1. An image processing device comprising: a processor which executes a program to perform processing including: segmentation processing which, when foreground pixels determined to be a foreground, background pixels determined to be a background, and undetermined pixels that remain undetermined as to whether they are a foreground or a background are designated in an image, further segments the undetermined pixels into foreground pixels and background pixels based on designation information; segment information acquisition processing which acquires segment information that is information for a case in which a first image is segmented into foreground pixels and background pixels by the segmentation processing; and segmentation control processing which, when controlling the segmentation processing to segment a second image having a predetermined relationship with the first image, specifies foreground pixels, background pixels, and undetermined pixels of the second image in the image, by using the segment information acquired by the segment information acquisition processing, wherein the segmentation control processing determines an area in which a predetermined width in accordance with the relationship between the first image and the second image is provided for a boundary line between the foreground pixels and the background pixels determined by the segment information, specifies an area in the second image corresponding to the determined area as undetermined pixels, and controls the segmentation processing to segment the second image.
2. The image processing device according to claim 1 , wherein the segmentation control processing specifies such that a width of the area corresponding to the undetermined pixels becomes thicker as the relationship between the first image and the second image is lower, and controls the segmentation processing to segment the second image.
3. The image processing device according to claim 2 , wherein the segmentation control processing specifies such that the width of the area corresponding to the undetermined pixels becomes thicker as a difference between a time when the first image is captured and a time when the second image is captured is larger, and controls the segmentation processing to segment the second image.
4. The image processing device according to claim 3 , wherein the segmentation control processing creates and displays a drawing line having a predetermined width by a drawing operation performed on the image by a user, and designates the created drawing line of the predetermined width as the boundary line.
5. The image processing device according to claim 1 , wherein the segmentation processing performs predetermined image segmentation processing for segmenting pixels of the whole image into foreground pixels and background pixels by using positional information of some of the foreground pixels or the background pixels specified in the image, and wherein the segmentation control processing designates pixels located at a predetermined distance away from the boundary line as some of the foreground pixels or the background pixels, and controls the segmentation processing to segment the image.
6. The image processing device according to claim 5 , wherein the segmentation processing performs predetermined image segmentation processing for segmenting pixels of the whole image into foreground pixels and background pixels by using positional information of some of the foreground pixels and the background pixels specified in the image, and wherein the segmentation control processing controls the segmentation processing to segment the image, with pixels located at the predetermined distance away on one side of an area segmented by the boundary line as the positional information of some of the foreground pixels, pixels located at the predetermined distance away on another side of the area segmented by the boundary line as the positional information of some of the background pixels, and pixels located within the predetermined distance from the boundary line as pixels that remain undetermined as to whether the pixels are a foreground or a background.
7. The image processing device according to claim 1 , wherein the processor further performs image acquisition processing which acquires, as the first image and the second image, two captured images showing a same photographic subject.
8. The image processing device according to claim 1 , wherein the processor further performs image acquisition processing which acquires two sequentially captured images as the first image and the second image, wherein the segmentation control processing specifies such that a width of the area corresponding to the undetermined pixels becomes thicker as a speed of the sequentially capturing slows down, and controls the segmentation processing to segment the second image.
9. The image processing device according to claim 8 , wherein the image acquisition processing sequentially acquires, as the first image and the second image, two sequentially captured images from among a plurality of frame images included in moving image data captured by the image acquisition device, wherein the segment information acquisition processing sequentially acquires positional information of a boundary line when sequential segmentation is performed by the segmentation processing with each frame image sequentially acquired by the image acquisition processing as a target, and wherein the segmentation control processing controls the segmentation processing to sequentially perform segmentation by using the positional information of the boundary line sequentially acquired by the segment information acquisition processing with each frame image sequentially acquired by the image acquisition processing as a target.
10. The image processing device according to claim 9 , wherein the segmentation control processing controls such that a boundary line for a first frame image to be segmented by the segmentation processing from among the plurality of frame images included in the moving image data is specified by a user operation performed on the image.
11. The image processing device according to claim 1 , wherein the segmentation processing performs predetermined image segmentation processing for (i) taking initially designated foreground pixels and background pixels as determined pixels for which determination as to whether pixels are a foreground or a background has been completed, (ii) and repeating, for target pixels that remain undetermined as to whether the target pixels are a foreground or a background, processing for improving a confidence level as to whether a target pixel is a foreground or a background based on information of a surrounding pixel located around the target pixel and having a higher confidence level as to whether the surrounding pixel is a foreground or a background as compared to a confidence level related to the target pixel, so that determination of all of the target pixels as to whether the target pixels are a foreground or a background is eventually completed.
12. The image processing device according to claim 11 , wherein the segmentation processing performs predetermined image segmentation processing in which, when surrounding pixels whose confidence levels as to whether the surrounding pixels are a foreground or a background are higher than confidence levels of the target pixels are present around the target pixels that remain undetermined as to whether the target pixels are a foreground or a background, and pixel values of the target pixels and pixel values of the surrounding pixels are close in distance, processing is repeated in which a label of a target pixel indicating whether the target pixel is a foreground or a background is changed to a same label as a label of a surrounding pixel and a confidence level of the target pixel is changed based on a confidence level of the surrounding pixel and the distance, so that determination of all of the target pixels as to whether the target pixels are a foreground or a background is eventually completed.
13. An image processing method for an image processing device, comprising: segmentation processing for, when foreground pixels determined to be a foreground, background pixels determined to be a background, and undetermined pixels that remain undetermined as to whether they are a foreground or a background are designated in an image, further segmenting the undetermined pixels into foreground pixels and background pixels based on designation information; segment information acquisition processing for acquiring segment information for a case in which a first image is segmented into foreground pixels and background pixels in the segmentation processing; and segmentation control processing for, when controlling the segmentation processing to segment a second image having a predetermined relationship with the first image, specifying foreground pixels, background pixels and undetermined pixels of the second image in the image, by using the segment information acquired in the boundary position acquisition processing, wherein the segmentation control processing determines an area in which a predetermined width in accordance with the relationship between the first image and the second image is provided for a boundary line between the foreground pixels and the background pixels determined by the segment information, specifies an area in the second image corresponding to the determined area as undetermined pixels, and controls the segmentation processing to segment the second image.
14. A non-transitory computer-readable storage medium having stored thereon a program that is executable by a computer, the program being executable by the computer to perform functions comprising: segmentation processing for, when foreground pixels determined to be a foreground, background pixels determined to be a background, and undetermined pixels that remain undetermined as to whether they are a foreground or a background are designated in an image, further segmenting the undetermined pixels into foreground pixels and background pixels based on designation information; segment information acquisition for acquiring segment information for a case in which a first image is segmented into foreground pixels and background pixels in the segmentation processing; and segmentation control for, when controlling the segmentation processing to segment a second image having a predetermined relationship with the first image, specifying foreground pixels, background pixels, and undetermined pixels of the second image in the image, by use of the segment information acquired in the segment acquisition, wherein the segmentation control determines an area in which a predetermined width in accordance with the relationship between the first image and the second image is provided for a boundary line between the foreground pixels and the background pixels determined by the segment information, specifies an area in the second image corresponding to the determined area as undetermined pixels, and controls the segmentation processing to segment the second image.
