Select Objective Measurements
The Objective Metrics can be used to calculate the perceived video quality (Sarnoff JND), QC a product when the results are known (PSNR with Thresholding), looking for artifacts when no reference is present (PSNR No Ref, Spatial and Temporal). In all cases, the metrics are displayed and written to a LOG file for off-line analysis.
JND
The Sarnoff JND model is a method of predicting the subjective rating of a group of human testers. JND measures the perceived differences between the original and processed video sequences and weights them based on where they are in the video sequence. The scale is 0 to 100; where 0 is perfect and 100 should never actually occur. For more information about JND, please refer to the Sarnoff JND document (a link is on our website at www.videoclarity.com/WhitePapers.html).PSNR
The following is quoted from "Fractal Image Compression", by Yuval Fisher et al.,(Springer Verlag, 1995), section 2.4, "Pixelized Data". "...PSNR is used to measure the difference between two images. It is defined as
The PSNR is given in decibel units (dB), which measure the ratio of the peak signal and the difference between two images. An increase of 20 dB corresponds to a ten-fold decrease in the RMS (root mean squared) difference between two images. For simplicity, we display 100 when the images are identical. The actual value is infinite.
PSNR measures all of the differences between the original and processed videos. It does not try to weight these. The calculation is very fast and is used to perform QA/QC when the perceived video quality is already known.
2 No Reference Metrics are achieved using this same formula:
- Chrominance - The average Color Hue (measure of PSNR relative to the CbCr baseline)
- Luminance -The average Luma Intensity (measure of PSNR relative to the Y baseline/Black)
Spatial
Spatial measures the activity within a video sequence. Spatial is a no-reference metric. Large values indicate a substantial change within an image - for example: panning stadium crowds would generate a large Spatial Index. A solid color would produce a low Spatial Index. For more information about the Spatial metric, please refer to ITU-T P.910 (a link is on our website at www.videoclarity.com/WhitePapers.html).
The Spatial Index is the STD deviation, and presented in Pixel Value units, which measure the value difference from one pixel to its neighbors across the image. If two sequences are being played, then the Spatial Index is calculated for both video sequences, and the differences are displayed and graphed. The Log file holds the actual values for both video sequences and the differences.
Temporal
Temporal measures the activity frame-to-frame within a video sequence. Temporal is a no-reference metric. Large values indicate a substantial change occurred during the video sequence - for example: a scene change would generate a large Temporal Index. A frozen frame would generate a Temporal Index of 0. For more information about the Temporal metric, please refer to ITU-T P.910 (a link is on our website at www.videoclarity.com/WhitePapers.html).
The Temporal Index is the STD deviation, and presented in Pixel Value units, which measure the value difference from one pixel across many frames. If two sequences are being played, then the Temporal Index is calculated for both video sequences, and the differences are displayed and graphed. The Log file holds the actual values for both video sequences and the differences.
Objective Measurement Graph
The Objective measurement graph is enabled by pressing the Graph button in the Objective Metric Controls pane. If all of the data has already been collected (which it does by playing through the 2 video sequences the first time after the Objective Metric is turned on), the graph will be displayed as shown above. If the data has not been collected, the Graph button will switch to Graphing mode, and a horizontal line will be drawn across the center of the screen. The video sequences are played from the start through the end points. To change the start and end points, use Clip Alignment to adjust the first and last positions for both Viewports. Once the video sequences have been played, the graph is scaled and the minimum, maximum, and average Objective Metrics are displayed along with the graph. Using the shuttle bar (slide bar), the user can display any frame to visually assess the 2 video sequences.
Note: The frame is associated with the right side of the slide bar.
Pixel Values
To display individual pixel values press the right mouse button.Note: the left mouse button will still control panning.
Scrolling to any X,Y location will show the pixel values for the same location for both video sequences. Holding the right button while moving the cursor will allow moving the cursor in increments of 4 pixels at a time.
Note 2: to get finer control of the X,Y location, use the Pixel Value Hotkeys, which increment in 1-pixel increments, or type in an X,Y location in the Pixel Value Controls
Note 3: you can also type in an X,Y location directly.
