What the filter can do / what the filter can be used for:

If dark areas of a video are too dark, the filter can be used to brighten them up, without changing light and mid tones. The filter can invert the color space like the internal invert filter and can apply coring, like the coring filter plugin that exists for VirtualDub. If the tone of a video is, for example too blue, the filter can be used to compensate this. The overall brightness can be changed also. Perhaps the filter can also be used for some scientific use, if there is a need to mask a certain color. And the good thing is that it can all be done at the same time with one filter instance only. The filter also gives the possibility to apply curves in other color spaces. Using the HSV space, individual colors can be exchanged by other ones or/and the saturation can be changed for example. But note that one filter instance can always only process one color space. If a manipulation in RGB and HSV space is needed, the filter has to be put into the filter chain two times: once for RGB processing and once for HSV processing. Other possible applications of the filter include:
• Use of the filter as prefilter of the deshaker filter for difficult (dark) DV Cam captures in the first pass of deshaker. Some colors can be masked, so the motion analysis works better in the first pass of deshaker.
• Use of the filter in the area of "film look for DV material". See posts in the doom9 forum or the VirtualDub forum.

The x axis of the curve represents the input color value, the y axis represents the output color value. Initially the filter will show a 45° left down to right up curve. This signifies that the output color will be the same as the input color for all color values. Raising the output value, results in a lightning up of the selected input color value. The filter always stores the settings of five gradation curves, even if only the RGB curve is used. This way the settings of the R/G/B curves are not lost if, for example temporarily RGB only mode is used and the processing settings are saved at that time. The fifth curve is only used for the K channel of the CMYK color space.
Info about gradation curves: Curve (tonality) - Wikipedia

The Gradation Curves window / the filter configuration dialogue:

The filter window on the left side is link senitve. You can either click on the areas of the filter windows or select a topic below:

Color Space

Channel

Filter Curve Editor

Smooth

Processing Mode

Reset

Invert

Import

Export

Space:
Selects the color space. If another than RGB color space is selected, two color space conversions are necessary: From RGB to the target color space. Then the curve is applied in the target color space and finally there is a color space transformation back to RGB. The color space conversions will slow down the filtering.
Note that not all the color space conversions are lossless. Conversions to HSV, YUV or Lab and back are not lossless. For YUV the average error is about 0,98. For a color sample e.g. R:23 G:43 B:12 transformed to YUV and back resulting in a sample of R:23 G:42 B:12 the error is 1 because the value of green differs in one (43 vs. 42). The average error is calculated by processing all the ~16 million colors (24 Bit). The average error of the HSV conversion is about 0,87 and for Lab 1,51.
A note to the YUV conversion: The YUV space in this case uses 4:4:4. That means Y, Cr and Cb all use the full resolution.
A note to the Lab color space: The filter uses two big lookup tables that contain the conversion values of all the 16 Million colors. When the Lab space is selected for the first time, the tables are built up and 128 MByte of memory will be used to do the conversion from RGB to Lab and back. The memory will be freed only when either the filter is removed from the filter chain or VirtualDub is closed. Also note that when Lab is selected for the first time, the filter dialogue will stall for a moment until the tables are built up. The time it takes, depends on the speed of the PC.
The RGB/Lab conversion uses D65 as reference white and sRGB as RGB model.
General info about color spaces: Color space - Wikipedia

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Channel:
Selects the color channel. RGB shows the curve for the channel that edits all the colors at the same time while Red / Green / Blue shows the curve for each color channel.
Note that for other color spaces, a joint channel curve (RGB curve) does not make sense and is for that reason not available in other color spaces.

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Filter Curve Editor:
Different drawing modes allow different ways to design the filter curve. The drawing mode can be selected on the right side.

selects the Pen(cil) or Freehand mode.
By pressing the left mouse button and drawing over the filter window, the curve can be edited. Pressing the right mouse button sets the input value to the current position of the cursor, displaying the corresponding output value below the curve.
The output buttons below the curve window can be used to fine tune the output value for a selected input value. The output value shows the transformation that is applied for the corresponding input value. With the input buttons the input value position can be selected. If the input value equals the output value, no processing is applied for the selected input value.

The other three modes use coordinates/points to design the filter curve:
selects the line mode.
Straight lines will be drawn between the points. This can be useful eg. for drawing a stair type curve.
selects the spline mode.
A spline interpolation will be applied for the given coordinates. This is the default mode.
selects the gamma mode.
This mode can be used to apply a gamma correction. The gamma value is displayed below the curve.

Points can be set with the left mouse button and deleted with the right mouse button except in gamma mode that has got a fixed number of three points. When holding the left mouse button pressed on a point and moving the mouse, the point can be moved. Releasing the left mouse button sets the point on the current position.
The first and last point are two special points. They cannot be deleted nor can a point be set before the first or after the last point.
With the Point buttons a point can be selected for fine tuning. The selected point is outlined in red. The input and output buttons can move a point in the x (input) and y (output) axis. Currently the number of points is limited to 16.
Switching from any coordinates drawing mode to pen mode keeps the raw curve but the coordinates are lost. Switching from pen mode to any coordinates drawing mode will reset the curve to a straight line with two coordinates (0/0) (255/255) respectively three coordinates for the gamma mode. Switching to gamma mode will reset the curve to a straight curve with three coordinates, except the case when the curve already contains exactly three coordinates.

