19. An image retrieval method based on an input second image, the image retrieval method comprising a learning step, a preliminary processing step, a graph update step, and a retrieval step, wherein the learning step includes: performing a segmentation operation on a first image having annotations to segment the first image into one or more image regions; extracting image feature vectors and text feature vectors from all the image regions to obtain an image feature matrix and a text feature matrix; projecting the image feature matrix and the text feature matrix into a sub-space so as to maximize covariance between an image feature and a text feature, thereby obtaining the projected image feature matrix and the text feature matrix; providing a storage device, and storing the projected image feature matrix and the text feature matrix, wherein the step of storing the projected image feature matrix and the text feature matrix occurs subsequent to the step of providing the storage device; establishing first links between the image regions based on the projected image feature matrix; establishing second links between the first image and the image regions based on a result of the segmentation operation; establishing third links between the first image and the annotations based on the first image having the annotations; establishing fourth links between the annotations based on the projected text feature matrix; calculating weights of all the links; and obtaining a graph showing a triangular relationship between the first image, the image regions, and the annotations based on all the links and the weights of the links corresponding to the links, the preliminary processing step includes: receiving the second image; performing the segmentation operation on the second image to segment the second image into one or more image regions; extracting image feature vectors from all the image regions to obtain an image feature matrix of the second image; and projecting the image feature matrix into the sub-space to obtain a projected image feature matrix of the second image, the graph update step includes: establishing fifth links between the image region nodes of the second image and the image region nodes in the graph based on the projected first image feature matrix and the second image feature matrix; establishing sixth links between the second image and the image region nodes based on a result of the segmentation operation; determining weights of the links of the fifth links and the sixth links; and updating the graph based on the fifth links and the sixth links and the weights of the links corresponding to the fifth links and the sixth links, and the retrieval step includes: generating a restart vector corresponding to the second image; and acquiring and outputting a predetermined number of annotations most closely related to the second image with a random walk with restart.
19. An image retrieval method based on an input second image, the image retrieval method comprising a learning step, a preliminary processing step, a graph update step, and a retrieval step, wherein the learning step includes: performing a segmentation operation on a first image having annotations to segment the first image into one or more image regions; extracting image feature vectors and text feature vectors from all the image regions to obtain an image feature matrix and a text feature matrix; projecting the image feature matrix and the text feature matrix into a sub-space so as to maximize covariance between an image feature and a text feature, thereby obtaining the projected image feature matrix and the text feature matrix; providing a storage device, and storing the projected image feature matrix and the text feature matrix, wherein the step of storing the projected image feature matrix and the text feature matrix occurs subsequent to the step of providing the storage device; establishing first links between the image regions based on the projected image feature matrix; establishing second links between the first image and the image regions based on a result of the segmentation operation; establishing third links between the first image and the annotations based on the first image having the annotations; establishing fourth links between the annotations based on the projected text feature matrix; calculating weights of all the links; and obtaining a graph showing a triangular relationship between the first image, the image regions, and the annotations based on all the links and the weights of the links corresponding to the links, the preliminary processing step includes: receiving the second image; performing the segmentation operation on the second image to segment the second image into one or more image regions; extracting image feature vectors from all the image regions to obtain an image feature matrix of the second image; and projecting the image feature matrix into the sub-space to obtain a projected image feature matrix of the second image, the graph update step includes: establishing fifth links between the image region nodes of the second image and the image region nodes in the graph based on the projected first image feature matrix and the second image feature matrix; establishing sixth links between the second image and the image region nodes based on a result of the segmentation operation; determining weights of the links of the fifth links and the sixth links; and updating the graph based on the fifth links and the sixth links and the weights of the links corresponding to the fifth links and the sixth links, and the retrieval step includes: generating a restart vector corresponding to the second image; and acquiring and outputting a predetermined number of annotations most closely related to the second image with a random walk with restart. 20. The image retrieval method according to claim 19 , wherein a keyword is further input, and in the retrieval step, the restart vector corresponding to the second image and the keyword is generated and a predetermined number of images most closely related to the second image and the keyword are acquired and output based on the updated graph.