What is a pixel?
You’ve no doubt heard of the word pixel, and you may have some idea what a pixel is, but if you don’t, this post will go into detail to describe a pixel.
The simplest way to describe a pixel, or ‘Picture Element‘, is to define it as the smallest physical element in a digital image, or the most finite level of detail in the image. If you look at the image below, this is a zoomed in view to show some individual pixels: 
Pixels are essentially like tiles, each one of a single hue of colour, and together they form an image when viewed from a distance. Each pixel can only have one colour value. So when you are looking at a digital image, such as in Adobe PhotoShop, what you’re seeing is an image composed on heaps of tiny coloured squares laid out in a grid. Pixel based images are referred to as raster images.
It’s not possible to get any further detail beyond this basic pixel, or to extract any further detail from within it.
If you watch those crime shows on TV, you’ll see some wonderful things happening with digital imaging technology that allow them to find hidden clues within the pixels of captured footage by zooming in and ‘enhancing’ individual pixels to reveal a word, or reflection of a person. It’s understandable that some people may think it is possible to do this to some degree. This is impossible. The image above is an eye and is very low resolution. The pixels above don’t hold anymore information to determine any further features of the eye whatsoever. This is a far as this image can go. To see further detail, you’d need to take a much higher resolution image.
I love this example below from a well known TV show of what can’t be done with pixels: http://youtu.be/IRBo5ZGcyVA
Time to get really technical with pixels!
Pixel Resolution
The number of pixels present within an image define the image’s pixel resolution. The final pixel resolution of an image is defined by the width expressed in pixels and height expressed in pixels.
Digital cameras capture images as pixel elements, and the more pixels per image, the higher the image resolution.
Megapixel ability is a term often used with digital cameras. A megapixel is about one million pixels. A camera that takes photos at three megapixels will have lower resolution images than one that can take images at 10 megapixels. This reference is to the resolution of the images, not the file size. In essence, the pixel resolution refers to the image quality, and the more pixels, the better the quality. Other mechanical factors with digital cameras can change this also – a cheap camera taking images at 10 megapixels will not capture images as well as a good quality 10 megapixel camera.
Resolution is also further determined by the required output use of the image and can take on more specific definitions. The above describes a finite image resolution in pixel size. Most designers will want their images supplied with a high pixel resolution.
Image Resolution
DPI Resolution
A lot of people refer to 72dpi, or 300dpi images, when in fact they mean 72ppi, or 300ppi (which is pixels per inch). This is again an image resolution expression, but in this case this is just one of three measurements required for an image resolution for a specific output size and use.
When a designer or publisher requires an image of a certain resolution, at this stage, they are not concerned with pixels per inch. It really doesn’t matter as that comes later. Their concern is for an image that is of a good enough pixel resolution for use in various subsequent outputs, and will more than likely request the original, high pixel resolution image that is larger than the required output.
The reality is, when producing an image for a specific output size and use, the actual PPI of an image means absolutely nothing for resolution without a width and height attribute. It must have a resolution AND dimensions. Saying an image is 72ppi or 300ppi alone really doesn’t provide enough information as to the actual physical resolution required for it’s specific output. You can have a 300ppi image that is 100pixels by 100pixels, no good for print, or the same image that’s 72ppi but 3500pixels x 3500pixels, perfect for print. The designer will definitely want the 72ppi version in this case as all they’re interested in is the 3500pixel x 3500 pixel dimensions, and they’ll adjust the size and ppi where necessary as part of their mechanical processes for the required output.
An image produced by a digital camera can be adjusted to be any ppi (even to 1,000ppi), and it’s actual pixel resolution will not change in the slightest if its width and height pixel proportions are constrained (ie, kept at their original pixel width and height dimensions).
What PPI does do is determine the image’s physical resolution and dimensions when viewed at 100%.
Lets take a one megapixel image, and change the PPI, but keep the pixel width and height attributes the same. A one megapixel image is about one million pixels, which will have a width of 1,000 pixels and a height of 1,000 pixels. That’s actually quite a small image for print use.
- Converting that image to 72 pixels per inch will provide an image which is 13.889 inches, or 35.28cm wide and tall at 100%. Bear in mind that this would still be the physical print size of the image at 100%. It’s also worth noting that at that width & height AND resolution for print, the image will appear fuzzy as that is not an optimised size for print. (Note the combining of resolution and dimensions – they go hand in hand when producing images for an output size.)
When viewed on screen, this image zoomed to display at 100% will display at the resolution of the screen, not the PPI output size, and will not be 35.28cm wide and tall at 100% (unless the screen is 72ppi). So on my Mac the 1000 x 1000 pixel image at 100% will always display at 109ppi, or 23.3cm wide and tall on the screen. It’s common practice to set images to around 72ppi-96ppi for digital use such as online. The advent of Retina display devices slowly coming onto the market will eventually change this.
- Converting that same image for print output (300ppi) whilst constraining the pixel width and height dimensions will change the physical size of the image to a smaller size, and increase the resolution of the image at that smaller size, but it will not change the overall actual finite pixel resolution of the image itself, as it is still 1000 pixels x 1000pixels in size.
1000 pixels will be 3.333inches, or an image printed at 8.47cm x 8.47cm. So when a graphic designer needs a 300dpi image for printing, don’t go to a website and convert that to 300ppi, as the print dimensions will be too small for the image to print at 300dpi plus the width and height dimensions that the designer needs. The designer needs an image at the correct resolution AND dimensions, or more importantly, a high pixel resolution original that can sized down to the correct resolution AND dimensions for the print output.
Changing an image resolution
Increasing an image size
OK, so can you just increase an image’s pixel dimensions to make a larger image?
No.
Well, you can, sort of! Adobe Photoshop and other image editing software will allow you to do this, and they will fill in the ‘gaps’ between each pixel by a process of interpolation. However, at best they need to create new pixels from nothing to increase the image’s pixel density, and can only guess what sits between the existing pixels based on the existing pixels. Remember at the start I mentioned that the pixel is the most finite level of detail present in an image, so increasing the image size will not allow for more detail to be added that doesn’t exist in the original image. The lower resolution image can only capture a certain amount of detail with finite number of pixels it possesses.
This process can be used to ‘get away’ with some up-sizing to some degree – the quality is not that great, but visually, if you’re not too concerned, then it may do the trick. Ideally, as a designer myself, I usually avoid this at all times.
If you were to do the above, you would end up with something similar to this below. The left image is 79px x 100px. The right one has been interpolated upwards in Adobe Photoshop to double the width and height. At best, it looks blurry. it seems some detail has been regained, but this is in fact just an optical illusion. It just seems more detailed is there as are more pixels, but it will always be blurry:
Resize the original 79px x 100px image to 740px x 937px size as below, and this is what you get below. It will not get better, no matter how hard you try:
There are some interpolation software products online that use a myriad of calculation processes to try and guess what the missing pixels could be like, based on the surrounding pixels, and some might produce semi-ish OK results, but they are still guessing. There’s no way at all to regain information that isn’t there in the first place. Interpolation is in no way a substitute for not having an original high pixel resolution image.
Decreasing an image size
In that case, if you do have a large image, can you decrease an image’s pixel dimensions to make a smaller image?
Yes!
Images can size down to any size with no issue. Just make sure you keep the original, full resolution image. Don’t save over it. If you do, you can’t increase the size again, as image data, and therefore pixel information is lost during the downsizing process.
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