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Diffstat (limited to 'Documentation/driver-api/media/drivers/sh_mobile_ceu_camera.rst')
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diff --git a/Documentation/driver-api/media/drivers/sh_mobile_ceu_camera.rst b/Documentation/driver-api/media/drivers/sh_mobile_ceu_camera.rst new file mode 100644 index 000000000000..822fcb8368ae --- /dev/null +++ b/Documentation/driver-api/media/drivers/sh_mobile_ceu_camera.rst @@ -0,0 +1,142 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver +======================================================================= + +Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> + +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 +--------------------------------- + +.. code-block:: none + + -1-- + | + -2-- -\ + | --\ + | --\ + +-5-- . -- -3-- -\ + | `... -\ + | `... -4-- . - -7.. + | `. + | `. .6-- + | + | . .6'- + | .´ + | ... -4'- .´ + | ...´ - -7'. + +-5'- .´ -/ + | -- -3'- -/ + | --/ + | --/ + -2'- -/ + | + | + -1'- + +In the above chart minuses and slashes represent "real" data amounts, points and +accents represent "useful" data, basically, CEU scaled and cropped output, +mapped back onto the client's source plane. + +Such a configuration can be 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 = ((2') - (2)) / ((3') - (3)) + +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 = ((7') - (7)) = ((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 +------ + +The :ref:`V4L2 crop API <crop-scale>` says: + +"...specification does not define an origin or units. However by convention +drivers should horizontally count unscaled samples relative to 0H." + +We choose to follow the advise and interpret cropping units as client input +pixels. + +Cropping is performed in the following 6 steps: + +1. Request exactly user rectangle from the sensor. + +2. If smaller - iterate until a larger one is obtained. Result: sensor cropped + to 2 : 2', target crop 5 : 5', current output format 6' - 6. + +3. In the previous step the sensor has tried to preserve its output frame as + good as possible, but it could have changed. Retrieve it again. + +4. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate + intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2) + +5. Calculate and apply host scale = (6' - 6) / (4' - 4) + +6. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5) |