图形学笔记《Fundamentals of Computer Graphics 4th Edition》——【3】Raster Images 光栅图像

• raster display: show images as rectangular arrays of pixels
• pixel: a short for “picture element”
• raster image: a 2D array that stores the pixel value for each pixel——R G B
• 因为显示设备与图片大小不能保证一致，可能会有拉伸、变形、变色等变换发生，因此it’s best to think of a raster image as a device-independent description of the image to be displayed
• vector image: 另一种显示方式，storing descriptions of shapes——areas of color bounded by lines or curves——with no reference to any particular pixel grid
  
  • advantage: resolution independent, can be displayed well on very high resolution devices
  • disadvantage: must be rasterized（光栅化） before displayed
  • often used for: text, diagrams, mechanical drawings——crispness and precision are important, photographic images and complex shading aren’t needed

Raster Devices

Displays

一般说来，显示设备有两种：

• emissive displays: 发射型显示器，use pixels that directly emit controllable amounts of light
  
  • 如，light-emitting diode (LED), 每个像素上都有半导体设备（可能多个颜色如R G B，在远方看起来就会形成一种颜色），根据当前电流决定其光强
• transmissive displays: 透射式显示器，the pixels themselves don’t emit light but instead vary the amount of the light that they allow to pass through them, require a light source to illuminate them
  
  • 如，liquid crystal displays (LCDs)，如图所示，偏振光会根据电压大小偏转不同的角度，偏转度为0时没有光线通过filter，当电压使其偏转90度时，所有光线都会通过filter，此时光强最大
    

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• 分辨率的含义：一张图片上有多少个像素点（长×宽）

Hardcopy Devices

• int-jet printer: 喷墨式打印机，一般只能打印binary images，可使用多颜色喷头打印彩色图片
• thermal dye transfer: 热敏式，在print head和dye receiver中间加入一层donor ribbon，可以根据热度产生不同强度的颜色

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• 分辨率的含义——pixel desity
  
  • ppi: 如热敏式的300 pixels per inch——has elements spaced 300 per inch across its head
  • dpi: 如喷墨式的1200 dots per inch——places dots on a grid with 1200 grid points per inch
  • 同等质量的图片，喷墨式通常需要比热敏式更高的分辨率（吧？）

Input Devices

• 相机中也有像素级别的感光元件，去获取该像素的颜色和光强
  
• 多数相机中会使用像上图一样的color-filter array or mosaic to allow each pixel to see only red, green or blue light, leaving the image processing software to fill in the missing values in a process known as demosaicking（去马赛克）
• 还有一些相机使用多层感光元件，每一层去感不同颜色的光（R G B），不需要多余的处理了

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• 分辨率的含义：
  
  • the fixed number of pixels in the array
  • is usually quoted using the total count of pixels：如3000×2000 = 6 million pixels——6 megapixel (MP) camera

Images, Pixels, and Geometry

• 首先，我们将图片的表示抽象为$I(x,y) : R \rightarrow V$，其中 R⊂R2 R ⊂ R 2