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Erlind Winger
Erlind Winger

Reputation: 31

BMP image header - biXPelsPerMeter

I have read a lot about BMP file format structure but I still cannot get what is the real meaning of the fields "biXPelsPermeter" and "biYPelsPermeter". I mean in practical way, how is it used or how it can be utilized. Any example or experience? Thanks a lot

Upvotes: 3

Views: 5202

Answers (2)

Rick C. Hodgin
Rick C. Hodgin

Reputation: 503

Think of it this way: The image bits within the BMP structure define the shape of the image using that much data (that much information describes the image), but that information must then be translated to a target device using a measuring system to indicate its applied resolution in practical use.

For example, if the BMP is 10,000 pixels wide, and 4,000 pixels high, that explains how much raw detail exists within the image bits. However, that image information must then be applied to some target. It uses the relationship to the dpi and its target to derive the applied resolution.

If it were printed at 1000 dpi then it's only going to give you an image with 10" x 4" but one with extremely high detail to the naked eye (more pixels per square inch). By contrast, if it's printed at only 100 dpi, then you'll get an image that's 100" x 40" with low detail (fewer pixels per square inch), but both of them have the same overall number of bits within. You can actually scale an image without scaling any of its internal image data by merely changing the dpi to non-standard values.

Also, using 72 dpi is a throwback to ancient printing techniques (https://en.wikipedia.org/wiki/Twip) which are not really relevant in moving forward (except to maintain compatibility with standards) as modern hardware devices often use other values for their fundamental relationships to image data. For video screens, for example, Macs use 72 dpi as the default. Windows uses 96 dpi. Others are similar. In theory you can set it to whatever you want, but be warned that not all software honors the internal settings and will instead assume a particular size. This can affect the way images are scaled within the app, even though the actual image data within hasn't changed.

Upvotes: 2

Edenia
Edenia

Reputation: 2488

biXPelsPermeter Specifies the horizontal print resolution, in pixels per meter, of the target device for the bitmap.

biYPelsPermeter Specifies the vertical print resolution.

Its not very important. You can leave them on 2835 its not going to ruin the image. (72 DPI × 39.3701 inches per meter yields 2834.6472)

Upvotes: 6

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