Why are bmps stored upside down?

Asked 1/12, 2011 at 18:6 Answered 1/12, 2011 at 19:20

Why are BMP images stored upside down and zero-padded so they are four-byte aligned?

Daft answered 1/12, 2011 at 18:6 Comment(5)

possible duplicate of Why must "stride" in the System.Drawing.Bitmap constructor be a multiple of 4? – Polack 1/12, 2011 at 18:9

support.microsoft.com/kb/q81498 claims the upsidedownness is an artifact of being backwards compatible with Presentation Manager, which means it's something that IBM did for OS/2. – Artur 1/12, 2011 at 18:13

They are upside-down for compatibility with OS/2 Presentation Manager. It can actually be either, top line first bitmaps have a negative height. – Polack 1/12, 2011 at 18:14

@HansPassant, the ability to have a negative height is a feature added well after the bitmap format was popularized. – Shekinah 1/12, 2011 at 19:38

Guys settle the negative height argument this BMP browser drop-in/png/jpg universal "uncompressed" replacement library ain't gonna write itself!! – Daft 1/12, 2011 at 19:57

Here's a quote from Petzold:

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.

Source: Charles Petzold, Programming for Windows 5th Edition, Chapter 15.

Tetrahedron answered 1/12, 2011 at 19:20 Comment(1)

Interesting that they kept this representation for DIBs but changed it for window coordinates, reintroducing the inconsistency. – Shekinah 1/12, 2011 at 19:36

-5

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.

As for "4 Byte question" see Why must "stride" in the System.Drawing.Bitmap constructor be a multiple of 4?

Dharma answered 1/12, 2011 at 18:10 Comment(1)

No they aren't. BMPs are stored from bottom to top, left to right. – Inflatable 19/8, 2015 at 23:18

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