NAME
Prima::ImageViewer - standard image, icon, and bitmap viewer class.
DESCRIPTION
The module contains "Prima::ImageViewer" class, which provides image
displaying functionality, including different zoom levels.
"Prima::ImageViewer" is a descendant of "Prima::ScrollWidget" and
inherits its document scrolling behavior and programming interface.
See Prima::ScrollWidget for details.
API
Properties
alignment INTEGER
One of the following "ta::XXX" constants:
ta::Left
ta::Center
ta::Right
Selects the horizontal image alignment.
Default value: "ta::Left"
image OBJECT
Selects the image object to be displayed. OBJECT can be an instance
of "Prima::Image", "Prima::Icon", or "Prima::DeviceBitmap" class.
imageFile FILE
Set the image FILE to be loaded and displayed. Is rarely used since
does not return a loading success flag.
quality BOOLEAN
A boolean flag, selecting if the palette of "image" is to be copied
into the widget palette, providing higher visual quality on
paletted displays. See also "palette" in Prima::Widget.
Default value: 1
valignment INTEGER
One of the following "ta::XXX" constants:
ta::Top
ta::Middle or ta::Center
ta::Bottom
Selects the vertical image alignment.
NB: "ta::Middle" value is not equal to "ta::Center"’s, however the
both constants produce equal effect here.
Default value: "ta::Bottom"
zoom FLOAT
Selects zoom level for image display. The acceptable value range is
between 0.01 and 100. The zoom value is rounded to the closest
value divisible by 1/"zoomPrecision". For example, is
"zoomPrecision" is 100, the zoom values will be rounded to the
precision of hundredth - to fiftieth and twentieth fractional
values - .02, .04, .05, .06, .08, and 0.1 . When "zoomPrecision" is
1000, the precision is one thousandth, and so on.
Default value: 1
zoomPrecision INTEGER
Zoom precision of "zoom" property. Minimal acceptable value is 10,
where zoom will be rounded to 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0 .
The reason behind this arithmetics is that when image of arbitrary
zoom factor is requested to be displayed, the image sometimes must
begin to be drawn from partial pixel - for example, 10x zoomed
image shifted 3 pixels left, must be displayed so the first image
pixel from the left occupies 7 screen pixels, and the next ones -
10 screen pixels. That means, that the correct image display
routine must ask the system to draw the image at offset -3 screen
pixels, where the first pixel column would correspond to that
pixel.
When zoom factor is fractional, the picture is getting more
complex. For example, with zoom factor 12.345, and zero screen
offset, first image pixel begins at 12th screen pixel, the next -
25th ( because of the roundoff ), then 37th etc etc. Also, for
example the image is 2000x2000 pixels wide, and is asked to be
drawn so that the image appears shifted 499 screen image pixels
left, beginning to be drawn from ~ 499/12.3456=40.42122 image
pixel. Is might seem that indeed it would be enough to ask system
to begin drawing from image pixel 40, and offset
int(0.42122*12.345)=5 screen pixels to the left, however, that
procedure will not account for the correct fixed point roundoff
that accumulates as system scales the image. For zoom factor 12.345
this roundoff sequence is, as we seen before,
(12,25,37,49,62,74,86,99,111,123) for first 10 pixels displayed,
that occupy (12,13,12,12,13,12,12,13,12,12) screen pixels. For
pixels starting at 499, this sequence is
(506,519,531,543,556,568,580,593,605,617) offsets or
(13,12,12,13,13,12,12,13,12,12) widths -- note the two subsequent
13s there. This sequence begins to repeat itself after 200
iterations (12.345*200=2469.000), which means that in order to
achieve correct display results, the image must be asked to be
displayed from image pixel 0 if image’s first pixel on the screen
is between 0 and 199 ( or for screen pixels 0-2468), from image
pixel 200 for offsets 200-399, ( screen pixels 2469-4937), and so
on.
Since system internally allocate memory for image scaling, that
means that up to
2*200*min(window_width,image_width)*bytes_per_pixel unneccessary
bytes will be allocated for each image drawing call (2 because the
calculations are valid for both the vertical and horizontal
strips), and this can lead to slowdown or even request failure when
image or window dimensions are large. The proposed solution is to
roundoff accepted zoom factors, so these offsets are kept small -
for example, N.25 zoom factors require only max 1/.25=4 extra
pixels. When "zoomPrecision" value is 100, zoom factors are rounded
to 0.X2, 0.X4, 0.X5, 0.X6, 0.X8, 0.X0, thus requiring max 50 extra
pixels.
NB. If, despite the efforts, the property gets in the way, increase
it to 1000 or even 10000, but note that this may lead to problems.
Default value: 100
Methods
on_paint SELF, CANVAS
The "Paint" notification handler is mentioned here for the specific
case of its return value, that is the return value of internal
"put_image" call. For those who might be interested in "put_image"
failures, that mostly occur when trying to draw an image that is
too big, the following code might be useful:
sub on_paint
{
my ( $self, $canvas) = @_;
warn "put_image() error:$@" unless $self-> SUPER::on_paint($canvas);
}
screen2point X, Y, [ X, Y, ... ]
Performs translation of integer pairs integers as (X,Y)-points from
widget coordinates to pixel offset in image coordinates. Takes in
account zoom level, image alignments, and offsets. Returns array of
same length as the input.
Useful for determining correspondence, for example, of a mouse
event to a image point.
The reverse function is "point2screen".
point2screen X, Y, [ X, Y, ... ]
Performs translation of integer pairs as (X,Y)-points from image
pixel offset to widget image coordinates. Takes in account zoom
level, image alignments, and offsets. Returns array of same length
as the input.
Useful for determining a screen location of an image point.
The reverse function is "screen2point".
watch_load_progress IMAGE
When called, image viewer watches as IMAGE is being loaded ( see
"load" in Prima::Image ) and displays the progress. As soon as
IMAGE begins to load, it replaces the existing "image" property.
Example:
$i = Prima::Image-> new;
$viewer-> watch_load_progress( $i);
$i-> load('huge.jpg');
$viewer-> unwatch_load_progress;
Similar functionality is present in Prima::ImageDialog.
unwatch_load_progress CLEAR_IMAGE=1
Stops monitoring of image loading progress. If CLEAR_IMAGE is 0,
the leftovers of the incremental loading stay intact in "image"
propery. Otherwise, "image" is set to "undef".
zoom_round ZOOM
Rounds the zoom factor to "zoomPrecision" precision, returns the
rounded zoom value. The algorithm is the same as used internally in
"zoom" property.
AUTHOR
Dmitry Karasik, <dmitry@karasik.eu.org>.
SEE ALSO
Prima, Prima::Image, Prima::ScrollWidget, Prima::ImageDialog,
examples/iv.pl.