DV Pix - Multigeneration test images
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Text: The DV Format

[updated 16 January 2000]

Multigeneration tests are useful for two purposes: they show off the strengths and limitations of the DV compression scheme, and they also show off any weak points in a particular implementation!

DVStressTest is my current test image. It's not perfect, but it does have a variety of test subjects:

After rendering one or more generations, the bars can be observed on a vectorscope; look for saturation & hue changes. Look at the ramps on a waveform monitor for breaks in continuity, for loss of linearity, and for changes in levels. Look at the pix on a picture monitor for all manner of degradations. Extract a frame and take it into you favorite image editor; blow it up and look at artifacts.

The image DVtest1 is specifically designed to be hard to compress; it has "infinite" bandwidth with unnaturally sharp transitions horizontally, vertically, and diagonally. It has separate luma and chroma test sections, so that when you pass it through a DV codec you'll clearly see the difference between luma and chroma resolutions and compression methods. Each 16x16 square and its line of text is displaced one pixel horizontally and vertically in the DCT block structure from the preceding line, so each line will compress with different artifacts. Due to the sharp edges, multiple compression cycles (generations) will cause an increasing amount of mosquito noise to appear.

SF, on the other hand, is a naturalistic camera-captured image, which despite its high detail is already bandwidth limited by optical low-pass filtering in the camera and by one stage of DV compression. You can take it through multiple generations with little degradation occurring (or try your own camera-captured images).

Both images can be downloaded for your own tests. They are saved as TIFF files with LZW compression: this is a lossless compression that still yields acceptable sizes for transmission. To use them, simply import them into Premiere or another editing application; if your editor won't import TIFFs, use PhotoShop or another image editor to convert them to BMPs, PICTs, or another native format that your editing application can use. These images are 720x480 for 525/59.94 (NTSC) use; for use in 625/50 (PAL) format, open DVtest1 in Photoshop or another editor and add another 96 scanlines to make a 720x576 image (do not stretch the original image to fit the larger vertical space, or you'll destroy the relationship of the lines to the DCT block boundaries). Don't use "SF" for PAL tests, as its 4:1:1 structure won't be representative of the 4:2:0 images native to PAL DV formats (except for DVCpro, which uses 4:1:1 in PAL).

I suggest the following two tests. All instructions assume Adobe Premiere 4.2 / 5.X, but other editing applications offer similar operations, so adjust the steps to suit your system. At each stage, I suggest saving the resulting clip or still with an easily-remembered name followed by a generation number, i.e. "test01", "test02", "test03", etc., so that you can easily keep track of all the stages of the test.

Test #1: Import/Export

  1. Create a new project, selecting your DV codec as the compression method.
  2. Import the still image, and place it on the timeline.
  3. "Make movie" or otherwise generate a DV-compressed AVI or Quicktime clip from the still.
  4. Open the clip you've just made, and export a still (File | Export | Frame as bitmap...). This is the first generation.
  5. Repeat steps 2 through 4 as many times as you want, each time using the result of the previous step as your source image.
  6. You can now string together each of the clips created at each stage of the test and make a new clip combining them all. Play the clip back in looping mode, and you'll see an "animated" demonstration of the codec's multigeneration performance.
This test strictly tests the compression/decompression of the codec. On the first generation, if you're using DVtest1 as your source, you'll see some artifacts around the text on the luma side, but the text will still be quite readable. The chroma side will be almost unreadable: 4:1:1 or 4:2:0 coding doesn't provide much color resolution!

Using the Adaptec DVsoft codec in Premiere 4.2, I found that going ten generations with DVtest1 adds a bit more mosquito noise in each generation, but the worst degradation happens with the first compression. Going ten generations on SF showed no visible difference at all.

