BitJazz
SheerVideo Fidelity
Artifacts

Approximating codecs produce a number of objectionable artifacts, some subtle, some obvious. Compression artifacts are flaws that show up in an image that shouldn't be there, and are there only because of flaws in the compression and decompression algorithms. Each time a movie is decompressed and recompressed with an approximating codec such as DV, JPEG, or MPEG, these artifacts get worse, an effect known as generational loss. Examples of common artifacts include

• ghosting
• pixellation
• blurring
• bleeding
• washout

In the tables below, we show magnified examples of actual artifacts obtained by using professional-quality commercial JPEG and DV codecs.

Ghosting

One of the most objectionable artifacts that approximating image codecs introduce is ghosting, also known as echoing, ringing. A particularly colorful name for ghosting is mosquito noise, so called because it's commonly seen along poorly compressed silhouettes of skin. In ghosting, high-contrast edges are flanked by parallel curves of alternating brightness. Here's a 4X magnified example where the diagonal edge from the original is flanked in the degraded image by light and dark ghosts that are echoes of the original edge.

original	JPEG/DV	SheerVideo
no ghosting	ghosting	no ghosting

Ghosting shows up to some degree in approximating codecs at all quality settings, though it gets worse at lower quality. The degraded image showing ghosting here is the output of a JPEG codec, but other DCT-based approximating codecs have the same problem. SheerVideo, as a lossless image compressor, never introduces any ghosting artifacts.

Pixellation

In pixellation, also known as blocking, the image looks like it's divided up into big blocks or tiles, as in a mosaic or a cubist painting, with sharp discontinuities between block boundaries. In the subtle, actual-size example shown here, the smooth gradient of the original appears in the degraded image to be divided into four blocks of distinct shades of grey.

original	JPEG/DV	SheerVideo
no pixellation	pixellation	no pixellation

The degree of pixellation varies from very subtle at low compression to egregious at high compression power. Pixellation is a problem with all tile-based approximating codecs, which is essentially all approximating codecs. SheerVideo, being a lossless video compressor, never pixellates the image at all.

Blurring

The least surprising artifact of approximating image compressors is blurring, fuzziness, or unsharpness. In blurring, fine image details become indistinct, as though seen through an unfocussed lens. In the unmagnified example shown here, the sharp text of the word "blurring" in the original becomes, in the degraded image, much harder to read or even to focus on. Note also the ghosting above and below the text, and the fading of the background.

original	JPEG/DV	SheerVideo
no blurring	blurring	no blurring

Blurring is a problem with all approximating codecs that reduce the data rate by ignoring fine details, which is to say, all approximating codecs. The blurring gets worse the higher the compression power. The perfect fidelity of lossless video codec SheerVideo ensures that it never blurs the image in any way.

Bleeding

Television audiences are so used to bleeding, also known as smearing, that they are generally completely unaware of this artifact. In bleeding, colors of contrasting hue or saturation bleed across sharp brightness boundaries, so that the colors appear to be sloppily painted on. The problem is worst acute in images with high color detail, such as blooming meadows. In the magnified example shown here, the red and yellow hues are smeared across the circular boundary, yielding oranges and browns. Note also the ghost circles in the center and the mosquito noise on the outside.

original	JPEG/DV	SheerVideo
no bleeding	bleeding	no bleeding

Bleeding is a problem with all compressors that reduce the data rate by converting the image to luma-chroma space (Y'C_BC_R) and subsampling the chroma components, whether to 4:1:1, 4:2:0, or 4:2:2. This includes just about all approximating compressors, including JPEG and DV. SheerVideo, as a nondestructive image codec, never introduces bleeding. However, SheerVideo does support the native luma-chroma video formats, including Y'C_BC_R 8bv 4:4:4, Y'C_BC_R 8bv 4:2:2, and Y'C_BC_R 8bw 4:2:2, so SheerVideo will faithfully reproduce any bleeding already in the video.

Washout

One of the worst artifacts resulting from the use of approximating image codecs is washout. In washout, a contrast in hue or saturation between colors having the same brightness (luma) is completely lost, both colors being replaced with an in-between color that may appear strikingly different from the original colors. In the 4X magnified example shown here, red and green hues of equal luma are washed out to a uniform brown.

original	JPEG/DV	SheerVideo
no washout	washout	no washout

Like bleeding, washout is a problem with all compressors that reduce the data rate by converting the image to luma-chroma space and subsampling the chroma components, which includes just about all approximating compressors, including JPEG and DV. SheerVideo, as a nondestructive video codec, never causes washout.