Objective Metric Controls
The Objective Metric Control Pane controls the behavior of the JND, PSNR, Spatial, and Temporal objective metric, displays the Pixel Values at a chosen location, and sets the A-B parameters.Objective Metrics Controls
JND Objective Metric
Objective Metrics Description
| PSNR | This tab selects the PSNR Metric settings Note: Moving among the Objective Metrics does not turn on or off the metric calculations. This simply displays the collected data. Note2: PSNR has an extra button called To Black. This turns PSNR into a No Reference Calculation. If you have 2 video sequences loaded, it will calculate the PSNR for both sequences independently and subtract the difference. The Log file will have 3 sets of data. The Graph will display the subtracted difference. |
| Spatial | This tab selects the Spatial Metric settings Note: Moving among the Objective Metrics does not turn on or off the metric calculations. This simply displays the collected data. Note2: If you have 2 video sequences loaded, it will calculate the Spatial for both sequences independently and subtract the difference. The Log file will have 3 sets of data. The Graph will display the subtracted difference. |
| Temporal | This tab selects the Temporal Metric settings Note: Moving among the Objective Metrics does not turn on or off the metric calculations. This simply displays the collected data. Note2: If you have 2 video sequences loaded, it will calculate the Temporal for both sequences independently and subtract the difference. The Log file will have 3 sets of data. The Graph will display the subtracted difference. |
| JND | This tab selects the JND Metric settings Note: Moving among the Objective Metrics does not turn on or off the metric calculations. This simply displays the collect data. Note2: JND takes a considerable amount of time so we do not allow moving to another pane while this calculation is processing. |
| PixVal | This tab enables you to view individual Pixel Values. Please refer to the Pixel Value Controls for more information. |
| A Minus B | This tab enables you to set the A-B parameters. Please refer to the A-B Controls for more information |
| On/td> | This enables/disables calculating the Objective Metrics. When checked, the Metric calculation is enabled. Data is collected while the sequence is playing. To play the sequence, please press the Play, Graph, or Log buttons. When Unchecked, the calculated data is removed from memory. |
| To Black | This is only relevant to PSNR. Selecting this performs a No Reference Metric of the Luminance Intensity and Color Hue. |
| Spatial | Selecting this applies the spatial alignment calculated in the Metric Adjust pane. |
| Norm. | Selecting this applies the color hue/luminance intensity offset calculated in the Metric Adjust pane. |
| Thrs | This allows a threshold to be set of Y, Cb and Cr. Each component can be turned on/of. PSNR values under this threshold are returned as failures (including Luminance and Chrominance). JND, Spatial, and Temporal values over this threshold are returned as failures. Note: JND combines Cb & Cr together so there is only 1 threshold for color. |
| Failures | These are informational messages and cannot be changed. It displays the number of frames that are outside the threshold (see not above). |
| Y/G | Show the Objective Metric data for the Y/G value |
| Cb/B | Show the Objective Metric data for the Cb/B value |
| Cr/R | Show the Objective Metric data for the Cr/R value |
| F1/Fr, F2, Current, min, max, avg | These are informational messages and cannot be changed. Current – the current metric value F1/Fr – the current Sarnoff JND metric value for Field #1 or Frame F2 – the current Sarnoff JND metric value for Field#2 Min – the minimum metric value Max – the maximum metric value Avg – the average as defined in the appropriate ANSI spec. For PSNR and JND, average is a quadratic average. For Spatial, Temporal it is a straight average. |
| Graph | Display the Y/G, Cb/B, or Cr/R values over time on the Objective Metric Graph. If the data has not been calculated, then pushing this button will play the video sequences, calculate the Objective Metrics, and display them. Note 1: Y/G data will be printed in Green. Cb/B data will be printed in Blue. Cr/R data will be printed in Red. Note 2: In the upper, right corner, the graph will display J for JND, P for PSNR, S for SPATIAL, and T for TEMPORAL. |
| Log | This creates a log file which includes:
When you press log, you will be prompted to enter a location to save the file along with a filename. Note 1: You can restore these files using File Import, and ClearView will automatically load the clips, restore the image format, video format, alignment, and read in the Objective Metrics. Note 2: Files are written as a 12 column, space-delimitated file.
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Pixel Value Controls
Pixel Value Descriptions
Y/G Cb/B Cr/R A B |
These are the pixel values in decimal at the same X,Y location for video sequence A & B.
The color is also displayed for reference. Note 1: if you zoom the original image, then you can see the cursor more clearly. Note 2: to get finer control of the X,Y location, use the Pixel Value Hotkeys, which increment in 1-pixel increments, or type in an X,Y location in the Pixel Value Controls. |
| X, Y | This is the X, Y location of the current pixel.
Pressing the right mouse button enables this mode. Note: This is referenced within the original image – not the location after the video sequences have been rendered. |
A Minus B Controls
A Minus B Descriptions
A-B Use Diff Threshold |
Checking this box performs a |A-B| > Threshold on either the Chroma or Y pixels Unchecking this box, performs an A-B > 0 on all pixel values If true, it displays the difference. If false, it displays black |
| Threshold | This is the threshold value for the A-B Use Diff Threshold above |
| Chroma | Check means that the A-B calculation will be performed only on the chroma values (Cr & Cb) Uncheck means that the A-B calculation will be performed only on the luminance value (Y) |
| A-B Addback | Checking this box performs a |A-B| > Threshold calculation.
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Metric Adjust Controls
Metric Adjust Descriptions
Spatial-Align |
Checking this box performs a Spatial Alignment in both the horizontal and vertical direction. The current video frame in Viewport B is compared to the current frame in Viewport A. The results are shown in X, Y. Note: The video sequence is not adjusted. This offset only applies to the objective metrics. |
| X, Y | These are the X,Y offset values after the Spatial-Alignment. They can be overridden. Note: the offsets are restricted to be +/- 8 (X) and +/- 16 (Y). |
| Normalize | Checking this box performs a Luminance Intensity and Chrominance Hue calculation. The current video frame in Viewport B is compared to the current frame in Viewport A. The results are show in Y/G, Cb/B, Cr/R. Note: The video sequence is not adjusted. This offset only applies to the objective metrics. Note2: The offset is a linear offset per frame and is used to make PSNR more robust. |
| Y/G, Cb/B, Cr/R | These are the Y/G, Cb/B, and Cr/R luminance/chrominance offsets after the Normalization. |
| X, Y, W, H | This allows you to set a window size for the objective metrics. By default it is full screen, but the size can be adjusted. Note: to remove borders with noise, this tool can be used to exclude the noise. |