Lecture 1/2 Mathematical background/Images and Colours Flashcards
What is the reflectance formula?
E: incident radiance
S: reflectance
R: outgoing radiance
lambda: wavelength
Visualize the visual spectrum
What are the purpose of rods and cones in the retina of the eye?
4 types of photoreceptors:
1 Rods: grey scale
3 Cones: RGB scale
Can RGB generate every
perceivable colour?
Yes, in theory, but some weights
would need to be negative …
In practice, R, G, B can generate a good
fraction of all visible colours
How are digital images perceived?
Digital images are 2D sampled representations (a 2D array)
of some continuous function, like a real scene
What are the sampled digital values of an image?
The sampled digital values are called pixels
– The smallest individual element in an image
- 1 pixel = single sampled colour
– Usually represented in
Red, Green, Blue (RGB)
– Can represent many
different colours
What is Rasterisation?
Rasterisation: convert a continuous or vector image
representation to a rectangular sampled grid of pixels
How is an analog signal sampled?
e.g. in a digital camera
Digital cameras have a CCD or CMOS sensor
with light sensitive elements, typically
arranged in a “Bayer” pattern
Visualise Image storage (grey scale)
Digital images are arranged in memory in scanline order:
– pixels from left to right within a row
– rows from top to bottom
Calculate the index in memory (row based index)
Calculate the index in memory (column based index)
Calculate the index in memory (row based index)
Colour images are arranged in memory with rows of RGB pixels:
Visualise Image storage (RGB)
Colour images are arranged in memory with rows of RGB pixels:
4 values per pixel (RGBA: RGB and “alpha” for opacity)
Calculate the index in memory (column based index)
Colour images are arranged in memory with rows of RGB pixels:
List the colour spaces and their applications
Consider a 10 megapixel image, stored
with RGB colours and 32 bits per pixel:
10×106 pixels × 4 bytes/pixel = 40 MB
How many bits in a byte?
How many pixels in a megapixel?
8 bits in a byte
1*10^6 pixels in a megapixel
What is a Lossless compression example?
Lossless compression: e.g. PNG/GIF using
LZW (Lempel-Ziv-Welch) coding (enumerate
frequent strings)
What is a Lossy compression example?
Lossy compression: e.g. JPEG uses the DCT
(discrete cosine transform) to encode highfrequency components with fewer bits
What are additive colours?
What are subrtactive colours?
Why might CMYK be used over CMY in a printer?
add black (K) to:
– accurately print black
(C+M+Y = muddy grey)
– save ink compared to
C+M+Y (lots of ink
soaks paper)
– print fine black text or
lines without having to
align colours carefully
– save money: black ink is
cheaper than coloured
What is spot colour used for?
reproduce one colour perfectly
What is a larger gamut used for?
reproduce more colours, giving a larger range
Beyond CMYK? …
larger gamut: reproduce more colours
accuracy: more precise colour reproduction
spot colour: reproduce one colour perfectly
Humans also perceive the brightness of different colours, how?
What colour is a traffic cone?
What do HSV and HSL stand for?
When are they useful?
Often used for colour selection
in applications
– Hue, Saturation, Value (HSV)
– Hue, Saturation, Lightness (HSL)
What is CIEXYZ?
A 3D space with XYZ axis, contributing wavelengths of RGB to the perceived colour.
X: red/green
Y: blue/yellow
Z: white/black
What is CIE xyY?
A 2D space with xy axis, derived from CIEXYZ
x: chromaticity of colour
y: luminance (brightness)
