pixel&resolution(像素和分辨率)

reference

• What is pixel? - Definition from WhatIs.com (techtarget.com)
• What is resolution? - Definition from WhatIs.com (techtarget.com)
• What is a Pixel? - Definition from TechopediaWhat is a Pixel? - Definition from Techopedia
• MDN

像素和分辨率

• A pixel is represented by a dot or square on a computer monitor display screen.
• Pixels are the basic building blocks of a digital image or display and are created using geometric coordinates.
• Depending on the graphics card and display monitor, the quantity, size and color combination of pixels varies and is measured in terms of(根据) the display resolution.
  • For example, a computer with a display resolution of 1280 x 768 will produce a maximum of 98,3040 pixels on a display screen.(当一个pixel大小等同于一个 dot的时候达到最多)
• The pixel resolution spread also determines the quality of display;
  • more pixels per inch of monitor screen yields better image results.
    • For example, a 2.1 megapixels(百万像素) picture contains 2,073,600 pixels since it has a resolution of 1920 x 1080.
• The physical size of a pixel varies, depending on the resolution of the display.
  • It will equal the size of the dot pitch if the display is set to its maximum resolution,
  • and will be larger if the resolution is lower since each pixel will use more dots.
  • Because of that, individual pixels may become visible, leading to a blocky and chunky image defined as “pixelated”.
    • 也就是说,如果当一个图像像素占据了过多的屏幕物理点(dot),此时的一个pixel的大小显然会大于一个屏幕dot
• Pixels are uniformly arranged in a two-dimensional grid, although some different sampling patterns are available.
  • For example, in LCD screens the three main colors are sampled at different locations of a staggered grid(交错网格), while digital color cameras use a more regular grid.
• In computer monitors, pixels are square-shaped, meaning that their vertical and horizontal sampling pitches are equal.
• In other systems such as the anamorphic(可变性的) widescreen format of the 601 digital video standard, the shape of a pixel is rectangular.
• Each pixel has a unique logical address, a size of eight bits or more and, in most high-end display devices, the ability to project millions of different colors.
• The color of each pixel is determined by the specific blending of the three main components of the RGB color spectrum.
• Depending on the color system, a different number of bytes can be allocated for specifying each color component of the pixel.
  • For example, in 8-bit color systems, only one byte is allocated per pixel, limiting the palette to just 256 colors.
  • In the common 24-bit color systems used for nearly all PC monitors and smartphone displays, three bytes are allocated, one for each color of the RGB scale, leading to a total of 16,777,216 color variations.
  • A 30-bit deep color system allocates 10 bits each of red, green, and blue for a total of 1.073 billion color variations.
• However, since the human eye cannot discriminate more than ten million colors, more color variations do not necessarily add more detail, and may even lead to color banding issues.

分辨率

• In computers, resolution is the number of pixels (individual points of color) contained on a display monitor, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis. The sharpness of the image on a display depends on the resolution and the size of the monitor.
• The same pixel resolution will be sharper on a smaller monitor and gradually lose sharpness on larger monitors because the same number of pixels are being spread out over a larger number of inches.(人口密度)
• A given computer display system will have a maximum resolution that depends on its physical ability to focus light (in which case the physical dot size - the dot pitch - matches the pixel size) and usually several lesser resolutions.
  • For example, a display system that supports a maximum resolution of 1280 by 1023 pixels may also support 1024 by 768, 800 by 600, and 640 by 480 resolutions.
  • Note that on a given size monitor, the maximum resolution may offer a sharper image but be spread across a space too small to read well.
  • Display resolution is not measured in dots per inch as it usually is with printers.
  • However, the resolution and the physical monitor size together do let you determine the pixels per inch.
  • Typically, PC monitors have somewhere between 50 and 100 pixels per inch.
    • For example, a 15-inch VGA (see display modes) monitor has a resolution of 640 pixels along a 12-inch horizontal line or about 53 pixels per inch.
    • A smaller VGA display would have more pixels per inch.

