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Heya fellow QGIS users<br>
<br>
I have a TIFF file like
<a class="moz-txt-link-freetext" href="https://drive.switch.ch/index.php/s/bfvkWCM1P6pbJ5E/download">https://drive.switch.ch/index.php/s/bfvkWCM1P6pbJ5E/download</a> that I
want to represent a black-and-white image with transparency. (Each
pixel is either opaque black, opaque white or fully transparent*.)<br>
<br>
In the TIFF at hand, this is represented by a 1-byte-per-pixel band
with a 4-component (so presumably RGBA-)palette with (opaque) black
and white entries and another 1-byte-per-pixel band for alpha.
gdalinfo output looks like this:<br>
<blockquote type="cite">
<pre><code>Driver: GTiff/GeoTIFF
Files: merged.tif
Size is 28346, 47244
Coordinate System is:
PROJCS["CH1903 / LV03",
GEOGCS["CH1903",
DATUM["CH1903",
SPHEROID["Bessel 1841",6377397.155,299.1528128,
AUTHORITY["EPSG","7004"]],
TOWGS84[674.374,15.056,405.346,0,0,0,0],
AUTHORITY["EPSG","6149"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4149"]],
PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"],
PARAMETER["latitude_of_center",46.95240555555556],
PARAMETER["longitude_of_center",7.439583333333333],
PARAMETER["azimuth",90],
PARAMETER["rectified_grid_angle",90],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",600000],
PARAMETER["false_northing",200000],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["Y",EAST],
AXIS["X",NORTH],
AUTHORITY["EPSG","21781"]]
Origin = (672173.366739814984612,233935.800211683992529)
Pixel Size = (0.105835038426470,-0.105835038426470)
Metadata:
AREA_OR_POINT=Area
Image Structure Metadata:
COMPRESSION=LZW
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left ( 672173.367, 233935.800) ( 8d23'35.46"E, 47d15'13.47"N)
Lower Left ( 672173.367, 228935.730) ( 8d23'32.57"E, 47d12'31.56"N)
Upper Right ( 675173.367, 233935.800) ( 8d25'58.14"E, 47d15'12.27"N)
Lower Right ( 675173.367, 228935.730) ( 8d25'55.13"E, 47d12'30.35"N)
Center ( 673673.367, 231435.765) ( 8d24'45.32"E, 47d13'51.92"N)
Band 1 Block=256x256 Type=Byte, ColorInterp=Palette
Mask Flags: PER_DATASET ALPHA
Color Table (RGB with 256 entries)
0: 255,255,255,255
1: 0,0,0,255
2: 0,0,0,255
3: 0,0,0,255
4: 0,0,0,255
5: 0,0,0,255
6: 0,0,0,255
7: 0,0,0,255
8: 0,0,0,255
9: 0,0,0,255
10: 0,0,0,255
11: 0,0,0,255
12: 0,0,0,255
13: 0,0,0,255
14: 0,0,0,255
15: 0,0,0,255
16: 0,0,0,255
17: 0,0,0,255
18: 0,0,0,255
19: 0,0,0,255
20: 0,0,0,255
21: 0,0,0,255
22: 0,0,0,255
23: 0,0,0,255
24: 0,0,0,255
25: 0,0,0,255
26: 0,0,0,255
27: 0,0,0,255
28: 0,0,0,255
29: 0,0,0,255
30: 0,0,0,255
31: 0,0,0,255
32: 0,0,0,255
33: 0,0,0,255
34: 0,0,0,255
35: 0,0,0,255
36: 0,0,0,255
37: 0,0,0,255
38: 0,0,0,255
39: 0,0,0,255
40: 0,0,0,255
41: 0,0,0,255
42: 0,0,0,255
43: 0,0,0,255
44: 0,0,0,255
45: 0,0,0,255
46: 0,0,0,255
47: 0,0,0,255
48: 0,0,0,255
49: 0,0,0,255
50: 0,0,0,255
51: 0,0,0,255
52: 0,0,0,255
53: 0,0,0,255
54: 0,0,0,255
55: 0,0,0,255
56: 0,0,0,255
57: 0,0,0,255
58: 0,0,0,255
59: 0,0,0,255
60: 0,0,0,255
61: 0,0,0,255
62: 0,0,0,255
63: 0,0,0,255
64: 0,0,0,255
65: 0,0,0,255
66: 0,0,0,255
67: 0,0,0,255
68: 0,0,0,255
69: 0,0,0,255
70: 0,0,0,255
71: 0,0,0,255
72: 0,0,0,255
73: 0,0,0,255
74: 0,0,0,255
75: 0,0,0,255
76: 0,0,0,255
77: 0,0,0,255
78: 0,0,0,255
79: 0,0,0,255
80: 0,0,0,255
81: 0,0,0,255
82: 0,0,0,255
83: 0,0,0,255
84: 0,0,0,255
85: 0,0,0,255
86: 0,0,0,255
87: 0,0,0,255
88: 0,0,0,255
89: 0,0,0,255
90: 0,0,0,255
91: 0,0,0,255
92: 0,0,0,255
93: 0,0,0,255
94: 0,0,0,255
95: 0,0,0,255
96: 0,0,0,255
97: 0,0,0,255
98: 0,0,0,255
99: 0,0,0,255
100: 0,0,0,255
101: 0,0,0,255
102: 0,0,0,255
103: 0,0,0,255
104: 0,0,0,255
105: 0,0,0,255
106: 0,0,0,255
107: 0,0,0,255
108: 0,0,0,255
