clarify that input with an alpha channel requires additional computation (closes: #142)

README.md

@@ -27,15 +27,15 @@ software, because the raw pixel data never has to be loaded into memory.
 The following table shows how img2pdf handles different input depending on the
 input file format and image color space.
 
-| Format               | Colorspace                     | Result        |
-| -------------------- | ------------------------------ | ------------- |
-| JPEG                 | any                            | direct        |
-| JPEG2000             | any                            | direct        |
-| PNG (non-interlaced) | any                            | direct        |
-| TIFF (CCITT Group 4) | monochrome                     | direct        |
-| any                  | any except CMYK and monochrome | PNG Paeth     |
-| any                  | monochrome                     | CCITT Group 4 |
-| any                  | CMYK                           | flate         |
+| Format                                | Colorspace                     | Result        |
+| ------------------------------------- | ------------------------------ | ------------- |
+| JPEG                                  | any                            | direct        |
+| JPEG2000                              | any                            | direct        |
+| PNG (non-interlaced, no transparency) | any                            | direct        |
+| TIFF (CCITT Group 4)                  | monochrome                     | direct        |
+| any                                   | any except CMYK and monochrome | PNG Paeth     |
+| any                                   | monochrome                     | CCITT Group 4 |
+| any                                   | CMYK                           | flate         |
 
 For JPEG, JPEG2000, non-interlaced PNG and TIFF images with CCITT Group 4
 encoded data, img2pdf directly embeds the image data into the PDF without

src/img2pdf.py

@@ -1823,8 +1823,8 @@ def read_images(rawdata, colorspace, first_frame_only=False, rot=None):
 
                 smaskidat, _, _ = to_png_data(a)
                 logger.warning(
-                    "Image contains an alpha channel which will be stored "
-                    "as a separate soft mask (/SMask) image in PDF."
+                    "Image contains an alpha channel. Computing a separate "
+                    "soft mask (/SMask) image to store transparency in PDF."
                 )
             elif color in [Colorspace.P, Colorspace.PA] and iccp is not None:
                 # PDF does not support palette images with icc profile