Posts Tagged ‘resolution’

Bitmaps, PPI and DPI

Hi folks,

As promised in the previous article, today we will try to untangle the frequent confusions generated among many computer users by this simple word : “resolution”.
To do that, we are going to explain screen resolution, printing resolution, pixel dimensions, physical dimensions and how everything relates with regard to bitmaps.

Bitmaps dimensions are expressed in pixels (image width x height) either separated (usually for image files) or as the effective result of the multiplication (usually to express digital cameras sensitivity).
For example, an “8 Megapixels digital camera” means the photos it produces (actually bitmaps) are made up of cca. 8 million pixels, a photo being, for example, 3456 pixels wide by 2304 pixels high, which gives 7,962,624 pixels (cca. 8MP).
Funny thing is that the pixel dimension of the image has little to do with how the bitmap image appears on screen but is has lots to do with how the image will print!
How comes?

Well, to begin with, a pixel is a logical unit, not a physical one.
In other words, a pixel doesn’t have a fixed measured size, its size depends on the screen it is displayed on.
For example, imagine an LCD monitor having a screen resolution of 1000×1000 pixels, the screen’s physical dimensions being 1000×1000 mm.
In such case, the dimension of a pixel will be 1 square milimeter (1×1 mm).
Now imagine that you change the output resolution setting of the same monitor to 500×500 pixels: as monitor’s physical dimensions don’t change, a pixel will now be 2×2 mm, meaning 4 square milimeters.
To continue this exercise of imagination, think about a bitmap image having pixel-dimensions of 1000×1000 pixels: in the first case (screen resolution of 1000×1000) the image will be rendered full screen.
And for the second, case (chaging the screen-resolution of the same monitor to 500×500 pixels), the image will appear bigger (because each pixel is bigger than before) and will be displayed only partially on screen.
Of course, there are no such monitors/resolutions standards in real life, we only gave these examples to show that image size in pixels is relative, not “absolute”.
So perhaps at this point it will be easier to understand what pixels-per-inch (PPI) is : it is a value expressing how many pixels “fit” inside an inch but its significance depends on the context.
-when used with regard to an image resolution, PPI (often reffered to as DPI in this context) is the number of pixels per inch in the bitmap grid and is meant for printers (to determine how the image is to be printed within a specified size);
-when used with regard to a screen resolution’s appearance, PPI is the number of pixels per inch (or pixels-per-centimeter (PPCM) when metric system is used) depending on screen’s physical size and screen’s resolution as set by user;
Here are a few real life facts to help understanding:

-a monitor in 800×600 mode has a lower PPI than the same monitor has in a 1024×768 or 1280×1024 mode;
-a monitor of 12 inches wide (horizontal) by 9 inches height (vertical) at a resolution of 1024×768 pixels has a PPI value of cca. 85 (1024 pixels / 12 inches = 768 pixels / 9 inches = 85.3);
-a monitor on a Windows Operating System typically displays at 96 PPI;
-a bitmap image of 1,000 × 1,000 pixels, if labeled as 250 PPI (DPI is frequently misused to replace PPI in such context), will instruct the printer to print it at a size of 4 × 4 inches.

With printers there’s a different story.
First of all, there is an important difference between how an image appears to the human eye on screen and how it appears when printed.
Due to visual perception physiology, an image don’t need to have a very high resolution in order to appear at a decent size and with good quality on a computer screen.
Unfortunately, this is not the case when it comes to printing it : what is seen on print simply doesn’t match the quality of what is seen on screen.
Screens displays colors out of red, green and blue (RGB color model), by mixing them together into a vast color palette and light is emitted directly to the eyes.
White is obtained when all 3 colors are displayed at full intensity, black is obtained by their absence (zero intensity), hence the name “additive color model” for RGB.
Printers work differently : they create colors by mixing cyan, magenta, yellow and black inks (CMYK color model but there’s also a CMY model), inks absorbe light and human eyes see light reflected from paper.
White is obtained by using none of the 4 colors (as it’s the paper’s color) while black is obtained by full combination of all 4 (or 3, for CMY) colors inks, hence the name “substractive color model” for CMYK.

Similar to how pixel is the picture element of an image on screen, a dot is the picture element of a printed image.
And similar to how PPI value describes the density of pixels on screen, the DPI (“dots-per-inch”) value expresses how many individually printed dots “acomodate” within an inch.
Of course, the higher the DPI value, the better quality of the printed image will be.
But printers have a limited range of colors for each dot and their color pallette is lesser than in the case of screens.
So in order to obtain similar output quality, a bitmap image has to be printed in much higher DPI value than the PPI value needed for good screen viewing.
It is said that the printing process “could require a region of four to six dots (measured across each side) in order to faithfully reproduce the colour contained in a single pixel” so if a 100×100-pixel image is to be printed inside a one inch square, the printer must be capable of 400 to 600 dots per inch in order to accurately reproduce the image.

Finally, as if the already described confusions wouldn’t be enough, DPI is used to also express the resolutions in scanning processes, whereas the correct term to use appears to be “samples-per-inch” (SPI).

Hope that from now on, when using PaperScan it will be easier for you to deal with resolution-related terminology in bitmaps!

See you next week !

Bogdan

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Bitmaps : some explanations

Hi folks,

As shown in a previous article, all raster (bitmap) files hold information on each and every pixel in the grid that makes an image.
Today we are going to describe some consequences of this storage principle and, hopefully, we will shed some light on some related, “esoteric” terms.

First, all bitmap images are rectangular, as there has to be a grid (vector images, being defined by mathematical equations not by pixels, aren’t “restricted” to rectangular shape)

Secondly, bitmaps are resolution-dependant.
That means the bitmap stores information on a given (fixed) number of pixels only.
If you want to rescale the image to a lower resolution, some pixels from the original grid will have to be discarded while rescaling to higher resolution means new pixels (not existing in the original grid) need to be created.
Down-scaling a bitmap (for example when creating a thumbnail) produces far less damages to the quality of the image as opposed to up-scaling it.
So let’s take a closer look at the up-scaling case: zooming-in a bitmap image (in order to get a better view on details or to enlarge the image) won’t reveal new information at all, because such information simply isn’t there, the bitmap contains information only for the given number of pixels and that’s it.
And beyond a certain threshold, each discretely visible pixel composing the original image will became a bigger and bigger rectangle, altering the image more and more.
To reduce (eliminating is impossible) this upscaling problem, new pixels are created based on original pixels color information and this process of estimating new intermediary values from known existing values is called “interpolation“.
There are several interpolation algorithms, 3 of which are most commonly used: “nearest-neighbour“, bilinear and bicubic interpolations.
Nearest-neighbour is is the most simple but less effective interpolation method, producing jagged edges (looking like stairs) on increased-sized image while bicubic is much more complex and generates smoother straight-lines and curves.
But in the end of the day, no matter their complexity, all algorithms are creating the new pixels by some more or less reasonable aproximation or “educated guess“, so to speak.

Resolution-dependency of bitmaps imposes for another important aspect to be discussed: screen and printer rendering.
This subject contains many confusing terms, some of them commonly misused, so we will dedicate our next blog article entirely to that matter, in an attempt to untangle frequent confusions.

A fourth thing about bitmaps is about converting a bitmap format to another one and is actually good news.
As they are all based on the same encoding principle (storing information on all pixels in the image) conversion between different bitmaps file formats is rather easy.
Most popular bitmap formats are non-proprietary (have open specifications) like PNG, TIFF, JPEG or GIF.
BMP format was invented by Microsoft but being a relatively simple, well documented and free of patents format, it became quite a common raster image format.

See you next week, folks!

Bogdan

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