About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / video4linux / sh_mobile_ceu_camera.txt




Custom Search

Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1		Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver
2		=======================================================================
3	
4	Terminology
5	-----------
6	
7	sensor scales: horizontal and vertical scales, configured by the sensor driver
8	host scales: -"- host driver
9	combined scales: sensor_scale * host_scale
10	
11	
12	Generic scaling / cropping scheme
13	---------------------------------
14	
15	-1--
16	|
17	-2-- -\
18	|      --\
19	|         --\
20	+-5-- .      -- -3-- -\
21	|      `...            -\
22	|          `... -4-- .   - -7..
23	|                     `.
24	|                       `. .6--
25	|
26	|                        . .6'-
27	|                      .´
28	|           ... -4'- .´
29	|       ...´             - -7'.
30	+-5'- .´               -/
31	|            -- -3'- -/
32	|         --/
33	|      --/
34	-2'- -/
35	|
36	|
37	-1'-
38	
39	In the above chart minuses and slashes represent "real" data amounts, points and
40	accents represent "useful" data, basically, CEU scaled and cropped output,
41	mapped back onto the client's source plane.
42	
43	Such a configuration can be produced by user requests:
44	
45	S_CROP(left / top = (5) - (1), width / height = (5') - (5))
46	S_FMT(width / height = (6') - (6))
47	
48	Here:
49	
50	(1) to (1') - whole max width or height
51	(1) to (2)  - sensor cropped left or top
52	(2) to (2') - sensor cropped width or height
53	(3) to (3') - sensor scale
54	(3) to (4)  - CEU cropped left or top
55	(4) to (4') - CEU cropped width or height
56	(5) to (5') - reverse sensor scale applied to CEU cropped width or height
57	(2) to (5)  - reverse sensor scale applied to CEU cropped left or top
58	(6) to (6') - CEU scale - user window
59	
60	
61	S_FMT
62	-----
63	
64	Do not touch input rectangle - it is already optimal.
65	
66	1. Calculate current sensor scales:
67	
68		scale_s = ((2') - (2)) / ((3') - (3))
69	
70	2. Calculate "effective" input crop (sensor subwindow) - CEU crop scaled back at
71	current sensor scales onto input window - this is user S_CROP:
72	
73		width_u = (5') - (5) = ((4') - (4)) * scale_s
74	
75	3. Calculate new combined scales from "effective" input window to requested user
76	window:
77	
78		scale_comb = width_u / ((6') - (6))
79	
80	4. Calculate sensor output window by applying combined scales to real input
81	window:
82	
83		width_s_out = ((7') - (7)) = ((2') - (2)) / scale_comb
84	
85	5. Apply iterative sensor S_FMT for sensor output window.
86	
87		subdev->video_ops->s_fmt(.width = width_s_out)
88	
89	6. Retrieve sensor output window (g_fmt)
90	
91	7. Calculate new sensor scales:
92	
93		scale_s_new = ((3')_new - (3)_new) / ((2') - (2))
94	
95	8. Calculate new CEU crop - apply sensor scales to previously calculated
96	"effective" crop:
97	
98		width_ceu = (4')_new - (4)_new = width_u / scale_s_new
99		left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new
100	
101	9. Use CEU cropping to crop to the new window:
102	
103		ceu_crop(.width = width_ceu, .left = left_ceu)
104	
105	10. Use CEU scaling to scale to the requested user window:
106	
107		scale_ceu = width_ceu / width
108	
109	
110	S_CROP
111	------
112	
113	The API at http://v4l2spec.bytesex.org/spec/x1904.htm says:
114	
115	"...specification does not define an origin or units. However by convention
116	drivers should horizontally count unscaled samples relative to 0H."
117	
118	We choose to follow the advise and interpret cropping units as client input
119	pixels.
120	
121	Cropping is performed in the following 6 steps:
122	
123	1. Request exactly user rectangle from the sensor.
124	
125	2. If smaller - iterate until a larger one is obtained. Result: sensor cropped
126	   to 2 : 2', target crop 5 : 5', current output format 6' - 6.
127	
128	3. In the previous step the sensor has tried to preserve its output frame as
129	   good as possible, but it could have changed. Retrieve it again.
130	
131	4. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate
132	   intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2)
133	
134	5. Calculate and apply host scale = (6' - 6) / (4' - 4)
135	
136	6. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5)
137	
138	--
139	Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.