Large Panoramic Images

The last versions of PTViewer are fast enough to display a panorama in a rather large window: as a result the panoramic image becomes larger too. This can cause problems because, with most Java virtual machines, applets can use only a relatively small amount of memory, even if the computer has much more RAM available.

PTViewer loads the panoramic image in memory, so if there is not enough memory available it cannot show the image. In order to know the maximum size of an image that can be shown by PTViewer we need to know how much memory is available to the applet. An empiric test has lead to the following results (please keep in mind that I cannot guarantee that they are absolutely correct):

The memory (in bytes) needed to store an image can be computed with this formula:

w * h * 4

where w if the width of the image in pixel and h is the height. When loading a jpeg image PTViewer will need for a small time twice as much memory. PTViewer will also need memory for other data structures, so available memory will be less than the theoretical maximum.

Other memory is needed for the viewer window: the number of bytes is

w * h * 5

where w is the width of the viewer window and h is the height. As an example a 1024*768 window will require 3.5Mb.

A 4000*2000 pixel image requires 30.5 Mb (1 Mb = 1024*1024 bytes...) just to store it. PTViewer temporarily requires 61Mb to load it so that image cannot be loaded in a Mac, but it should be possible to load it in Windows.

How to show larger images

• You can use dynamic progressive loading instead of loading a plain jpeg file. Dynamic progressive loading will not need twice as much memory while loading: it will require the memory needed to store the panoramic image plus the memory required to store the largest tile. If the image is divided in a high number of tiles the memory overhead will be very small. For example the same 4000*2000 pixel image as above, if divided in 10 tiles of the same size, will require (30.5 + 30.5/10) = 33.55 Mb and it should be possible to see it with a Mac.
• Sometimes you can crop the panoramic image, limiting the vertical field of view. For example in landscape images often there is not much interest in looking straight up or down. A 6000*1333 image will require the same memory as a 4000*2000 image and it will still have a vertical field of view of -40°/+40°.
• You can allocate more memory to Java. This must be done in the user's computer, so you cannot be sure that users will do it. Starting from version 2.8 beta4 PTViewer has a new parameter (outOfMemoryURL) that can be used to show a web page in case of an out-of-memory error. In that page you can explain what happened and how to allocate more memory , so users will see it only when (and if) they need it. Allocating more memory to Java is not a difficult task but most users do not like to fiddle with system settings, so I have developed a tiny Windows program that will do it automatically. You can put a link to that program in the web page so user will be able to choose between manually changing the memory setting or running the program.

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