1 Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver 2 ======================================================================= 3 4Terminology 5----------- 6 7sensor scales: horizontal and vertical scales, configured by the sensor driver 8host scales: -"- host driver 9combined scales: sensor_scale * host_scale 10 11 12Generic scaling / cropping scheme 13--------------------------------- 14 15-1-- 16| 17-2-- -\ 18| --\ 19| --\ 20+-5-- . -- -3-- -\ 21| `... -\ 22| `... -4-- . - -7.. 23| `. 24| `. .6-- 25| 26| . .6'- 27| .´ 28| ... -4'- .´ 29| ...´ - -7'. 30+-5'- .´ -/ 31| -- -3'- -/ 32| --/ 33| --/ 34-2'- -/ 35| 36| 37-1'- 38 39In the above chart minuses and slashes represent "real" data amounts, points and 40accents represent "useful" data, basically, CEU scaled and cropped output, 41mapped back onto the client's source plane. 42 43Such a configuration can be produced by user requests: 44 45S_CROP(left / top = (5) - (1), width / height = (5') - (5)) 46S_FMT(width / height = (6') - (6)) 47 48Here: 49 50(1) to (1') - whole max width or height 51(1) to (2) - sensor cropped left or top 52(2) to (2') - sensor cropped width or height 53(3) to (3') - sensor scale 54(3) to (4) - CEU cropped left or top 55(4) to (4') - CEU cropped width or height 56(5) to (5') - reverse sensor scale applied to CEU cropped width or height 57(2) to (5) - reverse sensor scale applied to CEU cropped left or top 58(6) to (6') - CEU scale - user window 59 60 61S_FMT 62----- 63 64Do not touch input rectangle - it is already optimal. 65 661. Calculate current sensor scales: 67 68 scale_s = ((2') - (2)) / ((3') - (3)) 69 702. Calculate "effective" input crop (sensor subwindow) - CEU crop scaled back at 71current sensor scales onto input window - this is user S_CROP: 72 73 width_u = (5') - (5) = ((4') - (4)) * scale_s 74 753. Calculate new combined scales from "effective" input window to requested user 76window: 77 78 scale_comb = width_u / ((6') - (6)) 79 804. Calculate sensor output window by applying combined scales to real input 81window: 82 83 width_s_out = ((7') - (7)) = ((2') - (2)) / scale_comb 84 855. Apply iterative sensor S_FMT for sensor output window. 86 87 subdev->video_ops->s_fmt(.width = width_s_out) 88 896. Retrieve sensor output window (g_fmt) 90 917. Calculate new sensor scales: 92 93 scale_s_new = ((3')_new - (3)_new) / ((2') - (2)) 94 958. Calculate new CEU crop - apply sensor scales to previously calculated 96"effective" crop: 97 98 width_ceu = (4')_new - (4)_new = width_u / scale_s_new 99 left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new 100 1019. Use CEU cropping to crop to the new window: 102 103 ceu_crop(.width = width_ceu, .left = left_ceu) 104 10510. Use CEU scaling to scale to the requested user window: 106 107 scale_ceu = width_ceu / width 108 109 110S_CROP 111------ 112 113The API at http://v4l2spec.bytesex.org/spec/x1904.htm says: 114 115"...specification does not define an origin or units. However by convention 116drivers should horizontally count unscaled samples relative to 0H." 117 118We choose to follow the advise and interpret cropping units as client input 119pixels. 120 121Cropping is performed in the following 6 steps: 122 1231. Request exactly user rectangle from the sensor. 124 1252. If smaller - iterate until a larger one is obtained. Result: sensor cropped 126 to 2 : 2', target crop 5 : 5', current output format 6' - 6. 127 1283. In the previous step the sensor has tried to preserve its output frame as 129 good as possible, but it could have changed. Retrieve it again. 130 1314. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate 132 intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2) 133 1345. Calculate and apply host scale = (6' - 6) / (4' - 4) 135 1366. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5) 137 138-- 139Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> 140