Question: I have found How to return a numpy array as an image using FastAPI?, however, I am still struggling to show the image, which appears just as a white square. I read an array into io.BytesIO like so: def iterarray(array): output = io.BytesIO() np.savez(output, array) yield output.get_value()  In my endpoint, my return is StreamingResponse(iterarray(), media_type='application/octet-stream') When I leave the media_type blank to be inferred a zipfile is downloaded. How do I get the array to be displayed as an image?

Answer: Option 1 - Return image as bytes The below examples show how to convert an image loaded from disk, or an in-memory image (in the form of numpy array), into bytes (using either PIL or OpenCV libraries) and return them using a custom Response directly. For the purposes of this demo, the below code is used to create the in-memory sample image (numpy array), which is based on this answer. # Function to create a sample RGB image def create_img(): w, h = 512, 512 arr = np.zeros((h, w, 3), dtype=np.uint8) arr[0:256, 0:256] = [255, 0, 0] # red patch in upper left return arr  Using PIL Server side: You can load an image from disk using Image.open, or use Image.fromarray to load an in-memory image (Note: For demo purposes, when the case is loading the image from disk, the below demonstrates that operation inside the route. However, if the same image is going to be served multiple times, one could load the image only once at startup and store it to the app instance, as described in this answer and this answer). Next, write the image to a buffered stream, i.e., BytesIO, and use the getvalue() method to get the entire contents of the buffer. Even though the buffered stream is garbage collected when goes out of scope, it is generally better to call close() or use the with statement, as shown here and in the example below. from fastapi import Response from PIL import Image import numpy as np import io @app.get('/image', response_class=Response) def get_image(): # loading image from disk # im = Image.open('test.png') # using an in-memory image arr = create_img() im = Image.fromarray(arr) # save image to an in-memory bytes buffer with io.BytesIO() as buf: im.save(buf, format='PNG') im_bytes = buf.getvalue() headers = {'Content-Disposition': 'inline; filename="test.png"'} return Response(im_bytes, headers=headers, media_type='image/png')  Client side: The below demonstrates how to send a request to the above endpoint using Python requests module, and write the received bytes to a file, or convert the bytes back into PIL Image, as described here. import requests from PIL import Image url = 'http://127.0.0.1:8000/image' r = requests.get(url=url) # write raw bytes to file with open('test.png', 'wb') as f: f.write(r.content) # or, convert back to PIL Image # im = Image.open(io.BytesIO(r.content)) # im.save('test.png')  Using OpenCV Server side: You can load an image from disk using cv2.imread() function, or use an in-memory image, which—if it is in RGB order, as in the example below—needs to be converted, as OpenCV uses BGR as its default colour order for images. Next, use cv2.imencode() function, which compresses the image data (based on the file extension you pass that defines the output format, i.e., .png, .jpg, etc.) and stores it in an in-memory buffer that is used to transfer the data over the network. import cv2 @app.get('/image', response_class=Response) def get_image(): # loading image from disk # arr = cv2.imread('test.png', cv2.IMREAD_UNCHANGED) # using an in-memory image arr = create_img() arr = cv2.cvtColor(arr, cv2.COLOR_RGB2BGR) # arr = cv2.cvtColor(arr, cv2.COLOR_RGBA2BGRA) # if dealing with 4-channel RGBA (transparent) image success, im = cv2.imencode('.png', arr) headers = {'Content-Disposition': 'inline; filename="test.png"'} return Response(im.tobytes(), headers=headers, media_type='image/png')  Client side: On client side, you can write the raw bytes to a file, or use the numpy.frombuffer() function and cv2.imdecode() function to decompress the buffer into an image format (similar to this)—cv2.imdecode() does not require a file extension, as the correct codec will be deduced from the first bytes of the compressed image in the buffer. url = 'http://127.0.0.1:8000/image' r = requests.get(url=url) # write raw bytes to file with open('test.png', 'wb') as f: f.write(r.content) # or, convert back to image format # arr = np.frombuffer(r.content, np.uint8) # img_np = cv2.imdecode(arr, cv2.IMREAD_UNCHANGED) # cv2.imwrite('test.png', img_np)   Useful Information Since you noted that you would like the image displayed similar to a FileResponse, using a custom Response to return the bytes should be the way to do this, instead of using StreamingResponse (as shown in your question). To indicate that the image should be viewed in the browser, the HTTP response should include the following Content-Disposition header, as described here and as shown in the above examples (the quotes around the filename are required, if the filename contains special characters): headers = {'Content-Disposition': 'inline; filename="test.png"'}  Whereas, to have the image downloaded rather than viewed (use attachment instead of inline): headers = {'Content-Disposition': 'attachment; filename="test.png"'}  If you would like to display (or download) the image using a JavaScript interface, such as Fetch API or Axios, have a look at the answers here and here. As for StreamingResponse, if the entire numpy array/image is already loaded into memory, StreamingResponse would not be necessary at all (and certainly, should not be the preferred choice for returning data that is already loaded in memory to the client). StreamingResponse streams by iterating over the chunks provided by your iter() function. As shown in the implementation of StreamingResponse class, if the iterator/generator you passed is not an AsyncIterable, a