mosaicing

mosaicing — build image mosaics

Stability Level

Stable, unless otherwise indicated

Functions

int vips_merge ()
int vips_mosaic ()
int vips_mosaic1 ()
int vips_match ()
int vips_globalbalance ()
int vips_remosaic ()

Includes

#include <vips/vips.h>

Description

These functions are useful for joining many small images together to make one large image. They can cope with unstable contrast and arbitary sub-image layout, but will not do any geometric correction. Geometric errors should be removed before using these functions.

The mosaicing functions can be grouped into layers:

The lowest level functions are im_correl() and vips_merge(). im_correl() searches a large image for a small sub-image, returning the position of the best sub-image match. vips_merge() joins two images together left-right or up-down with a smooth seam.

Next, vips_mosaic() use the search function plus the two low-level merge operations to join two images given just an approximate overlap as a start point.

The functions im_lrmosaic1() and im_tbmosaic1() are first-order analogues of the basic mosaic functions: they take two approximate tie-points and use them to rotate and scale the right-hand or bottom image before starting to join.

Finally, vips_globalbalance() can be used to remove contrast differences in a mosaic which has been assembled with these functions. It takes the mosaic apart, measures image contrast differences along the seams, finds a set of correction factors which will minimise these differences, and reassembles the mosaic. im_remosaic() uses the same techniques, but will reassemble the image from a different set of source images.

Functions

vips_merge ()

int
vips_merge (VipsImage *ref,
            VipsImage *sec,
            VipsImage **out,
            VipsDirection direction,
            int dx,
            int dy,
            ...);

Optional arguments:

  • mblend : gint, maximum blend size

This operation joins two images left-right (with ref on the left) or up-down (with ref above) with a smooth seam.

If the number of bands differs, one of the images must have one band. In this case, an n-band image is formed from the one-band image by joining n copies of the one-band image together, and then the two n-band images are operated upon.

The two input images are cast up to the smallest common type (see table Smallest common format in

arithmetic).

dx and dy give the displacement of sec relative to ref , in other words, the vector to get from the origin of sec to the origin of ref , in other words, dx will generally be a negative number.

mblend limits the maximum width of the blend area. A value of "-1" means "unlimited". The two images are blended with a raised cosine.

Pixels with all bands equal to zero are "transparent", that is, zero pixels in the overlap area do not contribute to the merge. This makes it possible to join non-rectangular images.

See also: vips_mosaic(), vips_insert().

Parameters

ref

reference image

 

sec

secondary image

 

out

output image

 

direction

horizontal or vertical merge

 

dx

displacement of ref from sec

 

dy

displacement of ref from sec

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error


vips_mosaic ()

int
vips_mosaic (VipsImage *ref,
             VipsImage *sec,
             VipsImage **out,
             VipsDirection direction,
             int xref,
             int yref,
             int xsec,
             int ysec,
             ...);

Optional arguments:

  • bandno : gint, band to search for features

  • hwindow : gint, half window size

  • harea : gint, half search size

  • mblend : gint, maximum blend size

This operation joins two images left-right (with ref on the left) or top-bottom (with ref above) given an approximate overlap.

sec is positioned so that the pixel (xsec , ysec ) in sec lies on top of the pixel (xref , yref ) in ref . The overlap area is divided into three sections, 20 high-contrast points in band bandno of image ref are found in each, and each high-contrast point is searched for in sec using hwindow and harea (see vips_correl()).

A linear model is fitted to the 60 tie-points, points a long way from the fit are discarded, and the model refitted until either too few points remain or the model reaches good agreement.

The detected displacement is used with vips_merge() to join the two images together.

See also: vips_merge(), vips_insert().

Parameters

ref

reference image

 

sec

secondary image

 

out

output image

 

direction

horizontal or vertical join

 

xref

position in reference image

 

yref

position in reference image

 

xsec

position in secondary image

 

ysec

position in secondary image

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error


vips_mosaic1 ()

int
vips_mosaic1 (VipsImage *ref,
              VipsImage *sec,
              VipsImage **out,
              VipsDirection direction,
              int xr1,
              int yr1,
              int xs1,
              int ys1,
              int xr2,
              int yr2,
              int xs2,
              int ys2,
              ...);

Optional arguments:

  • search : search to improve tie-points

  • hwindow : half window size

  • harea : half search size

  • interpolate : interpolate pixels with this

  • mblend : maximum blend size

  • bandno : band to search for features

This operation joins two images top-bottom (with sec on the right) or left-right (with sec at the bottom) given an approximate pair of tie-points. sec is scaled and rotated as necessary before the join.

