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About Image File Formats

This article deals with the subject of image file formats, mentioning the concept of file format in general, the difference between closed format and open format, and the most common universal image file formats, vector or bitmap, such as JPG, PNG, TIF, GIF, WEBP, EPS and SVG.

Closed And Open Formats

A file format corresponds to a certain coding standard established for storing information in a computer, that is, it establishes how bits are used to store types of data. Each file format corresponds to a different standard that can vary greatly in complexity depending on the content of the information stored. The most common contents are usually classified by the main and popular distinct groups: images, audio, video, document, data, font, internet and programming (although the growing demand of new applications has been increasing more and more format structures).

There are proprietary or closed formats as opposed to free or open formats. Open formats are specified and maintained by an organization established for that purpose and can be used on any system, by anyone and for any purpose, such as the JPG format (developed by the Joint Photographic Experts Group) which can be edited on any operating system (Windows, MAC OS, Linux, Android, among others) using any image application, whether free, proprietary or open-source.

Closed formats are considered trade secrets, involve special coding standards developed by the organizations that own them, implying specific software in their use, as for example the PSD image format that can only be edited in its program or package of origin, (Photoshop – Adobe package), although between proprietary programs there are often possibilities of importing formats from other proprietary ones, but not between free and open-source programs.

The file format is identified by its extension, which corresponds to the abbreviation after the dot separating it from the name given by the user (Filename.extension).

The file formats dealt with in this post are in the images group, subdivided into two types – bitmaps and vectors.

Image Types

In graphics editing, there are two basic and completely distinct types of image: the bitmap and the vector image.
Bitmap – The bitmap, also called raster image, is composed of a matrix of pixels whose color depth is stored in bits mapped into rows and columns (more knowledge in the previous post).
Vector – A vector image is not made up of pixels, but of vectors – mathematical calculations and instructions that define lines and shapes spatially, in two or three dimensions.

bitmap Bitmap image

Generally speaking and in computer graphics, bitmap is a series of bits representing a raster graphic image (an image converted into pixels), each pixel being represented as a group of bits (see image structure for better understanding).

raster image Raster Image

A raster image is obtained from a scanning pattern of parallel lines, a two-dimensional array of cells (pixels) that form the display of an image projected on a display screen. As an end result, in practical terms, when we refer to a raster image we end up referring to a bitmap because when we rasterize an image (whether printed or digital) we always get a bitmap – bits are directly related to pixels.

The optimal choice between using bitmap or vector is defined by the purpose of the image, whenever possible, and according to the following:

Bitmap

Resolution : Dependent on original resolution and resizing, no perfect definition
Resizing : Affects quality
File Size : Tends towards large (or heavy) sizes; depending on resolution, image dimensions, depth and color mode, and compression
Image control : Per pixel
Types : Color depth and compressed
Free popular file formats : EPS (mixed content), TIFF (usually color depth type, but depending on algorithm storage option), GIF, JPEG, PNG and WebP (compressed type); only EPS, GIF (free format since 2004), PNG and WebP support transparency
Purpose : Continuous tones – photography, digital painting and web environment; in printing, the bitmap is linked to the four-color process (CMYK)

Vector

Resolution : Independent of any factor, with permanent perfect definition
Resizing : Does not affect quality
File size : Tendency towards small (or light) sizes, depending on the number of objects contained and their complexity (filters applied, fill types, effects and embedded applications)
Image control : By nodes (in lines and curves) and fill shapes
Free popular file formats : EPS (mixed content) and SVG
Purpose : Solid colors – illustrations, symbols, logos and so-called line art (monochrome image, composed of lines – no shades or gradations of tone); in printing, vector is linked to the direct color technique (Pantone ®)

pantone color system
pantone palette

Resizing and Resampling

Do not confuse resizing with resampling as these are also concepts commonly represented confusingly in imaging software interfaces and vary from software to software. Resizing refers only to the print size, it does not change the number of pixels contained in the image and therefore the pixel dimension increases or decreases depending on the defined print size. Resampling refers to the image itself and its number of pixels, which is proportional to the resolution.

A correct resampling must be done using the resolution value so that the number of pixels of the width and height follow proportionally this value. Changing the values of these two measures without proportionally changing the resolution value is common, but causes pixel agglutination and degradation of the image, since in a bitmap the pixels have a fixed size. Therefore, images of this type should be scanned or constructed from the ideal resolution for their intended purpose, to avoid subsequent resampling or at least reduce the number of unavoidable resampling.

jpeg format JPG

JPG or JPEG – a format developed by the Joint Photographic Experts Group as a standard for storing large images in smaller files by permanently compressing sections of the image into blocks of pixels, with compression ratios ranging from 2:1 to 100:1 depending on the settings. Ideal for photography in general, but not recommended for images involving flat color such as illustrations, line art or typography. The image saved in JPG format also loses quality each time it is edited and rewritten.

png format PNG

Portable Network Graphic format by indexed color method and using lossless compression, similar to a GIF file but without copyright limitations and with the possibility of merging opacity with transparency, since PNG includes an 8-bit transparency channel that allows gradation from opaque tones to transparencies. Ideal for web use and spot color printing. This format does not support CMYK profiling and is therefore not suitable for four-colour printing.

tiff format TIF

Tagged Image File Format by the true-color and deep-color methods, very popular in the graphic, editorial and photographic areas. Created in the 1980s with the purpose of application in desktop publishing (pagination, editorial area), the great advantage of TIF is the possibility of not losing quality each time the image is edited and rewritten, which makes this format somewhat larger than JPG and therefore ideal for editing, storage, but also for printing – especially large format, which requires high resolution.

gif format GIF

Graphics Interchange Format by indexed color method and which, like PNG and unlike JPG, also uses lossless compression. Originally developed by CompuServe in the early 1990s, the format includes some important features such as compression, transparency, interlacing and storing multiple images in a single file, which allows a basic way to generate small animations. However, GIF lost popularity drastically due to copyright issues which led to the creation and development of the PNG format as an alternative. The copyright involved eventually expired in 2004, but already the PNG format had gained all the ground over GIF. PNG does not allow the animations part, but has the particularity of enabling gradation in transparencies, as mentioned above. GIF is therefore only suitable for the Web environment.

webp format WEBP

Modern image file format developed by Google as a replacement for JPEG, PNG, and GIF file formats, providing superior lossless and lossy compression for images on the web, as more than 60% of websites use images because more than 60% of web users claim to be visual learners. So keeping quality but reducing the image size files, this file format leads faster webpages load, less bandwidth use, and mobile battery power save, improving user experience and increasing websites performance regarding to search engine optimization. Announced in 2010, but first supported in 2018, by the end of 2021 more than 90% of browsers, image editors and operating systems already supported this format. WebP also supports animation and transparency.

eps format EPS

Encapsulated PostScript format structured in accordance with a set of standards for PostScript (files that can be printed directly by a PostScript printer without being open in an application) which in turn corresponds to a programming language that describes the appearance of the printed image. This is a completely different method to the bitmap formats, created by Adobe in the 1980s, capable of describing images, vectors and text in the same document. Ideal for the graphics industry; can be open and totally edited using an open-source desktop publishing program like Scribus.

svg format SVG

Scalable Vector Graphics format structured on XML (Extensible Markup Language), a markup language that establishes the rules for coding documents in a double human- and machine-readable format. This format describes two-dimensional vector graphics with support for interactivity and animation. Having been created by the W3C in the late 1990s, this format was specifically designed for application on the Web.

Find references and more knowledge in Learning Links

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