The download command can be used to download assets from the grid.

llInstantMessage(CORRADE,
    wasKeyValueEncode(
        [
            // Download a texture by UUID
            "command", "download",
            "group", wasURLEscape(GROUP),
            "password", wasURLEscape(PASSWORD),
            // or the path to an item in Corrade's inventory
            "item", "742c887b-a8ed-ba7b-6e7f-8ba8e9d1d76a",
            "type", "Texture",
            "callback", wasURLEscape(URL)
        ]
    )
);

and the data retrieved is compatible with the data supplied to the upload command.

In case the path optional parameter is not specified, then the asset will be downloaded, encoded as a Base64 string, then URI escaped and then placed as the value to the data key from the callback. If the path parameter is specified, then the data will be saved to the indicated path.

Grid clients such as Corrade download and upload textures to the grid in the JPEG2000 image format. The upload of textures in different formats other than JPEG2000 is provided as a convenience but Linden grids can only accept JPEG2000 images. In other words, regardless of the image format that you want to upload, Corrade will have to convert the image to the JPEG2000 format (with lossless compression) and only then upload it. You may want to rethink your application in case you really depend on some format provided here and convert your images to JPEG2000 yourself before uploading. For instance, under Windows, uploading a 24-bit PNG is meaningless from Corrade's perspective because the grid does not accept such a format (only JPEG2000) such that the 24-bit PNG will be first converted to JPEG2000 and then uploaded.

A counter-example when using any other format other than JPEG2000 is useful when you need to upload or download an image from PHP (or other environments) that do not have built-in support for the JPEG2000 image format. In such cases, in order to not over-complicate your script, you can use the formats listed here depending on your platform. However, as mentioned previously, be weary because grid clients can only upload textures in the JPEG2000 format such that you could probably convert your image to JPEG2000 better than having Corrade do it for you.

The supported image formats are the following:

Omitting the format parameter will result in a JPEG2000 image to be downloaded directly off the asset server. The JPEG2000 image format is defined with various profiles, such that if the downloaded image is saved to a file and then inspected by the file or exiftool command line utilities, the detected image description will be:

JPEG2000 codestream

which indicates a Profile 0 JPEG2000 which is what the SecondLife grid seems to be using. Profile 0 (subset of 1, 2) was probably chosen to maximize compatibility with tools that have not yet implemented full support for JPEG2000.

The proper file extension for such images is "j2c" which stands for JPEG2000 codestream. Some tools may not properly implement nor respect the file extension. For instance, using ffmpeg to convert an image to JPEG2000 codestream:

ffmpeg -i img.png img.j2c

will result in a Profile 1 JPEG2000 instead of a JPEG2000 Profile 0 codestream such that it is imperative to make sure that the proper tools are used when downloading or uploading images.

Furthermore, Corrade follows the SecondLife limits on texture uploads without any processing such that the user must ensure that the texture to be uploaded conforms to the specification.

By default sounds are stored on the grid in OGG-Vorbis format and Corrade can download the sound file as OGG.

Except for:

• notecards,
• scripts (LSLText)

all the other assets can be grabbed by Corrade's download command in case the UUID of the item is known.

Textures, sounds and animations are well-known formats that, once received, can be easily reused and converted to something else. On the other hand, clothing, landmarks, gestures, notecards and scripts (LSLText) are Linden-based formats that cannot be converted directly.

Some of these formats, for example Clothing, include references to images which will have to be pulled separately by first retrieving the clothing, going through the file and gathering UUIDs and then issuing another download to fetch the textures.