屏幕像素

• 是指,屏幕上的一个点

像素&分辨率示例

譬如,我的设备屏幕硬件分辨率为1920*1080

如果我调整分辨率

• 调整为1280*720:,那么此时屏幕上的各个元素(包括字体)都会变得很大,但这种放大会导致字体/窗口边界模糊,体验极差
  • windows输出的显示图像的信息量为1280*720,在不拉伸(不做铺满的情况下,虽然用不完整个屏幕,但是轮廓还是清晰的;但是windows将这 1280*720铺满至 1920*1080,就导致图像信息单位面积内的信息量(信息密度)很低,所以看起来边界不清晰,出现马赛克(像素)化)
• 这是因为,显示的图像上的每个pixel都被一定程度的放大了,来铺满真个屏幕,覆盖整个屏幕(1920*1080)后,这时候,图像的1pixel大于物理设备的一个点(dot)的大小

图像像素

• A pixel is also known as a picture element (pix = picture, el = element)
• A pixel is the basic logical unit in digital graphics. Pixels are combined to form a complete image, video, text, or any visible thing on a computer display.
• A pixel is the smallest unitof a digital image or graphic that can be displayed and represented on a digital display device.
• 像素是指由图像的小方格组成的，这些小方块都有一个明确的位置和被分配的色彩数值，小方格颜色和位置就决定该图像所呈现出来的样子。
• 可以将像素视为整个 图像中不可分割的单位或者是元素。
  • 不可分割的意思是它不能够再切割成更小单位抑或是元素，它是以 一个单一颜色的小格存在。
• 一个点阵图像包含了一定量的像素，这些像素决定图像在屏幕上所呈现的大小。

小结

• 一个像素到底有多大?

  • 和显示图像的设备联系起来,设备上的点(dot)是固定的size;而一幅图像所具有的有效信息量是固定的(像素数量是固定的,像素大小可以被放大,但是像素数量还是那些)
  • 当我们使用软件放大图像,图像上的像素就被放大,导致一个像素对应(占据)了更多的的物理dot;这些物理dot是实实在在的物理大小

• 具有高dpi的屏幕图像显示的关系

  • 当一幅图像(S)信息量充分大(像素充分多,对应的图像分辨率也大),在一块好的高dpi屏幕A(屏幕物理大小规格为WH)上就更充分的表现细节(假设铺满屏幕)(屏幕A的分辨率上限大)
  • 而一块低dpi屏幕B(屏幕物理大小规格也为WH),其分辨率上限小(整块屏幕含有的dot少于A屏幕);无法充分表现出原图片S的细节,同样铺满屏幕的情况下,这种情况下,屏幕B可能会采样来表示图片S
  • 图片的分辨率大于设备分辨率或者小于设备的分辨率的显示问题
    • 图像分辨率是一幅图片中像素的组成数量，如1024X768的图片，有1280X1024的图片，也有非常大的如2560X1600分辨率的图片等。
    • 当图像的分辨率大于显示器的分辨率时有两种显示方法，一种是局部显示，即屏幕的像素有多少就显示多少像素，这时只能看到图片的某一部分，可以上下左右的移动来看完整的内容。另一种方法是在屏幕内显示完整的图像，这时图片的像素会被压缩，如2560X1600的图片会删去一部分像素，以1920X1080的分辨率（显示屏的分辨率）来显示。
    • 这时可以看到完整的图片内容，不过在细节上是丢失一小部分像素的，如用 数码相机 的屏幕也可以看到完整的照片，不过感觉很模糊，放在电脑上看就好多了，因为屏幕的分辨率低。

css 像素

• 96pixel=1inch(这里的 pixel 和尺子上的长度可以直接挂钩)

开发中遇到的像素问题

使用密度无关像素(android)

• 您必须避免的第一个陷阱是使用像素来定义距离或尺寸。

• 使用像素来定义尺寸会带来问题，因为 不同的屏幕具有不同的像素密度(dpi:dot per inch;ppi:pixel per inch)，所以 同样数量的像素在不同的设备上 可能对应于不同的物理尺寸(本征尺寸))。

