1.. SPDX-License-Identifier: GPL-2.0 2 3Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver 4======================================================================= 5 6Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> 7 8Terminology 9----------- 10 11sensor scales: horizontal and vertical scales, configured by the sensor driver 12host scales: -"- host driver 13combined scales: sensor_scale * host_scale 14 15 16Generic scaling / cropping scheme 17--------------------------------- 18 19.. code-block:: none 20 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 and 46accents 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 height 57(1) to (2) - sensor cropped left or top 58(2) to (2') - sensor cropped width or height 59(3) to (3') - sensor scale 60(3) to (4) - CEU cropped left or top 61(4) to (4') - CEU cropped width or height 62(5) to (5') - reverse sensor scale applied to CEU cropped width or height 63(2) to (5) - reverse sensor scale applied to CEU cropped left or top 64(6) to (6') - CEU scale - user window 65 66 67S_FMT 68----- 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 at 77current sensor scales onto input window - this is user S_CROP: 78 79 width_u = (5') - (5) = ((4') - (4)) * scale_s 80 813. Calculate new combined scales from "effective" input window to requested user 82window: 83 84 scale_comb = width_u / ((6') - (6)) 85 864. Calculate sensor output window by applying combined scales to real input 87window: 88 89 width_s_out = ((7') - (7)) = ((2') - (2)) / scale_comb 90 915. 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 calculated 102"effective" crop: 103 104 width_ceu = (4')_new - (4)_new = width_u / scale_s_new 105 left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new 106 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 / width 114 115 116S_CROP 117------ 118 119The :ref:`V4L2 crop API <crop-scale>` says: 120 121"...specification does not define an origin or units. However by convention 122drivers should horizontally count unscaled samples relative to 0H." 123 124We choose to follow the advise and interpret cropping units as client input 125pixels. 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 cropped 132 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 as 135 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). Calculate 138 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