+$When you've been working in the design industry for a while, image resolution and DPI probably make perfect intuitive sense. But for folk just starting out – or those who work with designers but aren't themselves design professionals – it can be a hideously baffling mess.

+$Even if you happily use DPI values in your day to day work, answering the question 'what is DPI?' can quickly result in you tying yourself in knots.

+$So we're here to help. This guide is designed to help everyone understand image resolution, whether you're just starting out; work in a department such as marketing where you have to deal with design professionals; or even are a seasoned expert who needs to point a colleague to a guide that helps explain why, no, just changing the resolution to 300dpi inPhotoshop CC+$doesn't magically make it a 300dpi image (well, it不...+$but not in the way I mean, okay?)

+$Before we start, there are a couple of things to note. First, some of what I say will be a simplification, and though I'll add some caveats and detail later, I want to try to keep things simple early on.

+$Second, we'll be discussing bitmap formats here (broadly speaking, things like JPEG, PNG, GIF and PSD) because they're the tricky ones when talking about image size: they contain a finite amount of detail. Vector images such as EPS, SVG and AI are usually infinitely scaleable. They have their own problems, but we're ignoring them here!


What is DPI? The ultimate guide to image resolution


所以大家都知道这一点,但我们需要从头开始。无论它们保存的格式如何,位图图像都以相同的方式形成:彩色正方形的行和行堆叠在彼此之上。放大到足够远的位图图像,您可以看到这些正方形 - “图像元素”或像素;有点像罗马马赛克中的单个瓷砖或十字绣中的缝线 - 但是拉回来,你的眼睛只是将它们视为平滑的渐变和不间断的线条。




+$(Technically, you only use 'dots per inch' when discussing printed output, while 'pixels per inch' is for displays and digital images, and 'samples per inch' for scanners, but the concepts are all so closely related that at this stage in the explanation, you can treat them interchangeably. This is the most egregious lie to children in this piece; clarification coming later.)

+$How big are the pixels?

+$The above sounds pretty simple: a higher DPI equals a clearer picture, right? Well, there's a problem. The problem is that the size of the pixels – or tiles, in the analogy above – is only one of two factors that govern image size and quality. The other is the physical size of an image once it's printed, and the thing that ties them together is how many pixels there are in an image: the pixel dimensions, often expressed as a number of pixels wide and a number tall, such as 640×480.

+$(In photography, image size is usually expressed as megapixels, but that's just arrived at by multiplying the number of pixels wide by the number of pixels tall the images a camera produces are. 640×480=307200, and since that answer divided by a million – the 'mega' in megapixels – is 0.3, early cameras could be said to produce '0.3 megapixel' images.)

这两件事 - 每英寸多少点,以及图像最终的大小 - 完全取决于彼此(在例B之后会出现一个警告);改变一个,另一个改变。





+$The grid of pixels that makes up your computer screen never change size – 'a pixel' is an abstract, sizeless thing – but the pixels that make up an image can be printed at any size you like. Below, simplified, is what's happening:

What is DPI? The ultimate guide to image resolution


+$On the left are the original images. On the right, each has had the number of pixels reduced to about a quarter of the original values while maintaining the same physical size. This means that each individual pixel has got bigger and thus the DPI value has dropped (since there are fewer dots fitting into every inch) and the image becomes blurrier. Below, simplified, is what's happening:

What is DPI? The ultimate guide to image resolution

+$What Is Interpolation?

和我一起到目前为止?好,因为我要扔一个曲线球。虽然示例A不会更改图像中的信息量,而示例B会减少它,但您可以通过称为插值的过程添加更多信息。可以这样想:如果你的黄色像素旁边有一个蓝色像素,并希望在它们之间添加第三个像素 - 那么你可以在保持图像相同的物理尺寸的同时增加DPI,或者通常增加在保持类似DPI的同时可以输出的大小 - 然后计算机可以猜测该像素应该是绿色的。

+$Sounds great, but in practice, you can't just magic up loads of information that wasn't there in the first place. That's why taking, say, a 150dpi image and making it 300dpi (at the same physical size) isn't a solution to the problem of not having sufficient resolution.


What is DPI? The ultimate guide to image resolution


What is DPI? The ultimate guide to image resolution