differences between text/imagedata of image in LC6/8

Mark Waddingham mark at livecode.com
Fri Mar 24 06:58:28 EDT 2017

Hi Tiemo,

> in an old program I obviously tried to be very technically over correct 
> when
> copying an image and did:
> 1.       set the imagedata of img 2 to the imagedata of img 1
> This worked up to LC 6.7. In LC8/9 I get a corrupted image (black and
> stripes) in the target image.

The imageData of an image is the extracted color values for the pixels 
in the
image. Specifically, it is a sequence of bytes with each pixel 
represented as
xRGB. These are the bytes which are applied to the screen when the image 
rendered and does not include any alpha data (which is specified using 
alphaData property). To set the imageData you need to set the width and 
of the field first.

> What works in LC8/9 is
> 2.       set the text of img 2 to the text of img 1
> and
> 3.       put img 1 into img 2

Both of these do the same thing - (3) is short hand for (1).

The 'text' of an image is the original image data from a format such as 
PNG, GIF etc.
i.e. the content of a PNG file or GIF file.

> What is the correct syntax today? Is solution 2 and 3 the same and just 
> an
> alias (what it seems to me) or are there differences in details I do 
> not
> see?

In general, (2) or (3) is better than (1) as (1) is considered an 'image 
operation and as such will cause a recompression of the data in some 
fashion. Also
(2) and (3) will automatically set the size, and alpha channel of the 
image appropriately.

> And is there any explanation why the upward compatibility of solution 1 
> is
> broken?

If there is a difference here then it is a bug - although please do 
check that the width/height of the image is the same, and also transfer 
across alphaData too.

I'd recommend using (2) or (3) - or, indeed, using referenced files 
instead. The engine
will automatically cache referenced images and share the underlying 
decompressed data.

    set the filename of image 1 to "foo"
    set the filename of image 2 to "foo"

Will first resolve "foo" as an absolute path, load that image file into 
a cache if not
already present and attach the decompressed image data to the image 
object. Then, when
the second image's filename is set, it will find the image is already in 
the cache and
just use the decompressed image data attached to that file in the cache.
i.e. The two images share the same pixel data in memory.

Warmest Regards,


Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps

More information about the use-livecode mailing list