DScaler -- Histogram Filter

| Histogram Filter

This is a diagnostic filter which lets you see the colors used in the current image. This can help you choose your video settings.

The Settings

Display Mode

This filter can display

- The brightness (“Y”)
- The color (blue “U”, red “V”, or both)
- Or all three pixel components at once

The brightness is displayed in white, “V” color in pink, and “U” color in light blue. Regardless of the Display Mode, a dotted yellow line will appear below the graph, showing the graph’s full 256 value range.

Colors outside of the (CCIR-601) standard range will appear darker on the graph. That is meant to make calibration a little easier.

The left end of the line represents a color component value of 0; the right end a value of 255. At each position in the graph, the height of the histogram shows the logarithm of the frequency of that color component value. Where there aren’t any pixels with that color component value, the histogram’s dot will be just above the yellow line.

You may notice that the values toward the bottom of the graph are vertically separated by quite a bit. That’s just the effect of the log scale — It doesn’t say anything interesting about the picture.

Color Correction

This option causes the color components to be averaged vertically before inclusion in the histogram.

If you are using a composite connector or an internal tuner for NTSC or PAL (but not SECAM) video, this is a good idea. It gets rid of color crosstalk which would otherwise artifactually extend the range of the graph.

If you are using a PAL composite source, choose “Composite PAL.” For an NTSC composite source, pick “Composite NTSC.” Otherwise, choose “Disable.”

Fast Memory Access

This is a compatibility setting which should normally be turned on.

What’s it For?

Calibration

You can use this filter to manually calibrate signals for which you don’t have a calibration pattern.

Before you begin, you should make a decision about how you want to run DScaler — Using the standard color range, or using the full color range. The standard color range is easier to deal with, while the full color range gives slightly better color resolution.

If you are unfamiliar with calibration, it is strongly recommended that you use the standard range. That will let you use the same overlay settings in DScaler as you do in the rest of your applications.

First make sure that you have Full Luma Range (in the “Advanced Video Flags...” dialog) turned on — regardless of the color range you have decided on. Set the Histogram filter’s Display Mode to “brightness,” then open up the “Video Settings...” dialog.

If you’re going to use the standard color range, you should next move the Brightness slider until the lowest peak in the brightness distribution — which corresponds with black — is at the point where the brightness dots become dim. That is brightness = 16, which is what your overlay should expect for black.

If you have instead decided to use the full color range, then move the Brightness slider until the lowest peak is at the very left end of the graph.

Now wait until something really, really bright is on screen. If you are using the standard color range, move the Contrast slider until the whole brightness distribution is shown in the bright part of the histogram. To use the whole color range, set Contrast so the brightness distribution is entirely on the graph.

It’s important that something very bright be on the screen when you do this — Otherwise, you’ll be cutting off meaningful brightness information.

You may need to change your overlay brightness and contrast settings after you do this. This is especially important if you have chosen to use the full color range.

Though this procedure is better than not calibrating at all, it isn’t as nice as calibration based on a known pattern. Using the histogram maximizes the available information, but it does not give you any way to verify the accuracy of the result.

Diagnostics

This is a pretty general tool, so people will undoubtedly come up with other uses for it. It lets you see any changes to the signal which occur before DScaler processes the picture, so you can use it as a diagnostic for any early video transmission steps.

Note that steps which occur after DScaler processes the picture — in particular, any overlay or output device settings — will not show up in the histogram.

Filter Changes

DScaler version 4.1 — Values outside the standard color range are now dimmed in the histogram. The Vertical Averaging option has been renamed to Color Correction. (But it still averages vertically.)