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1.. SPDX-License-Identifier: GPL-2.02 3Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver4=======================================================================5 6Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>7 8Terminology9-----------10 11sensor scales: horizontal and vertical scales, configured by the sensor driver12host scales: -"- host driver13combined scales: sensor_scale * host_scale14 15 16Generic scaling / cropping scheme17---------------------------------18 19.. code-block:: none20 21 -1--22 |23 -2-- -\24 | --\25 | --\26 +-5-- . -- -3-- -\27 | `... -\28 | `... -4-- . - -7..29 | `.30 | `. .6--31 |32 | . .6'-33 | .´34 | ... -4'- .´35 | ...´ - -7'.36 +-5'- .´ -/37 | -- -3'- -/38 | --/39 | --/40 -2'- -/41 |42 |43 -1'-44 45In the above chart minuses and slashes represent "real" data amounts, points and46accents represent "useful" data, basically, CEU scaled and cropped output,47mapped back onto the client's source plane.48 49Such a configuration can be produced by user requests:50 51S_CROP(left / top = (5) - (1), width / height = (5') - (5))52S_FMT(width / height = (6') - (6))53 54Here:55 56(1) to (1') - whole max width or height57(1) to (2) - sensor cropped left or top58(2) to (2') - sensor cropped width or height59(3) to (3') - sensor scale60(3) to (4) - CEU cropped left or top61(4) to (4') - CEU cropped width or height62(5) to (5') - reverse sensor scale applied to CEU cropped width or height63(2) to (5) - reverse sensor scale applied to CEU cropped left or top64(6) to (6') - CEU scale - user window65 66 67S_FMT68-----69 70Do not touch input rectangle - it is already optimal.71 721. Calculate current sensor scales:73 74 scale_s = ((2') - (2)) / ((3') - (3))75 762. Calculate "effective" input crop (sensor subwindow) - CEU crop scaled back at77current sensor scales onto input window - this is user S_CROP:78 79 width_u = (5') - (5) = ((4') - (4)) * scale_s80 813. Calculate new combined scales from "effective" input window to requested user82window:83 84 scale_comb = width_u / ((6') - (6))85 864. Calculate sensor output window by applying combined scales to real input87window:88 89 width_s_out = ((7') - (7)) = ((2') - (2)) / scale_comb90 915. Apply iterative sensor S_FMT for sensor output window.92 93 subdev->video_ops->s_fmt(.width = width_s_out)94 956. Retrieve sensor output window (g_fmt)96 977. Calculate new sensor scales:98 99 scale_s_new = ((3')_new - (3)_new) / ((2') - (2))100 1018. Calculate new CEU crop - apply sensor scales to previously calculated102"effective" crop:103 104 width_ceu = (4')_new - (4)_new = width_u / scale_s_new105 left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new106 1079. Use CEU cropping to crop to the new window:108 109 ceu_crop(.width = width_ceu, .left = left_ceu)110 11110. Use CEU scaling to scale to the requested user window:112 113 scale_ceu = width_ceu / width114 115 116S_CROP117------118 119The :ref:`V4L2 crop API <crop-scale>` says:120 121"...specification does not define an origin or units. However by convention122drivers should horizontally count unscaled samples relative to 0H."123 124We choose to follow the advise and interpret cropping units as client input125pixels.126 127Cropping is performed in the following 6 steps:128 1291. Request exactly user rectangle from the sensor.130 1312. If smaller - iterate until a larger one is obtained. Result: sensor cropped132 to 2 : 2', target crop 5 : 5', current output format 6' - 6.133 1343. In the previous step the sensor has tried to preserve its output frame as135 good as possible, but it could have changed. Retrieve it again.136 1374. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate138 intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2)139 1405. Calculate and apply host scale = (6' - 6) / (4' - 4)141 1426. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5)143