109: 0,0,0,255
110: 0,0,0,255
111: 0,0,0,255
112: 0,0,0,255
113: 0,0,0,255
114: 0,0,0,255
115: 0,0,0,255
116: 0,0,0,255
117: 0,0,0,255
118: 0,0,0,255
119: 0,0,0,255
120: 0,0,0,255
121: 0,0,0,255
122: 0,0,0,255
123: 0,0,0,255
124: 0,0,0,255
125: 0,0,0,255
126: 0,0,0,255
127: 0,0,0,255
128: 0,0,0,255
129: 0,0,0,255
130: 0,0,0,255
131: 0,0,0,255
132: 0,0,0,255
133: 0,0,0,255
134: 0,0,0,255
135: 0,0,0,255
136: 0,0,0,255
137: 0,0,0,255
138: 0,0,0,255
139: 0,0,0,255
140: 0,0,0,255
141: 0,0,0,255
142: 0,0,0,255
143: 0,0,0,255
144: 0,0,0,255
145: 0,0,0,255
146: 0,0,0,255
147: 0,0,0,255
148: 0,0,0,255
149: 0,0,0,255
150: 0,0,0,255
151: 0,0,0,255
152: 0,0,0,255
153: 0,0,0,255
154: 0,0,0,255
155: 0,0,0,255
156: 0,0,0,255
157: 0,0,0,255
158: 0,0,0,255
159: 0,0,0,255
160: 0,0,0,255
161: 0,0,0,255
162: 0,0,0,255
163: 0,0,0,255
164: 0,0,0,255
165: 0,0,0,255
166: 0,0,0,255
167: 0,0,0,255
168: 0,0,0,255
169: 0,0,0,255
170: 0,0,0,255
171: 0,0,0,255
172: 0,0,0,255
173: 0,0,0,255
174: 0,0,0,255
175: 0,0,0,255
176: 0,0,0,255
177: 0,0,0,255
178: 0,0,0,255
179: 0,0,0,255
180: 0,0,0,255
181: 0,0,0,255
182: 0,0,0,255
183: 0,0,0,255
184: 0,0,0,255
185: 0,0,0,255
186: 0,0,0,255
187: 0,0,0,255
188: 0,0,0,255
189: 0,0,0,255
190: 0,0,0,255
191: 0,0,0,255
192: 0,0,0,255
193: 0,0,0,255
194: 0,0,0,255
195: 0,0,0,255
196: 0,0,0,255
197: 0,0,0,255
198: 0,0,0,255
199: 0,0,0,255
200: 0,0,0,255
201: 0,0,0,255
202: 0,0,0,255
203: 0,0,0,255
204: 0,0,0,255
205: 0,0,0,255
206: 0,0,0,255
207: 0,0,0,255
208: 0,0,0,255
209: 0,0,0,255
210: 0,0,0,255
211: 0,0,0,255
212: 0,0,0,255
213: 0,0,0,255
214: 0,0,0,255
215: 0,0,0,255
216: 0,0,0,255
217: 0,0,0,255
218: 0,0,0,255
219: 0,0,0,255
220: 0,0,0,255
221: 0,0,0,255
222: 0,0,0,255
223: 0,0,0,255
224: 0,0,0,255
225: 0,0,0,255
226: 0,0,0,255
227: 0,0,0,255
228: 0,0,0,255
229: 0,0,0,255
230: 0,0,0,255
231: 0,0,0,255
232: 0,0,0,255
233: 0,0,0,255
234: 0,0,0,255
235: 0,0,0,255
236: 0,0,0,255
237: 0,0,0,255
238: 0,0,0,255
239: 0,0,0,255
240: 0,0,0,255
241: 0,0,0,255
242: 0,0,0,255
243: 0,0,0,255
244: 0,0,0,255
245: 0,0,0,255
246: 0,0,0,255
247: 0,0,0,255
248: 0,0,0,255
249: 0,0,0,255
250: 0,0,0,255
251: 0,0,0,255
252: 0,0,0,255
253: 0,0,0,255
254: 0,0,0,255
255: 0,0,0,255
Band 2 Block=256x256 Type=Byte, ColorInterp=Alpha
</code></pre>
</blockquote>
<br>
When I drag-and-drop this file into QGIS (I tried 2.18 and 3 with
identical results) to add it as a raster layer, it is displayed in
bright green: <a class="moz-txt-link-freetext" href="https://i.stack.imgur.com/F6Op8.png">https://i.stack.imgur.com/F6Op8.png</a><br>
<br>
The automatically set display mode (to be found in the "style"
properties) is "multiband color". Only if I manually set this to
"paletted" is the TIFF displayed as intended:
<a class="moz-txt-link-freetext" href="https://i.stack.imgur.com/GNJGs.png">https://i.stack.imgur.com/GNJGs.png</a><br>
<br>
Other software (e.g. GIMP) displays the TIFF as intended without
need for manual color interpretation adjustment.<br>
<br>
Is that a bug in QGIS or am I doing something wrong? If a TIFF like
this is valid (I've asked at
<a class="moz-txt-link-freetext" href="https://gis.stackexchange.com/q/279584/51574">https://gis.stackexchange.com/q/279584/51574</a> whether it is), it
seems clear to me that a band with a palette should default to be
interpreted according to that palette and an alpha-band as
opacity/transparency, not as e.g. red and green channel.<br>
<br>
Regards,<br>
Raphael Das Gupta<br>
<br>
<hr width="100%" size="2">* the linked example image only contains
fully opaque pixels but has the same band structure as the real data
that also has fully transparent pixels<br>
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