• 尺寸相同的两个屏幕可能具有不同数量的像素
  

• android 提供的解决方案:

  • 要在密度不同的屏幕上保留界面的可见尺寸，您必须使用密度无关像素 (dp) 作为度量单位来设计界面。
  • dp 是一个 虚拟像素单位，1 dp 约等于中密度屏幕（160dpi；“基准”密度）上的 1 像素。
    • 对于其他每个密度，Android 会将此值转换为相应的 实际像素数。

将 dp 单位转换为像素单位

在某些情况下，您需要以 dp 表示尺寸，然后将其转换为像素。dp 单位转换为屏幕像素很简单：

px = dp * (dpi / 160)

• 假设在某一应用中，用户的手指至少移动 16 像素之后，系统才会识别出滚动或滑动手势。
• 在基线屏幕上，用户必须移动 16 pixels / 160 dpi（等于一英寸的 1/10 或 2.5 毫米,(基准密度屏幕上 160pixel=1inch=2.5cm;16pixel=0.25cm=2.5mm,)），系统才会识别该手势。
• 而在配备 高密度显示屏 (240dpi) 的设备上，用户的手指必须至少移动 16 pixels / 240 dpi，相当于 1 英寸的 1/15（1.7 毫米）。
  • 此距离短得多(1.75<2.5)，因此用户会感觉应用在 该设备上更灵敏。

css pixel & device pixel

• One pixel. For screen displays, it traditionally represents one device pixel (dot).

• However, for printers and high-resolution screens, one CSS pixel implies multiple device pixels. 1px = 1/96th of 1inch.

• css pixel:实际应用中，当使用单位 px 时，让 96px 的距离等同(屏幕上的)大约 1 英寸就合格了

  • 无论屏幕的实际像素密度是多少。

• The term CSS pixel is synonymous with theCSS unitofabsolute length px — which is normatively defined as being exactly 1/96th of 1 inch.

references

• See also
• CSS: Cascading Style Sheets | MDN (mozilla.org)
• length - CSS（层叠样式表） | MDN (mozilla.org)
• resolution | MDN (mozilla.org)
• CSS Length Explained on the MDN Hacks Blog

---

px&dpi(device pixel & css pixel)

• 如果一个屏幕的像素密度是 266 DPI，且屏幕上一个元素(比如一段文本<p>)的宽度是 96px(css pxiel)=1 inch(譬如通过属性width=96px来设置)，那么这个元素的实际宽度用设备像素(dot)来衡量是 266 设备像素(device pixel)。

  • dpi:图像每英寸长度内的像素点数。
    • DPI（Dots Per Inch，每英寸点数）是一个量度单位，用于点阵数码影像，
    • 指每一英寸长度中，取样、可显示或输出点的数目。
    • dpi越高,线条的表现可以越清晰(平滑)

Absolute length units

• Absolute length units represent a physical measurement when the physical properties of the output medium are known, such as for print layout.

• This is done by anchoring one of the units to a physical unit, and then defining the others relative to it.

• The anchor is done differently for low-resolution devices, such as screens, versus high-resolution devices, such as printers.

• For low-dpi devices, the unit px represents the physical reference pixel; other units are defined relative to it.

• Thus, 1in is defined as 96px, which equals 72pt.

• The consequence of this definition is that on such devices, dimensions described in inches (in), centimeters (cm), or millimeters (mm) don’t necessary match the size of the physical unit with the same name.

For high-dpi devices, inches (in), centimeters (cm), and millimeters (mm) are the same as their physical counterparts. Therefore, the px unit is defined relative to them (1/96 of 1 inch).

Note: Many users increase their user agent’s default font size to make text more legible.

• Absolute lengths can cause accessibility problems, since they are fixed and do not scale according to user settings.
• For this reason, prefer relative lengths (such as em or rem) when setting font-size.

detail list of absolute length unit

px

One pixel. For screen displays, it traditionally represents one device pixel (dot). However, for printers and high-resolution screens, one CSS pixel implies multiple device pixels. 1px = 1/96th of 1in.

cm

One centimeter. 1cm = 96px/2.54.

mm

One millimeter. 1mm = 1/10th of 1cm.

