use uint8 datatype for automatic output image format (#1281)

* use uint8 datatype for automatic output image format

* update changelog

Author: vincentsarago

CHANGES.md

@@ -5,18 +5,19 @@
 ### Misc 
 
 * update rio-tiler requirement to `>=8.0,<9.0`
+* return `UINT8` datatype JPEG/PNG when no output format is specified **breaking change**
 
 ### titiler.core
 
-* add `band_description` attribute to `Point` output model (returned by /point endpoints)  **breaking change**
+* add `band_description` attribute to `Point` output model (returned by /point endpoints) **breaking change**
 
 ### titiler.extensions
 
 * update rio-cogeo requirement to `7.0,<8.0`
 
 ### titiler.mosaic
 
-* change Response model for `/point` endpoint  **breaking change**
+* change Response model for `/point` endpoint **breaking change**
 
     ```python
     # before

src/titiler/core/tests/test_rendering.py

@@ -115,3 +115,46 @@ def test_rendering():
             assert data_converted[:, 2, 2].tolist() == [100, 100, 100, 50]
             # Non-masked from CMAP
             assert data_converted[:, 3, 3].tolist() == [255, 255, 255, 255]
+
+
+def test_rendering_auto_dtype():
+    """Test Automatic format selection and dtype"""
+    data = numpy.ma.zeros((1, 5, 5), dtype="uint16") + 1
+    data.mask = False
+    # add a masked value
+    data.mask[0, 0, 0] = True
+    im = ImageData(data)
+    with pytest.warns(InvalidDatatypeWarning):
+        content, media = render_image(im)
+        assert media == "image/png"
+
+        with MemoryFile(content) as mem:
+            with mem.open() as dst:
+                assert dst.count == 2
+                assert dst.dtypes == ("uint8", "uint8")
+
+    # Not Masked
+    data = numpy.ma.zeros((1, 5, 5), dtype="uint16") + 1
+    data.mask = False
+    im = ImageData(data)
+    with pytest.warns(InvalidDatatypeWarning):
+        content, media = render_image(im)
+        assert media == "image/jpeg"
+
+        with MemoryFile(content) as mem:
+            with mem.open() as dst:
+                assert dst.count == 1
+                assert dst.dtypes == ("uint8",)
+
+    # Full Masked
+    data = numpy.ma.zeros((1, 5, 5), dtype="uint16") + 1
+    data.mask = True
+    im = ImageData(data)
+    with pytest.warns(InvalidDatatypeWarning):
+        content, media = render_image(im)
+        assert media == "image/png"
+
+        with MemoryFile(content) as mem:
+            with mem.open() as dst:
+                assert dst.count == 2
+                assert dst.dtypes == ("uint8", "uint8")

src/titiler/core/titiler/core/utils.py

@@ -87,10 +87,6 @@ def render_image(  # noqa: C901
             mask != input_range[0], alpha_from_cmap, output_range[0][0]
         ).astype(data.dtype)
 
-    # If output_format is not set, we choose between JPEG and PNG
-    if not output_format:
-        output_format = ImageType.jpeg if mask.all() else ImageType.png
-
     # format-specific valid dtypes
     format_dtypes = {
         ImageType.png: ["uint8", "uint16"],
@@ -100,6 +96,15 @@ def render_image(  # noqa: C901
         ImageType.jp2: ["uint8", "int16", "uint16"],
     }
 
+    # If output_format is not set, we choose between JPEG and PNG
+    if not output_format:
+        # Check if any alpha value == min datatype value (== Masked)
+        is_masked = (mask == dtype_ranges[str(mask.dtype)][0]).any()
+        output_format = ImageType.png if is_masked else ImageType.jpeg
+        # For automatic format we make sure the output datatype
+        # will be the same for both JPEG and PNG
+        format_dtypes[ImageType.png] = ["uint8"]
+
     valid_dtypes = format_dtypes.get(output_format, [])
     if valid_dtypes and data.dtype not in valid_dtypes:
         warnings.warn(