RFR: 8251928: [macos] the printer DPI always be 72, cause some content lost when print out

[GennadiyKrivoshein]

On Fri, 30 May 2025 06:40:40 GMT, Prasanta Sadhukhan <psadhukhan at openjdk.org> wrote:

>> The fix for the https://bugs.openjdk.org/browse/JDK-8251928.
>> 
>> **Description**.
>> This PR contains changes to be able to print with DPI higher than 72 on macOS, set default CPrinterJob DPI is 300 like in the PSPrinterJob.
>> 
>> As described in the macOS drawing guide, the following steps are required to draw with high DPI (https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CocoaDrawingGuide/Transforms/Transforms.html#//apple_ref/doc/uid/TP40003290-CH204-BCICIJAJ):
>> 1. Convert the user-space point, size, or rectangle value to device space coordinates;
>> 2. Normalize the value in device space so that it is aligned to the appropriate pixel boundary;
>> 3. Convert the normalized value back to user space;
>> 4. Draw your content using the adjusted value. 
>>     
>> The 1-st step is now implemented in the CPrinterJob, a Graphics provided to the print method adjusted to a printer's DPI.
>> The 2-nd step is a drawing process in the java code (without changes).
>> The 3-rd step is now implemented in the PrinterView.m, the drawing scaled back to the 72 DPI.
>> The 4-th step is a drawing process in the native code (without changes).
>> 
>> **Tests**.
>> I run all tests from javax.print package and there is no any regression.
>> New test covers macOS and Linux only because we know its default DPI - 300.
>
> While running PDFBOX4935PrintDpi  testcase from the JBS, it is showing with your fix
> ScaleX: 4.166666
> ScaleY: 4.166666
> DPI = 300
> 
> while printing to pdf. Shouldn't the scale be shown as 2.0 for retina display while doing pdf printing?

@prsadhuk  , no, the scale should be 4.166666.

As described in the drawing guide
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CocoaDrawingGuide/Transforms/Transforms.html#//apple_ref/doc/uid/TP40003290-CH204-SW6

> Units in the user space are based on the printer's point, which was used in the publishing industry to measure the size of content on the printed page. A single point is equivalent to 1/72 of an inch. Points were adopted by earlier versions of Mac OS as the standard resolution for content on the screen. OS X continues to use the same effective “resolution” for user-space drawing.
> 
> Although a single point often corresponded directly to a pixel in the past, in OS X, that may not be the case. Points are not tied to the resolution of any particular device. If you draw a rectangle whose width and height are exactly three points, that does not mean it will be rendered on the screen as a three-pixel by three-pixel rectangle. On a 144 dpi screen, the rectangle might be rendered using six pixels per side, and on a 600-dpi printer, the rectangle would require 25 pixels per side. The actual translation from points to pixels is device dependent and handled for you automatically by OS X. 

So it does not matter what the actual device DPI is, we always use or see 72 on the java side.
I set the default printing DPI for macOS to 300 in this PR, so if you do not provide different value using javax.print.attribute.standard.PrinterResolution then the scaling should be 300/72 = 4.16666.

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PR Comment: https://git.openjdk.org/jdk/pull/25489#issuecomment-2922481400