force that point on the image to be neutral. This is done by adding curve points to the curves for the current working space.
Note: in Lab mode only one neutral point is used.


Used as a noun, this refers to a Color Pin, which is a shadow, highlight, neutral, or a pin from the Pin Palette.


In Curvemeister, Pinning is short for Color Pinning.

Pinning also describes the operation of automatically adding one or more control points to a curve, or adding control point to more than one curve. Pinning is used to automatically lock down its current position, or to modify the color curves in response to a color pinning operation. Examples are locking down the upper half of a curve, making the color value neutral, matching a color that you select using the color picker, or dragging a pin from the pin palette to the image..

Pinned Sample

A Pinned Sample is the end result of dragging a pin onto the image. The pin itself is not dropped onto the image, rather it provides the instructions for creating a pinned sample, which is an eyedropper sample point that has been set to create a particular color value at that point in the image by automatically creating curve points.

Highlight, Shadow, and Neutral points are examples of pinned samples, but this term is most often applied to a sample created using a Color Pinning operation. You may also use the commands in the image window right-click menu to pin a particular image color to match any selected color, or Photoshop’s foreground or background colors.

Because constraint points are retained when you switch color spaces, they are a good way to leverage your work across color spaces. For example, you may compare the effect of setting a particular neutral point in RGB mode versus Lab mode, and choose the more pleasant result before clicking the OK button.

See the discussion of Pinned Samples in the manual.


A brightness value, or region of an image corresponding to a medium light gray. This value is normally used in printing to denote CMYK(0,0,0,25), but it is also close to 25% gray, or Lab(25,0,0). See also midtone.


A brightness value , or region of an image, that is reasonably close to CMYK(0,0,0,75) 75% gray, or Lab(75,0,0). See also midtone.

White Threshold

[[Highlight and Shadow Thresholding ]] ;
[[Thresholding Using Highlight and Shadow Points ]]

The white threshold is a cutoff value that defines pure white colors. Adjust it using the curve interface’s threshold control.

Working Image and Color Space

Working Image denotes the current Curvemeister preview image or the color space of that image, and reflects the setting of Curvemeister’s color space radio buttons. For example if your image is originally a CMYK image, and you have selected RGB as the current color space, the native color space is CMYK, and the working color space is RGB. The opposite of working is native.

Zoom (in or out)

Zoom means changing the displayed size of your image. Zooming in makes the image larger, and zooming out makes it smaller.