| 24-bit image planes (was Colored MPEGs) [message #13470] |
Wed, 11 November 1998 00:00 |
Struan Gray
Messages: 178
Registered: December 1995
|
Senior Member |
|
|
David Fanning, davidf@dfanning.com writes:
> FOR j=0,frames-1 DO BEGIN
> image24[0,*,*] = r(data[*,*,j])
> image24[1,*,*] = g(data[*,*,j])
> image24[2,*,*] = b(data[*,*,j])
> MPEG_Put, mpegID, Image=image24, Frame=j
> ENDFOR
You probably know this already, and the posted code is what I call
pedagoptimal, but here's a handy speedup I like to use when creating
24-bit images from 8-bit ones. Notice that when you do an assignment:
image24[i,*,*] = 8_bit_image
you are using the least efficient way of accessing memory in IDL,
since you are keeping the first index fixed and stuffing array data
into the leftmost ones. You can define your image as a BYTEARR(xsize,
ysize, 3) to speed things up but I find that the fastest way to do
things is often to simply create the array from scratch and reform:
dataslice = data[*,*,j]
image24 = [r(dataslice), g(dataslice), b(dataslice)]
image24 = reform(image24, xsize, 3, ysize, /overwrite)
Creating the dataslice avoids the need to re-access the subarray
each time, giving me a factor of two speedup. Building the image and
reforming it gives a tenfold speedup on top of that. It doesn't make
a huge difference with a single small image, but with bigger images or
large numbers of movie frames it adds up.
If you are going to use the image with TV or other routines that
accept pixel/row/plane interleaved 24-bit images you can display the
image directly:
TV, image24, true=2
If not, as in the case here (MPEG_Put only accepts 3xnxm images),
you have to use TRANSPOSE to swap the indexing:
image24 = transpose(temporary(image24), [1,0,2])
This has a small time penalty, but nothing like that incurred
by stuffing images directly into the image planes.
Struan
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