Test #2: Processing Inside the Editor

  1. Open your editor, and create a new project using DV compression.
  2. Import the still, and place it on the timeline.
  3. Add a superimposed title, such as "1st generation", in one corner of the image. Put the title on a solid plinth (a box used as a background for the text), but leave the rest of the superimposed image totally transparent.
  4. "Make movie" or otherwise render this as a short (1/2 second) DV-compressed AVI or QuickTime clip. This is the first generation.
  5. Import the just-rendered clip, and place it on the timeline, replacing the original clip. Change the text of your superimposed title to read "2nd generation". Leave it in the same place, so that it covers the previous title but otherwise does not affect the image. "Make movie" or otherwise render this 2nd generation.
  6. Repeat step 5 for as many generations as you wish.
  7. String together all the rendered clips into a single clip, and play them out in looping mode to see an animation of multigeneration performance when processing is done inside your application.
 This test forces a decompression/recompression at each stage, but instead of simply exercising the codec this test keeps the image within the editor and adds processing to it. By adding a title in one corner you should be leaving the rest of the image intact, and it should degrade the same way as in the first test.

What I found using Premiere 4.2 and the Adaptec DVsoft codec on Windows 95 was that the luma information in this test was degraded exactly as in the first test, as you would expect. Chroma, however, underwent a leftwards shift, drifting across the image with every new generation -- and drifting across DCT block boundaries, too, causing further degradation. See the companion page Pix: Codecs for some 5th-generation tests on another graphic image.

DV Stress Test
Stressful image to test DV Codecs
(c) 2000 Origin8 Video
Note: JPEG compression shows artifacts not in the original
This "pathological" text image is designed to stress the DV codec. Ramps show luma accuracy & linearity; bars show chroma fidelity; text & graphics show mosquito noise, chroma accuracy, Y/C delay, etc.

Download DVStressTest.TIF (116 kBytes)

Pathological DV test image "DVtest1"Pathological text image to stress DV codec
Note: JPEG compression shows artifacts not in the original
This "pathological" text image is designed to stress the DV codec; all the text is rendered with no antialiasing and the sharpest possible transitions. 

The left side only has luma transitions; the right side has only chroma transitions; the luma value of the colors used is consistent (to a couple parts in a thousand) using the ITU-R BT601 equations converting RGB to YUV. 

On both sides, each line of text and its 16x16 pixel box is 31 pixels lower and one pixel to the right of the previous line. Thus each line is one pixel to the right and one pixel up, with respect to the DCT block structure*, compared to the previous line; each line will stress the DCT coding a bit differently. 

Download DVtest1.TIF (53 kBytes)

*DV DCT luma blocks are 8x8 pixels for luma. In 525/59.94, the chroma blocks are 32 pixels wide and 8 tall; in 625/50 the chroma blocks are 16x16 (in both cases, the chroma blocks are 8x8 samples; the odd sizes are due to the 4:1:1 chroma subsampling in 525, and 4:2:0 subsampling in 625 systems)

"DVtest1" details
detail of pathological text image
Note: JPEG compression shows artifacts not in the original
Camera-captured image "SF"
Camera-captured image of PacBell building, San Francisco
(c) 1995 Origin8 Video
Note: JPEG compression shows artifacts not in the original
This camera-captured image, shot with the 3-chip DCR-VX1000 and exported from Adobe Premiere using the Adaptec DVsoft codec under Windows 95, is more typical of the imagery found in video. While "sharp" edges are present, they are all prefiltered and bandwidth-limited by the optical and electronic processing of the video camera and circuitry -- and it's already been DV-compressed once. 

This image shows little if any degradation in multigeneration tests: I've run it through the codec ten times, intercut 1st and 10th generation images in a looped clip, and been unable to see where the cuts occur. A mathematical comparison showed only minor differences between the two images, mostly around the edge of the building and the sun glare. 

Download SF.TIF (526 kBytes!)

Suggestion: instead of using this image, use one of your own. I only provide this one as a reference standard, so that when I say that I've done the test with a real image, that image is available for others to play with to verify my results.

Copyright (c) 1998, 2000 by Adam J. Wilt.
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Last updated 16 January 2000.