使用GIMP 2.8 Script-Fu实现图片叠加失败：小图无法显示

我想用GIMP 2.8的Script-Fu语言自动将一张小型PNG图片叠加在另一张大型PNG图片的中心位置，但调用如下脚本时，第一张图片（大图）能正常显示，第二张图片（小图）却无法显示：

(overlay-images "/tmp/bigImage.png" "/tmp/smallImage.png" "/tmp/output.png")

以下是我用于排查问题的带调试语句的脚本：

(define (overlay-images image-file-1 image-file-2 output-file)
  (let* (
      (image (car (file-png-load RUN-NONINTERACTIVE image-file-1 image-file-1)))
      (drawable (car (gimp-image-get-active-layer image)))
      (overlay-layer (car (gimp-file-load-layer RUN-NONINTERACTIVE image image-file-2)))
    )
    ;; Add a debugging statement here
    (gimp-message (string-append "overlay-layer: " (number->string overlay-layer)))
    (gimp-image-add-layer image overlay-layer -1)
    (gimp-message (string-append "Number of layers: " (number->string (car (gimp-image-get-layers image)))))
    (gimp-message (string-append "Overlay layer visible: " (if (= (car (gimp-drawable-get-visible overlay-layer)) TRUE) "yes" "no")))
    
    
    
    (let* (
        (image-width (car (gimp-image-width image)))
        (image-height (car (gimp-image-height image)))
        (overlay-width (car (gimp-drawable-width overlay-layer)))
        (overlay-height (car (gimp-drawable-height overlay-layer)))
        (offset-x (/ (- image-width overlay-width) 2))
        (offset-y (/ (- image-height overlay-height) 2))
      )
      (gimp-layer-set-offsets overlay-layer offset-x offset-y)
    )
    (let* (
    (overlay-offsets (gimp-drawable-offsets overlay-layer))
    (overlay-offset-x (car overlay-offsets))
    (overlay-offset-y (cadr overlay-offsets))
  )
  (gimp-message (string-append "Overlay layer size: " (number->string (car (gimp-drawable-width overlay-layer))) "x" (number->string (car (gimp-drawable-height overlay-layer)))))
  (gimp-message (string-append "Overlay layer position: (" (number->string overlay-offset-x) ", " (number->string overlay-offset-y) ")"))
  )
  (gimp-message (string-append "Number of layers before saving: " (number->string (car (gimp-image-get-layers image)))))
  (gimp-message (string-append "Overlay layer visible before saving: " (if (= (car (gimp-drawable-get-visible overlay-layer)) TRUE) "yes" "no")))
    (file-png-save RUN-NONINTERACTIVE image drawable output-file output-file 0 9 0 0 0 0 0)
    (gimp-image-delete image)
  )
)

我的预期是第二张图片（smallImage.png）能显示在第一张图片（bigImage.png）的中心位置。尝试交换两张图片的位置后：

(overlay-images "/tmp/smallImage.png" "/tmp/bigImage.png" "/tmp/output.png")

输出文件中能显示smallImage.png，说明不存在文件读取错误。请问我忽略了什么问题？

问题核心有两个：

1. 保存时仅导出了初始活跃图层：Script-Fu的file-png-save函数只会导出你指定的单个drawable，你代码里的drawable变量始终是第一张加载的大图图层，小图图层虽已添加，但没被包含到导出内容里。交换图片位置时，小图是初始图层，所以能被导出显示。
2. 图层顺序错误：你用gimp-image-add-layer把小图放在了-1位置（图层栈最底部），会被大图完全遮挡，即使解决保存问题也看不到小图。

解决步骤 #

1. 调整图层顺序：把小图图层放在大图上方，确保能被看到，修改添加图层的代码：
   将(gimp-image-add-layer image overlay-layer -1)改为：

   (gimp-image-add-layer image overlay-layer 0)
   

   （0表示放在图层栈最顶部，保证小图在视觉上覆盖大图）

2. 合并可见图层后保存：在导出前合并所有可见图层，更新drawable变量为合并后的图层，在file-png-save前添加：

   (set! drawable (car (gimp-image-merge-visible-layers image CLIP-TO-IMAGE)))
   

修改后的关键代码片段：

(gimp-image-add-layer image overlay-layer 0) ;; 调整图层顺序到顶部
    ;; ... 原有调试、位置计算代码 ...
    (gimp-message (string-append "Overlay layer visible before saving: " (if (= (car (gimp-drawable-get-visible overlay-layer)) TRUE) "yes" "no")))
    ;; 合并可见图层
    (set! drawable (car (gimp-image-merge-visible-layers image CLIP-TO-IMAGE)))
    (file-png-save RUN-NONINTERACTIVE image drawable output-file output-file 0 9 0 0 0 0 0)
    (gimp-image-delete image)

这样修改后，脚本会把小图放在大图上方，合并图层后再导出，就能得到小图居中显示在大图上的结果。

内容的提问来源于stack exchange，提问作者Isaac