thread from the external threadpool—see this answer for more details on that threadpool—will be spawned to run the synchronous iterator you passed, using Starlette's iterate_in_threadpool() function, in order to avoid blocking the event loop. It should also be noted that the Content-Length response header is not set when using StreamingResponse (which makes sense, since StreamingResponse is supposed to be used when you don't know the size of the response beforehand), unlike other Response classes of FastAPI/Starlette that set that header for you, so that the browser will know where the data ends. It should be kept that way, as if the Content-Length header is included (of which its value should match the overall response body size in bytes), then to the server StreamingResponse would look the same as Response, as the server would not use transfer-encoding: chunked in that case (even though at the application level the two would still differ)—take a look at Uvicorn's documentation on response headers and MDN'S documentation on Transfer-Encoding: chunked for further details. Even in cases where you know the body size beforehand, but would still need using StreamingResponse, as it would allow you to load and transfer data by specifying the chunk size of your choice, unlike FileResponse (see later on for more details), you should ensure not setting the Content-Length header on your own, e.g., StreamingResponse(iterfile(), headers={'Content-Length': str(content_length)}), as this would result in the server not using transfer-encoding: chunked (regardless of the application delivering the data to the web server in chunks, as shown in the relevant implementation). As described in this answer:  Chunked transfer encoding makes sense when you don't know the size of your output ahead of time, and you don't want to wait to collect it all to find out before you start sending it to the client. That can apply to stuff like serving the results of slow database queries, but it doesn't generally apply to serving images.  Even if you would like to stream an image file that is saved on the disk, file-like objects, such as those created by open(), are normal iterators; thus, you could return them directly in a StreamingResponse, as described in the documentation and as shown below (if you find yield from f being rather slow, when using StreamingResponse, please have a look at this answer on how to read the file in chunks with the chunk size of your choice—which should be set based on your needs, as well as your server's resources). It should be noted that using FileResponse would also read the file contents into memory in chunks, instead of the entire contents at once. However, as can be seen in the implementation of FileResponse class, the chunk size used is pre-defined and set to 64KB. Thus, based on one's requirements, they should decide on which of the two Response classes they should use. @app.get('/image') def get_image(): def iterfile(): with open('test.png', mode='rb') as f: yield from f return StreamingResponse(iterfile(), media_type='image/png')  Or, if the image was already loaded into memory instead, and then saved into a BytesIO buffered stream, since BytesIO is a file-like object (like all the concrete classes of io module), you could return it directly in a StreamingResponse (or, preferably, simply call buf.getvalue() to get the entire image bytes and return them using a custom Response directly, as shown earlier). In case of returning the buffered stream, as shown in the example below, please remember to call buf.seek(0), in order to rewind the cursor to the start of the buffer, as well as call close() inside a background task, in order to discard the buffer, once the response has been sent to the client. from fastapi import BackgroundTasks @app.get('/image') def get_image(background_tasks: BackgroundTasks): # supposedly, the buffer already existed in memory arr = create_img() im = Image.fromarray(arr) buf = BytesIO() im.save(buf, format='PNG') # rewind the cursor to the start of the buffer buf.seek(0) # discard the buffer, after the response is returned background_tasks.add_task(buf.close) return StreamingResponse(buf, media_type='image/png')  Thus, in your case scenario, the most suited approach would be to return a custom Response directly, including your custom content and media_type, as well as setting the Content-Disposition header, as described earlier, so that the image is viewed in the browser. Option 2 - Return image as JSON-encoded numpy array The below should not be used for displaying the image in the browser, but it is rather added here for the sake of completeness, showing how to convert an image into a numpy array (preferably, using asarray() function), then return the data in JSON format, and finally, convert the data back to image on client side, as described in this and this answer. For faster alternatives to the standard Python json library, see this answer. Using PIL Server side: from PIL import Image import numpy as np import json @app.get('/image') def get_image(): im = Image.open('test.png') # im = Image.open('test.png').convert('RGBA') # if dealing with 4-channel RGBA (transparent) image arr = np.asarray(im) return json.dumps(arr.tolist())  Client side: import requests from PIL import Image import numpy as np import json url = 'http://127.0.0.1:8000/image' r = requests.get(url=url) arr = np.asarray(json.loads(r.json())).astype(np.uint8) im = Image.fromarray(arr) im.save('test_received.png')  Using OpenCV Server side: import cv2 import json @app.get('/image') def get_image(): arr = cv2.imread('test.png', cv2.IMREAD_UNCHANGED) return json.dumps(arr.tolist())  Client side: import requests import numpy as np import cv2 import json url = 'http://127.0.0.1:8000/image' r = requests.get(url=url) arr = np.asarray(json.loads(r.json())).astype(np.uint8) cv2.imwrite('test_received.png', arr)