code by Tim (#451), only the stacking approach

Author: Sonja Stockhaus

src/spatialdata_plot/pl/render.py

@@ -338,7 +338,7 @@ def _render_shapes(
         cax = None
         if aggregate_with_reduction is not None:
             vmin = aggregate_with_reduction[0].values if norm.vmin is None else norm.vmin
-            vmax = aggregate_with_reduction[1].values if norm.vmin is None else norm.vmax
+            vmax = aggregate_with_reduction[1].values if norm.vmax is None else norm.vmax
             if (norm.vmin is not None or norm.vmax is not None) and norm.vmin == norm.vmax:
                 # value (vmin=vmax) is placed in the middle of the colorbar so that we can distinguish it from over and
                 # under values in case clip=True or clip=False with cmap(under)=cmap(0) & cmap(over)=cmap(1)
@@ -846,20 +846,22 @@ def _render_images(
     # 2) Image has any number of channels but 1
     else:
         layers = {}
-        for ch_index, c in enumerate(channels):
-            layers[c] = img.sel(c=c).copy(deep=True).squeeze()
-
-            if not isinstance(render_params.cmap_params, list):
-                if render_params.cmap_params.norm is not None:
-                    layers[c] = render_params.cmap_params.norm(layers[c])
+        for ch_idx, ch in enumerate(channels):
+            layers[ch] = img.sel(c=ch).copy(deep=True).squeeze()
+            if isinstance(render_params.cmap_params, list):
+                ch_norm = render_params.cmap_params[ch_idx].norm
+                ch_cmap_is_default = render_params.cmap_params[ch_idx].cmap_is_default
             else:
-                if render_params.cmap_params[ch_index].norm is not None:
-                    layers[c] = render_params.cmap_params[ch_index].norm(layers[c])
+                ch_norm = render_params.cmap_params.norm
+                ch_cmap_is_default = render_params.cmap_params.cmap_is_default
+
+            if not ch_cmap_is_default and ch_norm is not None:
+                layers[ch_idx] = ch_norm(layers[ch_idx])
 
         # 2A) Image has 3 channels, no palette info, and no/only one cmap was given
         if palette is None and n_channels == 3 and not isinstance(render_params.cmap_params, list):
             if render_params.cmap_params.cmap_is_default:  # -> use RGB
-                stacked = np.stack([layers[c] for c in channels], axis=-1)
+                stacked = np.stack([layers[ch] for ch in layers], axis=-1)
             else:  # -> use given cmap for each channel
                 channel_cmaps = [render_params.cmap_params.cmap] * n_channels
                 stacked = (
@@ -892,12 +894,54 @@ def _render_images(
             # overwrite if n_channels == 2 for intuitive result
             if n_channels == 2:
                 seed_colors = ["#ff0000ff", "#00ff00ff"]
-            else:
+                channel_cmaps = [_get_linear_colormap([c], "k")[0] for c in seed_colors]
+                colored = np.stack(
+                    [channel_cmaps[ch_ind](layers[ch]) for ch_ind, ch in enumerate(channels)],
+                    0,
+                ).sum(0)
+                colored = colored[:, :, :3]
+            elif n_channels == 3:
                 seed_colors = _get_colors_for_categorical_obs(list(range(n_channels)))
+                channel_cmaps = [_get_linear_colormap([c], "k")[0] for c in seed_colors]
+                colored = np.stack(
+                    [channel_cmaps[ind](layers[ch]) for ind, ch in enumerate(channels)],
+                    0,
+                ).sum(0)
+                colored = colored[:, :, :3]
+            else:
+                if isinstance(render_params.cmap_params, list):
+                    cmap_is_default = render_params.cmap_params[0].cmap_is_default
+                else:
+                    cmap_is_default = render_params.cmap_params.cmap_is_default
 
-            channel_cmaps = [_get_linear_colormap([c], "k")[0] for c in seed_colors]
-            colored = np.stack([channel_cmaps[ind](layers[ch]) for ind, ch in enumerate(channels)], 0).sum(0)
-            colored = colored[:, :, :3]
+                if cmap_is_default:
+                    seed_colors = _get_colors_for_categorical_obs(list(range(n_channels)))
+                else:
+                    # Sample n_channels colors evenly from the colormap
+                    if isinstance(render_params.cmap_params, list):
+                        seed_colors = [
+                            render_params.cmap_params[i].cmap(i / (n_channels - 1)) for i in range(n_channels)
+                        ]
+                    else:
+                        seed_colors = [render_params.cmap_params.cmap(i / (n_channels - 1)) for i in range(n_channels)]
+                channel_cmaps = [_get_linear_colormap([c], "k")[0] for c in seed_colors]
+
+                # Stack (n_channels, height, width) → (height*width, n_channels)
+                H, W = next(iter(layers.values())).shape
+                comp_rgb = np.zeros((H, W, 3), dtype=float)
+
+                # For each channel: map to RGBA, apply constant alpha, then add
+                for ch_idx, ch in enumerate(channels):
+                    layer_arr = layers[ch]
+                    rgba = channel_cmaps[ch_idx](layer_arr)
+                    rgba[..., 3] = render_params.alpha
+                    comp_rgb += rgba[..., :3] * rgba[..., 3][..., None]
+
+                colored = np.clip(comp_rgb, 0, 1)
+                logger.info(
+                    f"Your image has {n_channels} channels. Sampling categorical colors and using "
+                    f"multichannel strategy 'stack' to render."
+                )  # TODO: update when pca is added as strategy
 
