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This pull request fixes incorrect RGB outputs in i.his.rgb GRASS module for certain combinations of hue, intensity, and saturation values, particularly:

• When saturation = 0 (expected grayscale output)
• When intensity = 0 (expected black output)
• When intensity = 255 and saturation = 255 (expected pure hue)

The prior implementation failed to correctly compute RGB for these edge cases due to formula limitations, incorrect assumptions, and redundancy in the his2rgb.c logic.

This patch:

• Correctly computes pure grayscale and black cases
• Fixes hue interpolation logic with proper clamping and rounding
• Replaces the core conversion logic with a mathematically accurate implementation
• Preserves the rowbuffer overwrite style used in main.c

File Changed

• his2rgb.c — fixed all cases via modular hue2rgb() logic and improved edge-case handling.

Validation Table (Old vs. Reference RGB Values)

IDX	H	I	S	OLD_RGB	REF_RGB	Match	Justification
0	0	0	0	[0, 0, 0]	[0, 0, 0]	Yes	S=0: grayscale expected
1	0	0	128	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
2	0	0	255	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
3	0	128	0	[0, 0, 0]	[128,128,128]	No	S=0: grayscale expected
4	0	128	128	[192, 64, 64]	[192, 64, 64]	Yes	Match
5	0	128	255	[255, 1, 1]	[255, 1, 1]	Yes	Match
6	0	255	0	[0, 0, 0]	[255,255,255]	No	S=0: grayscale expected
7	0	255	128	[255,255,255]	[255,255,255]	Yes	Match
8	0	255	255	[255,255,255]	[255, 0, 0]	No	Expected pure hue
9	60	0	0	[0, 0, 0]	[0, 0, 0]	Yes	S=0: grayscale expected
10	60	0	128	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
11	60	0	255	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
12	60	128	0	[0, 0, 0]	[128,128,128]	No	S=0: grayscale expected
13	60	128	128	[139,192, 64]	[139,192, 64]	Yes	Match
14	60	128	255	[150,255, 1]	[150,255, 1]	Yes	Match
15	60	255	0	[0, 0, 0]	[255,255,255]	No	S=0: grayscale expected
16	60	255	128	[255,255,255]	[255,255,255]	Yes	Match
17	60	255	255	[255,255,255]	[0,255,0]	No	Expected pure hue
18	120	0	0	[0, 0, 0]	[0, 0, 0]	Yes	S=0: grayscale expected
19	120	0	128	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
20	120	0	255	[0, 0, 0]	[0, 0, 0]	Yes	I=0: black expected
21	120	128	0	[0, 0, 0]	[128,128,128]	No	S=0: grayscale expected
22	120	128	128	[64,192,169]	[64,192,169]	Yes	Match
23	120	128	255	[1,255,210]	[1,255,210]	Yes	Match
24	120	255	0	[0, 0, 0]	[255,255,255]	No	S=0: grayscale expected
25	120	255	128	[255,255,255]	[255,255,255]	Yes	Match
26	120	255	255	[255,255,255]	[0,255,0]	No	Expected pure hue
27	45	64	192	[107,112, 16]	[107,112, 16]	Yes	Match
28	90	128	64	[96,160,104]	[96,160,104]	Yes	Match
29	135	192	255	[129,233,255]	[129,233,255]	Yes	Match
30	180	128	128	[94,64,192]	[94,64,192]	Yes	Match
31	210	64	255	[120,0,128]	[120,0,128]	Yes	Match
32	240	192	64	[208,176,187]	[208,176,187]	Yes	Match
33	270	128	192	[224,100, 32]	[128,32,224]	No	Hue interpolation mismatch
34	300	255	128	[255,255,255]	[255,255,255]	Yes	Match
35	330	192	255	[159,255,129]	[255,129,192]	No	Hue interpolation mismatch
36	30	192	192	[239,212,145]	[239,212,145]	Yes	Match

Fixes https://github.com/OSGeo/grass/issues/5659

Please review and suggest further improvements if necessary.