Photoshop

SUBATRACTIVE COLOURS
All substances absorb, transmit or reflect specific wavelengths of white light. When an object absorbs some light, only the remaining mixture of reflected or transmitted wavelengths is detected by our eyer. An opaque white material reflects all wavelengths, whilst a black one absorbs them. Translucent or transparent materials absorb or subtract certain wavelengths of whitelight and transmit the others. All of the spectral colours can be produced from a white light source by passing it through single or pairs of translucent CMY filters. This is a subtractive process since the transmitted light will be less intense than the light source. A cyan filter, which transmits blue and green light but subtracts red light , followed by a magenta filter, which subtracts greenlight, result in only the blue light being transmitted. Weakening the cyan filter allows some red light to be transmitted, producing violet light.
Colour photography materials incorporate variable density, subtractive CMY dyes, which filter light to reproduce life like images. In printing techniques such as offset lithography the density of the CMY process inks cannot be continuously varied acrossan image, so a range of colours is produced by a halftone technique, where CMY dots of variable size are printed in overlapping grids. The smaller the dot, the less light it will absorb, decreasing apparent density by increasing the amount of reflected light. Process ink pigments are less pure than photographic dyes, so pure black cannot be obtained by over printing solid CMY inks. For thisreason, black (K) ink is printed in addition to, or instead of dense CMY combinations.
Process ink impurities, combined with the incomplete reflectance of printing paper generally result in a smaller colour gamut than photographic materials.

COLOUR BASICS
Our perception of colours in nature is determinate by three factors the type of light source, how substances change the reflected or transmittedlight, and the sensitivity of our eyes to the resulting light.
The sun radiates a wide variation of electromagnetic waves, each having a different wavelength. The human eye is sensitive to only a small range of these wavelengths, known as with light.
Rainbows are created when white light is split up by droplets of water. Passing a beam of with light through a glass prism produces a similareffect. The shorter wavelengths are bent (refrected) more than the longer ones, splitting the with light into its component spectrum of visible colours. Each colour causes a specific reaction in the eye is red, green and blue cones or receptors. Yellow is perceived by both the red and green cones, for example.
The spectrum colours are the basic building blocks of a much wider range or gamut ofcolours. When selections of these pure wavelengths are mixed or added together in differing proportions, thousands of different colour sensations can be perceived.
ADDITIVE COLOURS
Colour monitors an TV is mirror the function of the eye by emitting red, green and blue colours (RGB) the three primary colurs can be composed by adding these primaries in different proportions and intensities, giving riseto the term additive mixing. Green and blue light result in cyan (C), red and blue light make magenta (M), and red and green light form yellow (Y). C, M and Y are known as the secondary colours of light, or the primary colorants when referring to pigments. White light is produced when red, green and blue are added in similar proportions, where as black results from their total absence. In reality,the black displayed on colour monitors is likely to be a dark green or brown grey due to stray light emissions. The gamut of colours that can be displayed on a monitor is smaller than that seen in nature because it is limited by the characteristics of the phosphor screen coatings that emit the light.

RESIZING BITMAPS
Any bitmapped image has specific resolution or number of pixels per inch....