Matric image

A matric image (or image as a mode not , or English a “bitmap” or “”) is a Digital image stored in a file in a Format of data which is composed of a table of Pixel S or points of Couleur, generally rectangular, which can be visualized on a Moniteur of computer, any other device display, or simply a sheet of paper.

History

In 1672, Isaac Newton shows using the prism that the white light proceeds of the addition of all the colors (Newton will count seven colors, arbitrary number and culturally marked). In 1839, the year of the birth of photography, Michel Eugene Chevreul publishes a book explaining the optical effects produced by the colors and their juxtaposition, i.e. not the superposition of coloured layers (filters) or mixes it colors, but the effect produced by different colors put side-by-side and seen by far. In 1869 (the May 7th precisely), without knowing and without to have worked together, Louis Ducos of Hauron and Charles Cros propose at the French company of photography a process of their invention which makes it possible to obtain stereotypes colors. All this work will mark Georges Seurat, the creator of the Pointillisme (or néo- Impressionnisme), and will be at the origin of color printing or of television colors.

It would also be necessary to speak about the textile processes, in particular those of Jacquard, which consider the images as dot matrices (textile industry was the first to use besides the programming by perforated cards). One can also think of the technique of the mosaic already practiced by the Greeks and the Romans where the images are represented by the juxtaposition of small earthenware or coloured stone squares (the tesselles ones). All this is at the origin of the images “in chart of point”, or English, “ bitmap ”.

Principle

The coding or its data-processing representation of an image implies its Numérisation. This digitalization is done in two spaces:

space space where the image is digitized along the axis of the Abscisse S and the Ordonnée S: one speaks about sampling. The samples in this space are named pixels and their number will constitute the definition image.
the space of the colors where the various values of Luminosité which a pixel can take are digitized to represent its Couleur and its intensity; one speaks about quantification. The precision in this space depends on the number of bits on which one codes the luminosity and is called Depth image.

The quality of a matric image is determined by the full number of pixels (called its definition) and the quantity of contained information in each pixel (often called depth of digitalization of the colors).

Definition of image

The definition of an image with its related concept of resolution defines the level of details which will be visible in the image. The more there will be pixels, the more there will be visible fine details. It is said that the more one image has pixels, the more it is of great quality. An image digitized with a resolution of 640×480 pixels (thus container: 307200 pixels) will appear very approximate and in the form of a paving of small squares of color, by comparison with a digitized image with 1280×1024 (: 1310720 pixels).

Since it costs a great quantity of data to store an image of very great quality, techniques of Data compression are often employed to reduce the size of the images stored on a disc. Some of these techniques lose information, and thus impoverish the quality of the image, in order to carry out a file occupying much less place on disc. The techniques of compression which lose information are known as destroying.

Coding of the colors

The colors of the images can be is coded according to a pallet, i.e. that information of color of each pixel indicates the row of a color in a predetermined list. The format GIF is a format of image using a pallet.

One can also make correspond directly a color by coding a red triplet of colors , Vert and Bleu in information associated with a pixel, of which the number of bits used will indicate the maximum number of possible colors. For example 16 million colors for a standard posting on 24 bits including 8 for each component of the space of color RVB or 2 only for one posting Monochrome with only one bit by pixel.
Le green component has sometimes more bits than the two others, to offer to the human eye a greater understanding of this color.

Various representations

It is necessary to distinguish the various representations of a matric image.

In a file, for storage and the exchange. In this case, the image is generally compressed and stored in a graphic format. The principal matric formats are BMP, GIF, tiff, png and JPEG.
In graphic memory of the computer or the Graphics card. This format is generally without any compression to be able to be directly exploitable and displayable on the screen.

Comparison with the vectorial images

When a matric image is enlarged, since one adds no information which would not be already present, that induces a visible loss of quality. More exactly, once an image is digitized, its definition is fixed and its visual aspect cannot improve, even by using better display devices or easy ways of posting. An increased digital image is known as pixellized .

Principle of posting

How are posted the matric images: let us give an example, with the letter “J”: J

By looking at this letter well on the screen of the monitor, and by approaching a magnifying glass, you can observe a chromatic aberration on the edges of the magnifying glass. You see a “J”, the computer sees anything else only “. ” which represents one zero and one “X” which represents a one:

….X ….X ….X ….X X… X .XXX.

Where there is one zero, the computer orders with its video material to paint the fill color and when it meets a one he asks him to fix the current color of foreground. It is actually complicated a little more, but all is brought back basically to a course of bit out of bit, by making the distinction between the colors of the pixel adjacent, to form an image together. It is the basic principle of the posting of a drawing on a computer.

Restitution

At the end of the 20th century, the monitors of computer could post approximately between 72 and 96 points by inch (dpi, is 28 to 38 point per centimetre), whereas the modern Imprimante S can reach resolutions of 600 dpi even more (236 points per cm); thus to work with images intended for the impression can prove to be difficult or require large monitors and very powerful Ordinateur S. The monitors with resolutions of 200 dpi (79 point per cm) were available for the general public towards the end of 2001 and of the higher resolutions are awaited in the years to come.

The images intended for the professional impression are worked with 300 dpi (118 points per cm) and in CMJN (cyan, magenta, yellow, black, representation by subtractive Synthèse). They occupy between a score of megabytes (~20 Mo) and more than 100. RVB is a pallet of posting which corresponds to the additive Synthèse.

Let us note that an image of 640×480 on a screen of 36 cm (14") a resolution of 22,2 points per cm has, that is to say 56 dpi. An image of: 1600×: 1200 on a screen of 53 cm (21") a resolution of 38 points per cm has, that is to say 96 dpi.

Analogy in 3D

In computer graphics 3D (in three dimensions), the concept of a plane screen of pixels is sometimes extended to a three-dimensional volume formed of small paving stones called “voxels”. In this case, there is a regular grid in three-dimensional space with elements containing the information of color for each point of the grid. Although the “voxels” are powerful abstractions to treat the forms 3D complexes, they ask much memory to be stored in a table of rather big size. Consequently, the vectorial images are more often used than the “voxels” to produce images in three dimensions.

Use

The matric images were initially patented by Texas Instruments in the Années 1970, and are now omnipresent.

The matric format was used to send a message in space bound for an extraterrestrial intelligence: it is about an image black and white representing the Solar system, of dimensions N on m , N and m being prime numbers. Only the series of N × m bits is transmitted, dimensions of the image must be deduced from the Décomposition in factors first; the transmitters hope thus that the receivers will be able to break up N × m into its two factors, and to thus recompose the image.

See too

vectorial Image
Cast iron bitmap
editor of image bitmap
Typography

JPEG
GIF
png
tiff
EBX
Format of data for the images

For practical informations for the use of the matric images:

graphic Programs
Formats of graphic file

External bonds

bitmap on FOLDOC (in English)

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