Q

One quarter of a millimeter. 1Q = 1/40th of 1cm.

in

One inch. 1in = 2.54cm = 96px.

pc

One pica. 1pc = 12pt = 1/6th of 1in.

pt

One point. 1pt = 1/72nd of 1in.

Relative length units

Relative lengths represent a measurement in terms of some other distance. Depending on the unit, this can be the size of a specific character, the line height, or the size of the viewport.

Font-relative lengths

Font-relative lengths define the <length> value in terms of the size of a particular character or font attribute in the font currently in effect in an element or its parent.

Note: These units, especially em and rem, are often used to create scalable layouts, which maintain the vertical rhythm of the page even when the user changes the font size.

• cap

  Represents the “cap height” (nominal height of capital letters) of the element’s font.

• ch

  Represents the width, or more precisely the advance measure, of the glyph “0” (zero, the Unicode character U+0030) in the element’s font.In the cases where it is impossible or impractical to determine the measure of the “0” glyph, it must be assumed to be 0.5em wide by 1em tall.

• em

  Represents the calculated font-size of the element. If used on the font-size property itself, it represents the inherited font-size of the element.

• ex

  Represents the x-height of the element’s font. On fonts with the “x” letter, this is generally the height of lowercase letters in the font; 1ex ≈ 0.5em in many fonts.

• ic

  Equal to the used advance measure of the “水” glyph (CJK water ideograph, U+6C34), found in the font used to render it.

• lh

  Equal to the computed value of the line-height property of the element on which it is used, converted to an absolute length.

• remRepresents the font-size of the root element (typically <html>). When used within the root element font-size, it represents its initial value (a common browser default is 16px, but user-defined preferences may modify this).

• rlh Equal to the computed value of the line-height property on the root element (typically <html>), converted to an absolute length. When used on the font-size or line-height properties of the root element, it refers to the properties’ initial value.

Viewport-percentage lengths

Viewport-percentage lengths define the <length> value relative to the size of the viewport, i.e., the visible portion of the document. Viewport lengths are invalid in @page declaration blocks.

• vh

  Equal to 1% of the height of the viewport’s initial containing block.

• vw

  Equal to 1% of the width of the viewport’s initial containing block.

• vi

  Equal to 1% of the size of the initial containing block, in the direction of the root element’s inline axis.

• vb

  Equal to 1% of the size of the initial containing block, in the direction of the root element’s block axis.

• vmin

  Equal to the smaller of vw and vh.

• vmax

  Equal to the larger of vw and vh.

CSS resolution

分辨率：用于描述媒体查询中的分辨率的 <resolution> CSS数据类型表示输出设备的像素密度。

在屏幕上，单位与CSS英寸，厘米或像素有关，而与物理值无关。

Syntax

数据类型由严格为正<number>组成，后跟下面列出的单位之一。与所有CSS维度一样，单位字面值与数字之间没有空格。

单位

• dpi

  表示每英寸的点数。屏幕通常每英寸包含72或96个点，但打印文档的dpi通常要大得多。 1英寸是2.54厘米，1dpi≈0.39dpcm。

• dpcm

  每厘米上的点数。1英寸是2.54厘米, 1dpcm ≈ 2.54dpi.

• dppx

  表示每个px的点数。 由于CSS px的固定比率为1:96，因此1dppx相当于96dpi。

  • which corresponds to the default resolution of images displayed in CSS as defined by image-resolution (en-US).

• x

  dppx 的别名

Note: Although the number 0 is always the same regardless of unit, the unit may not be omitted. In other words, 0 is invalid and does not represent 0dpi, 0dpcm, or 0dppx.

Examples

• Examples

Use in a media query

• Use in a media query

@media print and (min-resolution: 300dpi) { ... }

Valid resolutions

• Valid resolutions

96dpi
50.82dpcm
3dppx

Invalid resolutions

• Invalid resolutions

72 dpi     Spaces are not allowed between the number and the unit.
ten dpi    The number must use digits only.
0          The unit is required.