PixelsPerInch in Horizontal?

Posted on 2003-04-01
Medium Priority
Last Modified: 2010-04-06
Hi Guys.
I want to output a graphic that needs to be in an exact size - something like 10inch over 20inch.

I know I can convert vertically - Screen.PixelsPerInch gives me the amount of pixels in an inch. But only vertically.

So what are my options if I want the horizontal property as well?

Question by:ShadowHawk071998
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LVL 26

Accepted Solution

Russell Libby earned 1000 total points
ID: 8248685

Try this

 hdcScreen: HDC;
 vertppi:   Integer;
 horzppi:   Integer;

 vertppi:=GetDeviceCaps(hdcScreen, LOGPIXELSY);
 horzppi:=GetDeviceCaps(hdcScreen, LOGPIXELSX);



Expert Comment

ID: 8248887
Hi rllibby

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LVL 12

Expert Comment

ID: 8252483
I thought that you typically always got '72' for the PixelsPerInch for a screen - hence you don't actually get 100% accuracy? Is that wrong, then? Printers are fine. Screens are approximate???
LVL 26

Expert Comment

by:Russell Libby
ID: 8255036

Delphi's Screen.PixelsPerInch is returned via a call to GetDeviceCaps, i gave you the corresponding call to get  the horizontal. As far as being an apprx, yes that is the case.


Notes from MSDN:

Sometimes you need an application to display things in terms of a real unit of measurement such as an inch or millimeter. When dealing with a printer, resolution is usually given in dots per inch (DPI), which makes it easy to convert pixels to real inches. However, on a video display, resolution is given only in pixels. A given video mode will be some X pixels wide with no information as to the real dimensions of the display area.

Because there is no way to programmatically determine the real dimensions of the viewable area on a video display, it is impossible for a program to determine real output dimensions. Two manual methods for determining real output dimensions are given in this article.

When output is destined for a printer, the application can call GetDeviceCaps() using LOGPIXELSX and LOGPIXELSY to determine dots per real inch. However, for a video display, LOGPIXELSX and LOGPIXELSY are defined by the video driver and may vary wildly. These numbers define a logical inch, which is almost never equal to a real inch.

Applications that need to output real sizes to the video display can use one of the following two methods for determining output size:

The application can ask the user what size monitor is attached. Using this value, an application can approximate the actual viewable area, and given the resolution of the output (GetDeviceCaps, HORZRES, VERTRES), the application can approximate real inches. This solution gives only an approximation of a real inch. Several factors can introduce errors into this approximation including the size adjustments on digital monitors.

The application can ask the user to hold a measuring device to the screen and measure a given line. This is the only way to guarantee that output on a video display is exactly the expected size, and recalibration would be necessary after any adjustment to the monitor.


Expert Comment

ID: 9316498
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