откопал старый патч watermark patch by Vadym Zakovinko
причесал, добавил проверок, убрал memory leaking (надеюсь), переделал с JPG на PNG, добавил полноценную работу с альфаканалом. Работает с версией 1.2.6.

пример использования
location /images/ {
image_filter_buffer 10M;
image_filter_watermark "/srv/www/htdocs/images/swtor.png";
image_filter_watermark_position top-right; # top-left|top-right|bottom-right|bottom-left
image_filter watermark;
expires -1;
}

сам патч:

--- src/http/modules/ngx_http_image_filter_module.c
+++ src/http/modules/ngx_http_image_filter_module.c
@@ -2,6 +2,7 @@
/*
* Copyright (C) Igor Sysoev
* Copyright (C) Nginx, Inc.
+ * watermark patch: Vadym Zakovinko <vp.zakovinko@gmail.com> updated by Trurl McByte <trurl@mcbyte.net>
*/


@@ -18,6 +19,7 @@
#define NGX_HTTP_IMAGE_RESIZE 3
#define NGX_HTTP_IMAGE_CROP 4
#define NGX_HTTP_IMAGE_ROTATE 5
+#define NGX_HTTP_IMAGE_WATERMARK 6


#define NGX_HTTP_IMAGE_START 0
@@ -46,6 +48,9 @@

ngx_flag_t transparency;

+ ngx_str_t watermark;
+ ngx_str_t watermark_position;
+
ngx_http_complex_value_t *wcv;
ngx_http_complex_value_t *hcv;
ngx_http_complex_value_t *acv;
@@ -150,6 +155,20 @@
offsetof(ngx_http_image_filter_conf_t, buffer_size),
NULL },

+ { ngx_string("image_filter_watermark"),
+ NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+ ngx_conf_set_str_slot,
+ NGX_HTTP_LOC_CONF_OFFSET,
+ offsetof(ngx_http_image_filter_conf_t, watermark),
+ NULL },
+
+ { ngx_string("image_filter_watermark_position"),
+ NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+ ngx_conf_set_str_slot,
+ NGX_HTTP_LOC_CONF_OFFSET,
+ offsetof(ngx_http_image_filter_conf_t, watermark_position),
+ NULL },
+
ngx_null_command
};

@@ -518,6 +537,15 @@
return ngx_http_image_resize(r, ctx);
}

+ if (conf->filter == NGX_HTTP_IMAGE_WATERMARK) {
+
+ if (!conf->watermark.data) {
+ return NULL;
+ }
+
+ return ngx_http_image_resize(r, ctx);
+ }
+
ctx->max_width = ngx_http_image_filter_get_value(r, conf->wcv, conf->width);
if (ctx->max_width == 0) {
return NULL;
@@ -813,6 +841,10 @@

resize = 0;

+ } else if (conf->filter == NGX_HTTP_IMAGE_WATERMARK) {
+
+ resize = 0;
+
} else { /* NGX_HTTP_IMAGE_CROP */

resize = 0;
@@ -958,6 +990,40 @@
gdImageColorTransparent(dst, gdImageColorExact(dst, red, green, blue));
}

+ if (conf->filter == NGX_HTTP_IMAGE_WATERMARK && conf->watermark.data) {
+ FILE *watermark_file = fopen((const char *)conf->watermark.data, "r");
+ if (watermark_file) {
+ gdImagePtr watermark, watermark_mix;
+ ngx_int_t wdx = 0, wdy = 0;
+
+ watermark = gdImageCreateFromPng(watermark_file);
+
+ if(watermark != NULL) {
+ watermark_mix = gdImageCreateTrueColor(watermark->sx, watermark->sy);
+ if (ngx_strcmp(conf->watermark_position.data,
+ "bottom-right") == 0) {
+ wdx = dx - watermark->sx - 10;
+ wdy = dy - watermark->sy - 10;
+ } else if (ngx_strcmp(conf->watermark_position.data, "top-left") == 0) {
+ wdx = wdy = 10;
+ } else if (ngx_strcmp(conf->watermark_position.data, "top-right") == 0) {
+ wdx = dx - watermark->sx - 10;
+ wdy = 10;
+ } else if (ngx_strcmp(conf->watermark_position.data, "bottom-left") == 0) {
+ wdx = 10;
+ wdy = dy - watermark->sy - 10;
+ }
+ gdImageCopy(watermark_mix, dst, 0, 0, wdx, wdy, watermark->sx, watermark->sy);
+ gdImageCopy(watermark_mix, watermark, 0, 0, 0, 0, watermark->sx, watermark->sy);
+ gdImageCopyMerge(dst, watermark_mix, wdx, wdy, 0, 0, watermark->sx, watermark->sy, 75);
+ gdFree(watermark);
+ gdFree(watermark_mix);
+ } else { ngx_log_error(NGX_LOG_ERR, r->connection->log, 0, "watermark file '%s' is not PNG", conf->watermark.data);}
+ } else {
+ ngx_log_error(NGX_LOG_ERR, r->connection->log, 0, "watermark file '%s' not found", conf->watermark.data);
+ }
+ }
+
sharpen = ngx_http_image_filter_get_value(r, conf->shcv, conf->sharpen);
if (sharpen > 0) {
gdImageSharpen(dst, sharpen);
@@ -1223,6 +1289,11 @@
ngx_conf_merge_size_value(conf->buffer_size, prev->buffer_size,
1 * 1024 * 1024);

+ ngx_conf_merge_str_value(conf->watermark, prev->watermark, "");
+
+ ngx_conf_merge_str_value(conf->watermark_position,
+ prev->watermark_position, "bottom-right");
+
return NGX_CONF_OK;
}

@@ -1252,6 +1323,9 @@
} else if (ngx_strcmp(value[i].data, "size") == 0) {
imcf->filter = NGX_HTTP_IMAGE_SIZE;

+ } else if (ngx_strcmp(value[i].data, "watermark") == 0) {
+ imcf->filter = NGX_HTTP_IMAGE_WATERMARK;
+
} else {
goto failed;
}