Tk_PhotoPutBlock无法正确渲染SVG文件的技术问题

我尝试用C语言结合Tk API，基于resvg库实现SVG转Tk Photo对象并在Tk Frame中显示的功能。

使用的SVG内容如下：

<svg id="svg1" viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">  
  <circle r="32" cx="35" cy="65" fill="#F00" opacity="0.5"/>  
  <circle r="32" cx="65" cy="65" fill="#0F0" opacity="0.5"/>  
  <circle r="32" cx="50" cy="35" fill="#00F" opacity="0.5"/>
</svg>

对应的C代码实现：

Tk_PhotoHandle source;
unsigned char *imgData;
// PhotoObj : set photo [image create photo]; # tcl code
// FileObj  : file.svg
source = Tk_FindPhoto(interp, Tcl_GetString(PhotoObj));
if (source == NULL) {
    Tcl_SetResult(interp, "photo not found", TCL_STATIC);
    return TCL_ERROR;
}
char *sp = Tcl_GetString(FileObj);
resvg_init_log;
resvg_options *opt = resvg_options_create();
resvg_options_load_system_fonts(opt);
resvg_render_tree *tree;
int err = resvg_parse_tree_from_file(sp, opt, &tree);
resvg_options_destroy(opt);
if (err != RESVG_OK) {
    printf("Error id: %i\n", err);
    return TCL_ERROR;
}
resvg_size size = resvg_get_image_size(tree);
int width = (int)size.width;
int height = (int)size.height;

imgData = (unsigned char *)Tcl_Alloc(width*height*4);
if (imgData == NULL) {
    Tcl_SetResult(interp, "cannot alloc image buffer", TCL_STATIC);
    return TCL_ERROR;
}
resvg_render(tree, resvg_transform_identity(), width, height, imgData);
Tk_PhotoImageBlock block;
Tk_PhotoGetImage(source, &block);
block.pixelPtr   = imgData;
block.width      = width;
block.height     = height;
block.pitch      = width * 4;
block.pixelSize  = 4;
block.offset[0]  = 0;
block.offset[1]  = 1;
block.offset[2]  = 2;
block.offset[3]  = 3;
if (Tk_PhotoSetSize(interp, source, width, height) != TCL_OK) {
    return TCL_ERROR;
}
Tk_PhotoPutBlock(interp, source, &block, 0, 0, width, height, TK_PHOTO_COMPOSITE_SET);
resvg_tree_destroy(tree);
return TCL_OK;

如图所示，左侧原SVG的颜色与右侧Tk中显示的图像颜色不一致。我怀疑问题出在imgData的数据格式上，原resvg示例中针对cairo表面做了数据转换处理，但我对图像格式转换了解有限，不知道Tk端需要如何调整数据才能匹配正确的颜色。

内容的提问来源于stack exchange，提问作者n.robert