windows 为什么 bmps 存储颠倒？
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Why are bmps stored upside down?
提问by bobobobo
Why are BMP images stored upside down and zero-padded so they are four-byte aligned?
为什么 BMP 图像颠倒存储并用零填充，因此它们是四字节对齐的？
回答by pezcode
Here's a quote from Petzold:
这是来自 Petzold 的引述：
So, in DIBs, the bottom row of the image is the first row of the file, and the top row of the image is the last row in the file. This is called a bottom-up organization. Because this organization is counterintuitive, you may ask why it's done this way.
Well, it all goes back to the OS/2 Presentation Manager. Someone at IBM decided that all coordinate systems in PM—including those for windows, graphics, and bitmaps—should be consistent. This provoked a debate: Most people, including programmers who have worked with full-screen text programming or windowing environments, think in terms of vertical coordinates that increase going down the screen. However, hardcore computer graphics programmers approach the video display from a perspective that originates in the mathematics of analytic geometry. This involves a rectangular (or Cartesian) coordinate system where increasing vertical coordinates go up in space.
In short, the mathematicians won. Everything in PM was saddled with a bottom-left origin, including window coordinates. And that's how DIBs came to be this way.
因此，在 DIB 中，图像的底行是文件的第一行，图像的顶行是文件的最后一行。这称为自下而上的组织。因为这个组织是违反直觉的，你可能会问为什么这样做。
好吧，这一切都可以追溯到 OS/2 Presentation Manager。IBM 的某个人决定 PM 中的所有坐标系——包括那些用于窗口、图形和位图的坐标系——应该是一致的。这引发了一场争论：大多数人，包括在全屏文本编程或窗口环境中工作过的程序员，都认为垂直坐标会在屏幕下方增加。然而，核心计算机图形程序员从源自解析几何数学的角度来处理视频显示。这涉及一个矩形（或笛卡尔）坐标系，其中垂直坐标在空间中增加。
简而言之，数学家赢了。PM 中的所有内容都带有左下角的原点，包括窗口坐标。这就是 DIB 是如何变成这样的。
Source: Charles Petzold, Programming for Windows 5th Edition, Chapter 15.
来源：Charles Petzold，Windows 编程第 5 版，第 15 章。
回答by Beginner
They are stored accordingly to a display coordinate system. (0, 0) is at the upper left corner. X (corresponds to width) goes from left to right. Y (corresponds to height) goes from up to bottom.
它们相应地存储到显示坐标系中。(0, 0) 位于左上角。X（对应于宽度）从左到右。Y（对应于高度）从上到下。
As for "4 Byte question" see Why must "stride" in the System.Drawing.Bitmap constructor be a multiple of 4?
至于“4 字节问题”，请参阅为什么 System.Drawing.Bitmap 构造函数中的“步幅”必须是 4 的倍数？
