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Pixels, Dots, and Lines per Inch

Megapixels

A digital camera uses pixel elements (also known as a pixel) to capture images. More number of pixels makes the better resolution of that image.

Formula to calculate megapixel of a camera:

Example:

Let’s take dimension of an image: 2500 X 3192

So, according to formula

(2500 * 3192) / 1 Million = 7.9 = 8 megapixel (approx).

Aspect ratio

Aspect ratio is the ratio between height and width of an image. A colon is used to separate two numbers. Different images on different screens have a different ratio.

Common aspect ratio is as follows:

1.33:1, 1.37:1, 1.43:1, 1.50:1, 1.56:1, 1.66:1, 1.75:1, 1.78:1, 1.85:1, 2.00:1, e.t.c

Advantage

1. It maintains balance in the appearance of an image (ratio between horizontal and vertical pixel).
2. When the aspect ratio is increased, images do not distort.

For Example:

Above image has 100 rows and 100 columns. If we resize the image, the image quality will remain the same.

Now changing the value of row and column, we get the image in balanced resolution.

For example

Here is a grayscale image given with aspect ratio 5:2, and the resolution of the image is 480000 pixels.

Calculate the following things:

1. Dimensions of the image.
2. Size of the image.

Given

Aspect ratio: c:r = 5:2
Pixel resolution: c * r = 480000
Bits per pixel: grayscale image = 8bpp

Solution 1:

Equation 1 —————————–

Equation 2 —————————–

Comparing equation 1 and equation 2

Putting value of r in equation 1

So, row = 438 and column = 1095

Solution 2:

Pixel Resolution

In pixel resolution, the term resolution refers to the total number of pixels in a digital image. If an image has M rows and N columns then a resolution is defined as M X N. The quality of the image depends on the pixel resolution.

As we can see pixel resolution of the above image is 300 X 168.

Pixels per inch

Pixel per inch is used to measure the density of a pixel of an electronic device such as a computer monitor or image digitizing device such as a camera.

Horizontal and vertical density is usually the same because most devices have square pixels or non-square pixels depends on the device.

To calculate the PPI first, we have to calculate the number of pixels that fit on the diagonal:

Where,

w = Number of pixels on the horizontal line

h = Number of pixels on the vertical line.

Now, to calculate PPI as the ratio

Where,

d_i =diagonal screen size in inches.

Example:

Calculate PPI of a smartphone.

Let the diagonal size of smartphone be 6 inches. And vertical and horizontal pixel count be 650 and 450 pixels.

d_o = √(w² + h²) = √(1440² + 2560²) = 2937

PPI = d_o/d_i = 2937/6 = 489

Dots per inch

The definition of DPI is the same as PPI. DPI is used to measure the number of points which is equivalent to 1 inch.

In an ink printer DPI setting is used instead of PPI. Because in a printer, dots per inch means the number of ink dots per inch. DPI can be higher and lower depending on the printer’s characteristics.

Formula to calculate DPI

Example

Let PPI be 489
DPI = 1 / 489 = 0.0020 mm

Lines per inch

Lines per inch are used to measure the resolution of images which are printed in halftones (prints image in series of dots). Higher the LPI number, dots will be denser which result in excellent resolution.

Newspapers are printed in the resolution of 85 Lines per inch whereas magazines are printed in a resolution of 133 Lines per inch.

Below table shows the lines per inch capacity of the printers:

Printer	LPI
Screen printing	45-65 lpi
Laser printer (300 dpi)	65 lpi
Laser printer (600 dpi)	85-105 lpi
Offset Press (newsprint paper)	85 lpi
Offset Press (coated paper)	85-185 lpi

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