Block Based DCT Compression: Typical Artefacts
Below is a single frame of a 720 pixels by 576 line, interlaced, 25 frame/sec sequence coded at 352 x 288 pixels, 25 frames/sec so as to achieve a video bitstream of 388 Kbits/second using MPEG1.
Considering that field dropping is used to convert the interlaced video to non-interlaced, and 4:2:0 color subsampling is used, this gives a compression ratio of:
1.5*(720*576/2 *25*8 )/388,000 = 160
As you can see, this compression ratio is way too high, but it does show up some of the problems that start to occur with these techniques when they push the limits.
The numbers in red indicate some typical artefacts caused by the algorithm,
which can be found in all Block-Based DCT coding methods.
- Blocking, where the block boundaries become obvious due to discontinuities in texture, luminance and color.
- Mosquito effect, where edges seem to buzz, due to the ringing effect and also high frequency noise generated by motion compensation mismatch. It can be particularly
annoying because it changes from frame to frame and viewers notice it more than if it were stationary.
- Aliasing and ringing on sharp edges. Aliasing, or staircase effect, is caused both by the conversion of interlaced video to non-interlaced progressive by field dropping and also by
quantizing higher frequency DCT components which leaves only a horizontal or vertical representation for diagonal lines close to horizontal or vertical. Ringing is caused by the DCT transform
because removing higher frequency components reveals low frequency components that are normally compensated for and invisible. [Yeun and Wu '96]
- Color bleeding and mismatch, caused by coarse quantization of color blocks and spatial color subsampling. Also caused by motion compensation, which uses only luminance information to
find a prediction from previous or following frames, and hence may match to a nearby block with quite different color information
- Blurring, because most of the high frequency texture has been lost.
Reference: Micheal Yuen and H.R. Wu.
Identification of Coding Distortions in Hybrid MC/DPCM/DCT Codecs.
Monash University Dept. of Digital Systems. 1996