Before performing the transformation, the tie-points are improved by searching band bandno in an area of sec of size hsearchsize for a match of size hwindowsize to ref .

If search is TRUE, before performing the transformation, the tie-points are improved by searching an area of sec of size harea for a mosaic1 of size hwindow to ref .

mblend limits the maximum size of the blend area. A value of "-1" means "unlimited". The two images are blended with a raised cosine.

Pixels with all bands equal to zero are "transparent", that is, zero pixels in the overlap area do not contribute to the merge. This makes it possible to join non-rectangular images.

If the number of bands differs, one of the images must have one band. In this case, an n-band image is formed from the one-band image by joining n copies of the one-band image together, and then the two n-band images are operated upon.

The two input images are cast up to the smallest common type (see table Smallest common format in

arithmetic).

See also: vips_merge(), vips_insert(), vips_globalbalance().

Parameters

ref

reference image

 

sec

secondary image

 

out

output image

 

direction

horizontal or vertical join

 

xr1

first reference tie-point

 

yr1

first reference tie-point

 

xs1

first secondary tie-point

 

ys1

first secondary tie-point

 

xr2

second reference tie-point

 

yr2

second reference tie-point

 

xs2

second secondary tie-point

 

ys2

second secondary tie-point

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error


vips_match ()

int
vips_match (VipsImage *ref,
            VipsImage *sec,
            VipsImage **out,
            int xr1,
            int yr1,
            int xs1,
            int ys1,
            int xr2,
            int yr2,
            int xs2,
            int ys2,
            ...);

Optional arguments:

  • search : search to improve tie-points

  • hwindow : half window size

  • harea : half search size

  • interpolate : interpolate pixels with this

Scale, rotate and translate sec so that the tie-points line up.

If search is TRUE, before performing the transformation, the tie-points are improved by searching an area of sec of size harea for a match of size hwindow to ref .

This function will only work well for small rotates and scales.

Parameters

ref

reference image

 

sec

secondary image

 

out

output image

 

xr1

first reference tie-point

 

yr1

first reference tie-point

 

xs1

first secondary tie-point

 

ys1

first secondary tie-point

 

xr2

second reference tie-point

 

yr2

second reference tie-point

 

xs2

second secondary tie-point

 

ys2

second secondary tie-point

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error


vips_globalbalance ()

int
vips_globalbalance (VipsImage *in,
                    VipsImage **out,
                    ...);

Optional arguments:

  • gamma : gamma of source images

  • int_output : TRUE for integer image output

vips_globalbalance() can be used to remove contrast differences in an assembled mosaic.

It reads the History field attached to in and builds a list of the source images that were used to make the mosaic and the position that each ended up at in the final image.

It opens each of the source images in turn and extracts all parts which overlap with any of the other images. It finds the average values in the overlap areas and uses least-mean-square to find a set of correction factors which will minimise overlap differences. It uses gamma to gamma-correct the source images before calculating the factors. A value of 1.0 will stop this.

Each of the source images is transformed with the appropriate correction factor, then the mosaic is reassembled. out is VIPS_FORMAT_FLOAT, but if int_output is set, the output image is the same format as the input images.

There are some conditions that must be met before this operation can work: the source images must all be present under the filenames recorded in the history on in , and the mosaic must have been built using only operations in this package.

See also: vips_remosaic().

Parameters

in

mosaic to rebuild

 

out

output image

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error


vips_remosaic ()

int
vips_remosaic (VipsImage *in,
               VipsImage **out,
               const char *old_str,
               const char *new_str,
               ...);

vips_remosaic() works rather as vips_globalbalance(). It takes apart the mosaiced image in and rebuilds it, substituting images.

Unlike vips_globalbalance(), images are substituted based on their file‐ names. The rightmost occurence of the string old_str is swapped for new_str , that file is opened, and that image substituted for the old image.

It's convenient for multispectral images. You can mosaic one band, then use that mosaic as a template for mosaicing the others automatically.

See also: vips_globalbalance().

Parameters

in

mosaic to rebuild

 

out

output image

 

old_str

gamma of source images

 

new_str

gamma of source images

 

...

NULL-terminated list of optional named arguments

 

Returns

0 on success, -1 on error

Types and Values