> Or, at least define a "null" value that can be "overwritten" by the
> subsequent picture.
> Can a picture have a "null" value, or does any # represent a color?
> If I have a VI with a picture in, and I want to "overlay" it with a
> picture out, I'm going to need to analyze the data in the picture in.
> But I don't seem to be able to extract the data by any means. That
> is, unlike most data types such as enum, I can't multiply it by a #, I
> can't slice it, I can't do anything with it. Is there any way to "get
> at" the data in a picture control/indicator?
There is a datatype in LV called the picture datatype, and a somewhat
new addition which is a cluster of pixmap information.
The picture data is a command stream and is you can easily add a new
drawing command either below or above any other picture by prepending or
appending to a given picture. If you open one of the picture VIs you
will see that there is some string casting going on and the picture is
basically treated as a string. In reality, it is a derived type from
string and is just a binary opcode/opdata stream. Others have reverse
engineered the picture VIs and recoded them over the years for various
performance reasons. You can do this also, though it is a bit tedious,
and it doesn't really sound like you need to do this.
If you are really talking about the image cluster, then this is a
different story. The image cluster is a combination of data including a
mask that can be used to record or draw bitmaps with transparency. It
doesn't look like it supports alpha blending, pixels are either the
source or the destination pixel values, but this will allow for holes in
your pixmap.
To directly answer your original set of questions, LV does have a
defined value for a transparent color. Unfortunately, there is no good
standard for this and this is not something we can pass on to the OSes.
For this reason, the transparent color can be used for many
operations, but it doesn't work inside of pixmaps. The cost of
searching every pixmap looking for transparent colors, when most of the
time it won't be found, would be pretty expensive and isn't supported.
Placing transparent colors in a pixmap will result in the pixel going
black since that is how the OS will interpret the transparent color LV uses.
For more info on pictures, you might want to look at the examples such
as the graphing demos or the robot arm.
Greg McKaskle