             _ax_show_and_transform(
                 colored,
@@ -943,6 +987,7 @@ def _render_images(
                 zorder=render_params.zorder,
             )
 
+        # 2D) Image has n channels, no palette but cmap info
         elif palette is not None and got_multiple_cmaps:
             raise ValueError("If 'palette' is provided, 'cmap' must be None.")
 

src/spatialdata_plot/pl/utils.py

@@ -2006,7 +2006,7 @@ def _validate_col_for_column_table(
             table_name = next(iter(tables))
             if len(tables) > 1:
                 warnings.warn(
-                    f"Multiple tables contain color column, using {table_name}",
+                    f"Multiple tables contain column '{col_for_color}', using table '{table_name}'.",
                     UserWarning,
                     stacklevel=2,
                 )
@@ -2042,25 +2042,49 @@ def _validate_image_render_params(
         element_params[el] = {}
         spatial_element = param_dict["sdata"][el]
 
+        # robustly get channel names from image or multiscale image
         spatial_element_ch = (
-            spatial_element.c if isinstance(spatial_element, DataArray) else spatial_element["scale0"].c
+            spatial_element.c.values if isinstance(spatial_element, DataArray) else spatial_element["scale0"].c.values
         )
-        if (channel := param_dict["channel"]) is not None and (
-            (isinstance(channel[0], int) and max([abs(ch) for ch in channel]) <= len(spatial_element_ch))
-            or all(ch in spatial_element_ch for ch in channel)
-        ):
+        channel = param_dict["channel"]
+        if channel is not None:
+            # Normalize channel to always be a list of str or a list of int
+            if isinstance(channel, str):
+                channel = [channel]
+
+            if isinstance(channel, int):
+                channel = [channel]
+
+            # If channel is a list, ensure all elements are the same type
+            if not (isinstance(channel, list) and channel and all(isinstance(c, type(channel[0])) for c in channel)):
+                raise TypeError("Each item in 'channel' list must be of the same type, either string or integer.")
+
+            invalid = [c for c in channel if c not in spatial_element_ch]
+            if invalid:
+                raise ValueError(
+                    f"Invalid channel(s): {', '.join(str(c) for c in invalid)}. Valid choices are: {spatial_element_ch}"
+                )
             element_params[el]["channel"] = channel
         else:
             element_params[el]["channel"] = None
 
         element_params[el]["alpha"] = param_dict["alpha"]
 
-        if isinstance(palette := param_dict["palette"], list):
+        palette = param_dict["palette"]
+        assert isinstance(palette, list | type(None))  # if present, was converted to list, just to make sure
+
+        if isinstance(palette, list):
+            # case A: single palette for all channels
             if len(palette) == 1:
                 palette_length = len(channel) if channel is not None else len(spatial_element_ch)
                 palette = palette * palette_length
-            if (channel is not None and len(palette) != len(channel)) and len(palette) != len(spatial_element_ch):
-                palette = None
+            # case B: one palette per channel (either given or derived from channel length)
+            channels_to_use = spatial_element_ch if element_params[el]["channel"] is None else channel
+            if channels_to_use is not None and len(palette) != len(channels_to_use):
+                raise ValueError(
+                    f"Palette length ({len(palette)}) does not match channel length "
+                    f"({', '.join(str(c) for c in channels_to_use)})."
+                )
         element_params[el]["palette"] = palette
         element_params[el]["na_color"] = param_dict["na_color"]
 
@@ -2086,7 +2110,7 @@ def _validate_image_render_params(
 def _get_wanted_render_elements(
     sdata: SpatialData,
     sdata_wanted_elements: list[str],
-    params: (ImageRenderParams | LabelsRenderParams | PointsRenderParams | ShapesRenderParams),
+    params: ImageRenderParams | LabelsRenderParams | PointsRenderParams | ShapesRenderParams,
     cs: str,
     element_type: Literal["images", "labels", "points", "shapes"],
 ) -> tuple[list[str], list[str], bool]:
@@ -2243,7 +2267,7 @@ def _create_image_from_datashader_result(
 
 
 def _datashader_aggregate_with_function(
-    reduction: (Literal["sum", "mean", "any", "count", "std", "var", "max", "min"] | None),
+    reduction: Literal["sum", "mean", "any", "count", "std", "var", "max", "min"] | None,
     cvs: Canvas,
     spatial_element: GeoDataFrame | dask.dataframe.core.DataFrame,
     col_for_color: str | None,
@@ -2307,7 +2331,7 @@ def _datashader_aggregate_with_function(
 
 
 def _datshader_get_how_kw_for_spread(
-    reduction: (Literal["sum", "mean", "any", "count", "std", "var", "max", "min"] | None),
+    reduction: Literal["sum", "mean", "any", "count", "std", "var", "max", "min"] | None,
 ) -> str:
     # Get the best input for the how argument of ds.tf.spread(), needed for numerical values
     reduction = reduction or "sum"