140 lines
3.7 KiB
Plaintext
140 lines
3.7 KiB
Plaintext
Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver
|
||
=======================================================================
|
||
|
||
Terminology
|
||
-----------
|
||
|
||
sensor scales: horizontal and vertical scales, configured by the sensor driver
|
||
host scales: -"- host driver
|
||
combined scales: sensor_scale * host_scale
|
||
|
||
|
||
Generic scaling / cropping scheme
|
||
---------------------------------
|
||
|
||
-1--
|
||
|
|
||
-2-- -\
|
||
| --\
|
||
| --\
|
||
+-5-- . -- -3-- -\
|
||
| `... -\
|
||
| `... -4-- . - -7..
|
||
| `.
|
||
| `. .6--
|
||
|
|
||
| . .6'-
|
||
| .´
|
||
| ... -4'- .´
|
||
| ...´ - -7'.
|
||
+-5'- .´ -/
|
||
| -- -3'- -/
|
||
| --/
|
||
| --/
|
||
-2'- -/
|
||
|
|
||
|
|
||
-1'-
|
||
|
||
In the above chart minuses and slashes represent "real" data amounts, points and
|
||
accents represent "useful" data, basically, CEU scaled amd cropped output,
|
||
mapped back onto the client's source plane.
|
||
|
||
Such a configuration can be produced by user requests:
|
||
|
||
S_CROP(left / top = (5) - (1), width / height = (5') - (5))
|
||
S_FMT(width / height = (6') - (6))
|
||
|
||
Here:
|
||
|
||
(1) to (1') - whole max width or height
|
||
(1) to (2) - sensor cropped left or top
|
||
(2) to (2') - sensor cropped width or height
|
||
(3) to (3') - sensor scale
|
||
(3) to (4) - CEU cropped left or top
|
||
(4) to (4') - CEU cropped width or height
|
||
(5) to (5') - reverse sensor scale applied to CEU cropped width or height
|
||
(2) to (5) - reverse sensor scale applied to CEU cropped left or top
|
||
(6) to (6') - CEU scale - user window
|
||
|
||
|
||
S_FMT
|
||
-----
|
||
|
||
Do not touch input rectangle - it is already optimal.
|
||
|
||
1. Calculate current sensor scales:
|
||
|
||
scale_s = ((3') - (3)) / ((2') - (2))
|
||
|
||
2. Calculate "effective" input crop (sensor subwindow) - CEU crop scaled back at
|
||
current sensor scales onto input window - this is user S_CROP:
|
||
|
||
width_u = (5') - (5) = ((4') - (4)) * scale_s
|
||
|
||
3. Calculate new combined scales from "effective" input window to requested user
|
||
window:
|
||
|
||
scale_comb = width_u / ((6') - (6))
|
||
|
||
4. Calculate sensor output window by applying combined scales to real input
|
||
window:
|
||
|
||
width_s_out = ((2') - (2)) / scale_comb
|
||
|
||
5. Apply iterative sensor S_FMT for sensor output window.
|
||
|
||
subdev->video_ops->s_fmt(.width = width_s_out)
|
||
|
||
6. Retrieve sensor output window (g_fmt)
|
||
|
||
7. Calculate new sensor scales:
|
||
|
||
scale_s_new = ((3')_new - (3)_new) / ((2') - (2))
|
||
|
||
8. Calculate new CEU crop - apply sensor scales to previously calculated
|
||
"effective" crop:
|
||
|
||
width_ceu = (4')_new - (4)_new = width_u / scale_s_new
|
||
left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new
|
||
|
||
9. Use CEU cropping to crop to the new window:
|
||
|
||
ceu_crop(.width = width_ceu, .left = left_ceu)
|
||
|
||
10. Use CEU scaling to scale to the requested user window:
|
||
|
||
scale_ceu = width_ceu / width
|
||
|
||
|
||
S_CROP
|
||
------
|
||
|
||
The API at http://v4l2spec.bytesex.org/spec/x1904.htm says:
|
||
|
||
"...specification does not define an origin or units. However by convention
|
||
drivers should horizontally count unscaled samples relative to 0H."
|
||
|
||
We choose to follow the advise and interpret cropping units as client input
|
||
pixels.
|
||
|
||
Cropping is performed in the following 6 steps:
|
||
|
||
1. Request exactly user rectangle from the sensor.
|
||
|
||
2. If smaller - iterate until a larger one is obtained. Result: sensor cropped
|
||
to 2 : 2', target crop 5 : 5', current output format 6' - 6.
|
||
|
||
3. In the previous step the sensor has tried to preserve its output frame as
|
||
good as possible, but it could have changed. Retrieve it again.
|
||
|
||
4. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate
|
||
intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2)
|
||
|
||
5. Calculate and apply host scale = (6' - 6) / (4' - 4)
|
||
|
||
6. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5)
|
||
|
||
--
|
||
Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
|