Also use alpha channel from img2img input image as mask

modules/img2img.py

 import math
-from PIL import Image
+from PIL import Image, ImageOps, ImageChops
 
 from modules.processing import Processed, StableDiffusionProcessingImg2Img, process_images
 from modules.shared import opts, state
@@ -16,7 +16,9 @@ def img2img(prompt: str, init_img, init_img_with_mask, steps: int, sampler_index
 
     if is_inpaint:
         image = init_img_with_mask['image']
-        mask = init_img_with_mask['mask']
+        alpha_mask = ImageOps.invert(image.split()[-1]).convert('L').point(lambda x: 255 if x > 0 else 0, mode='1')
+        mask = ImageChops.lighter(alpha_mask, init_img_with_mask['mask'].convert('L')).convert('RGBA')
+        image = image.convert('RGB')
     else:
         image = init_img
         mask = None

modules/ui.py

@@ -323,7 +323,7 @@ def create_ui(txt2img, img2img, run_extras, run_pnginfo):
                 with gr.Group():
                     switch_mode = gr.Radio(label='Mode', elem_id="img2img_mode", choices=['Redraw whole image', 'Inpaint a part of image', 'Loopback', 'SD upscale'], value='Redraw whole image', type="index", show_label=False)
                     init_img = gr.Image(label="Image for img2img", source="upload", interactive=True, type="pil")
-                    init_img_with_mask = gr.Image(label="Image for inpainting with mask", elem_id="img2maskimg", source="upload", interactive=True, type="pil", tool="sketch", visible=False)
+                    init_img_with_mask = gr.Image(label="Image for inpainting with mask", elem_id="img2maskimg", source="upload", interactive=True, type="pil", tool="sketch", visible=False, image_mode="RGBA")
                     resize_mode = gr.Radio(label="Resize mode", show_label=False, choices=["Just resize", "Crop and resize", "Resize and fill"], type="index", value="Just resize")
 
                 steps = gr.Slider(minimum=1, maximum=150, step=1, label="Sampling Steps", value=20)