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 (red “U”, blue “V”, or both)
- Or all three pixel componenents at once
The brightness is displayed in white, “U” color in pink, and “V” 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.
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.
Vertical averaging
This option causes the color components to be averaged vertically before inclusion in the histogram.
In general, this is a good idea. It gets rid of color crosstalk, which would otherwise artifactually extend the range of the graph. On the other hand, it does cause inaccuracy at vertical edges in the picture.
Fast memory access
You’ll probably want to keep this setting turned on. With it enabled, the filter will run up to twice as fast.
However, there are a few motherboards — mostly made by VIA — for which Fast memory access will cause problems. If you see fleeting horizontal lines in your picture, try turning this off in any filters which offer it. If that fixes the lines, you’ll know to keep this setting disabled.
What’s it For?
Calibration
You can use this filter to manually calibrate signals for which you don’t have a calibration pattern. First make sure that you have Full Luma Range (in the “Advanced Video Flags...” dialog) turned on. Set the Histogram filter’s display mode to “brightness,” then open up the “Video Settings...” dialog.
Move the Brightness slider until the lowest peak in the brightness distribution — which corresponds with black — is at the very left end of the graph.
Then wait until something really, really bright is on screen. Move the Contrast slider until the whole brightness distribution is shown in the histogram. 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.
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.