android_kernel_xiaomi_sm8350/Documentation/video4linux/sh_mobile_ceu_camera.txt

	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>