Image compression Flashcards
What is 100 photo quality?
Perfect image quality with no loss of information
When the compression ratio is high…
The image ratio is low
What is non-lossy compression?
- Non-lossy (lossless) compression – original image can be recovered exactly from compressed image file.
- E.g. Portable Network Graphics (.png)
What is lossy compression?
- Lossy compression – Information lost when file is compressed. Therefore only an approximation to the original image is possible.
- Eg. Joint Experts Photographic Group JPEG (.jpg)
What are the 3 types of data redundancy?
- Coding redundancy
- Inter-pixel redundancy
- Psycho-visual redundancy
What causes coding redundancy?
It is caused by sub-optimal code words for symbol encoding - a symbol typically (but not exclusively) represents grey-level.
What causes inter-pixel redundancy?
Due to grey-level correlations between neighbouring pixels.
What is psycho-visual redundancy?
Information contained within an image that is superfluous to the interpretation or aesthetics of an image.
What does clarity depend on?
- Spatial frequency
- Amplitude
What is the compression ratio?
A measure of the the compactness of a compressed image file
How is compression ratio (Cr) defined?
CR = b1 / b2
b1 = the number of bits in the original image
b2 = the number of bits in the encoded image
What are the steps from A to the compressed image?
- Transformation
- Quantization
- Symbol encoder
What are the steps from compressed image to A?
- Symbol decoder
- Dequantizer
- Inverse transformation
What does symbol refer to?
Symbol is a general term that can refer to pixel values or transformation coefficient values.
What happens in the transformation step?
- Converts the input image A into a format that is better suited to encoding.
- This transforms the pixels into coefficients.
- For most natural images a significant number of transformation coefficients have negligible magnitude and can be discarded.
- The remaining coefficients can be coarsely quantized – thereby reducing the number of bits required to encode them.
What happens in the quantization step?
- The quantizer achieves compression by approximating the transformation coefficients.
- Quantization is an inherently lossy process whereas the image transform and coding processes are not.
- Therefore a non-lossy compression method can not comprise a quantization component.
What happens in the symbol coding step?
Symbol coding compresses the image by exploiting the fact that in natural images some grey-levels occur more frequently than others (also applies to transformation coefficients).
What is the JPEG image compression recipe?
- Split into 8x8 blocks and treat each block separately.
- Apply Discrete Cosine Transformation (DCT) to each image block.
- Quantise the DCT coefficients (transformed 8x8 pixel block) .
- Apply Huffman coding scheme (symbol coder) to quantised coefficients.
In JPEG image compression transformations where are the significant coefficients found?
The significant coefficients of the DCT are usually located in the top left-hand corner
What does a quantizer do and how is it achieved?
- A quantizer reduces the number of bits needed to store the coefficients that result from an image transformation.
- This is achieved by reducing the precision of the coefficients.
What is cosine referred to?
The basis function
What does weighting refer to?
Coefficient and relates to pixel values