Editing the curve with preview open and some (slow) filters active in the filter chain is not recommended because responsiveness is lost. Either edit the curve with preview closed in such a case or edit the curve with preview active only with this filter active.

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Smooth:
Smoothes the filer curve. The smoothing algorithm is not perfect, but it helps to remove some jagginess of the curve. It is only available in pen drawing mode.

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Processing mode:
"RGB only" will only process the image by the common RGB channel. Any edited color channel will be ignored. "RGB + R/G/B" processes the individual color settings first and then applies the RGB settings. These two modes behave similar to the curves function of painting programs.
The weighted modes are just like the first two modes differing only in way RGB is processed. First a Y (gray) value of each pixel is calculated. Then the output value of the RGB curve of this Y (RGB input) value is taken and applied to all three colors of the pixel the same way. An example to demonstrate the difference of the two modes: Let's assume that the output setting for the input value of 30 in the RGB curve is 50. In the mode "RGB only" all pixels that have got a R or G or B value of 30 will be changed to 50, while the other colors of the pixel stay the same (if they are not 30 as well). A pixel with the color R30 G10 B15 will be changed to R50 G10 B15 (if the values for 10 and 15 are unchanged in the curve). While in the weighted mode a pixel that has got a Y value of 30 ex. R10 G38 B50 will be changed to R30 G58 B70. 50-30 = 20 and 20 is summed up to the individual R G B values.
The mode "no processing" applies no change and can be used to spot the difference between the activated filter and no processing in the preview window.
Other color spaces than RGB only offer the individual channel processing (e.g. Y/U/V) and the no processing option.

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Reset:
Changes back the currently selected curve to the initial settings, where no processing is applied at all.

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Invert:
Inverts the currently selected curve. Inverting an unmodified (45°) RGB curve results in the same effect as applying the internal invert filter.

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Import:
Imports external gradation curves. In the dialog box the file containing the curves can be selected and imported.
The standard file type is .amp which is used by Adobe Photoshop. But in general any file containing a curve that is structured like .amp files can be imported into the filter. If the import function detects one curve in the file, the data is loaded into the RGB curve. If there are three curves in the import file, they get loaded into the R / G / B curves or the other individual curves of the other color spaces (e.g. Y / U / V). With four or more curves within the import file, the first curve gets loaded into the RGB curve then followed by R / G / B. If the file contains more than five curves all curves above the fifth are ignored. The fifth curve is only used for the K channel of the CMYK space. Importing an .amp file sets the drawing mode to pen mode. Map files (.amp) can be saved using the curves function of Adobe Photoshop by selecting pen mode.
Another file type is .acv also used by Adobe Photoshop. Only the coordinates are stored in this format. Importing an .acv file sets the drawing mode of all curves to spline mode.
Other import formats are: Comma Separated Values (*.csv) - see under export, Tone Curve File (*.crv), Tone Map File (*.map) and Smartcurve HSV (*.amp).
The crv and map formats are used by Corel Draw. These formats contain the drawing mode, the coordinates and the raw curve data of each curve.
The SmartCurve HSV import allows a correct import of *.amp HSV color space files generated by the Photoshop Smartcurve plugin. The plugin stores the curve data similar to the Photoshop *.amp files, but uses a different order for the curves.

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Export:
The default format exports a file with the structure of a map file. The export function always stores five gradation curves. The exported file can be loaded within the curves function of Adobe Photoshop. The file structure is very similar to the way the filter data gets stored in the processing files of VirtualDub. The data in the processing file is stored with hex values FF=255 straight forward. The input value 0 of the RGB curve is in first position, were as only the output value is stored. So it goes from the first output value to the last (255) and then followed by the data of the R / G / B curves. For other color spaces than RGB the first curve is stored as an empty curve (0-255 or 45° filter curve), then followed by the curves of the individual channels (e.g. Y / U / V). The channels are stored in the order of their appearance (e.g. HSV 1:empty 2:H 3:S 4:V 5:empty). If you look at a map file with a hex editor you will see the same thing as in the .vcf processing file of VirtualDub. Exporting curves that use the coordinates drawing modes will store only the raw curve data but not the coordinates.
The second export format is .acv that only stores the coordinates. The drawing mode is not stored. If pen mode for a curve is active, the raw data will not get stored, but two coordinates (0/0) (255/255). That results in a straight line when importing the curves later.
As a third export format Comma Separated Values (*.csv) can be selected. Each line in the file contains one decimal value of the curves. In 1280 lines the data of the five curves is stored. The file can e.g. be imported in Excel or opened and edited in Notepad. This format currently only stores the raw curve data and does not store the coordinates if a non pen mode is selected.

Alternatively all the filter data (drawing modes / coordinates / raw curve data / space & processing mode) can be stored in a .vcf VirtualDub processing file.

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