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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-- -\
| -\
| - -6--
|
| - -6'-
| -/
| --- -4'- -/
| ---/
+-5'- -/
| -- -3'-
| --/
| --/
-2'- -/
|
|
-1'-
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
------
If old scale applied to new crop is invalid produce nearest new scale possible
1. Calculate current combined scales.
scale_comb = (((4') - (4)) / ((6') - (6))) * (((2') - (2)) / ((3') - (3)))
2. Apply iterative sensor S_CROP for new input window.
3. If old combined scales applied to new crop produce an impossible user window,
adjust scales to produce nearest possible window.
width_u_out = ((5') - (5)) / scale_comb
if (width_u_out > max)
scale_comb = ((5') - (5)) / max;
else if (width_u_out < min)
scale_comb = ((5') - (5)) / min;
4. Issue G_CROP to retrieve actual input window.
5. Using actual input window and calculated combined scales calculate sensor
target output window.
width_s_out = ((3') - (3)) = ((2') - (2)) / scale_comb
6. Apply iterative S_FMT for new sensor target output window.
7. Issue G_FMT to retrieve the actual sensor output window.
8. Calculate sensor scales.
scale_s = ((3') - (3)) / ((2') - (2))
9. Calculate sensor output subwindow to be cropped on CEU by applying sensor
scales to the requested window.
width_ceu = ((5') - (5)) / scale_s
10. Use CEU cropping for above calculated window.
11. Calculate CEU scales from sensor scales from results of (10) and user window
from (3)
scale_ceu = calc_scale(((5') - (5)), &width_u_out)
12. Apply CEU scales.
--
Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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