143 lines
3.8 KiB
ReStructuredText
143 lines
3.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver
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=======================================================================
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Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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Terminology
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-----------
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sensor scales: horizontal and vertical scales, configured by the sensor driver
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host scales: -"- host driver
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combined scales: sensor_scale * host_scale
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Generic scaling / cropping scheme
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---------------------------------
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.. code-block:: none
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-1--
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-2-- -\
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| --\
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| --\
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+-5-- . -- -3-- -\
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| `... -\
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| `... -4-- . - -7..
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| `.
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| `. .6--
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| . .6'-
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| .´
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| ... -4'- .´
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| ...´ - -7'.
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+-5'- .´ -/
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| -- -3'- -/
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| --/
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| --/
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-2'- -/
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-1'-
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In the above chart minuses and slashes represent "real" data amounts, points and
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accents represent "useful" data, basically, CEU scaled and cropped output,
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mapped back onto the client's source plane.
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Such a configuration can be produced by user requests:
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S_CROP(left / top = (5) - (1), width / height = (5') - (5))
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S_FMT(width / height = (6') - (6))
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Here:
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(1) to (1') - whole max width or height
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(1) to (2) - sensor cropped left or top
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(2) to (2') - sensor cropped width or height
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(3) to (3') - sensor scale
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(3) to (4) - CEU cropped left or top
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(4) to (4') - CEU cropped width or height
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(5) to (5') - reverse sensor scale applied to CEU cropped width or height
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(2) to (5) - reverse sensor scale applied to CEU cropped left or top
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(6) to (6') - CEU scale - user window
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S_FMT
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-----
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Do not touch input rectangle - it is already optimal.
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1. Calculate current sensor scales:
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scale_s = ((2') - (2)) / ((3') - (3))
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2. Calculate "effective" input crop (sensor subwindow) - CEU crop scaled back at
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current sensor scales onto input window - this is user S_CROP:
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width_u = (5') - (5) = ((4') - (4)) * scale_s
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3. Calculate new combined scales from "effective" input window to requested user
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window:
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scale_comb = width_u / ((6') - (6))
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4. Calculate sensor output window by applying combined scales to real input
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window:
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width_s_out = ((7') - (7)) = ((2') - (2)) / scale_comb
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5. Apply iterative sensor S_FMT for sensor output window.
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subdev->video_ops->s_fmt(.width = width_s_out)
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6. Retrieve sensor output window (g_fmt)
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7. Calculate new sensor scales:
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scale_s_new = ((3')_new - (3)_new) / ((2') - (2))
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8. Calculate new CEU crop - apply sensor scales to previously calculated
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"effective" crop:
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width_ceu = (4')_new - (4)_new = width_u / scale_s_new
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left_ceu = (4)_new - (3)_new = ((5) - (2)) / scale_s_new
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9. Use CEU cropping to crop to the new window:
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ceu_crop(.width = width_ceu, .left = left_ceu)
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10. Use CEU scaling to scale to the requested user window:
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scale_ceu = width_ceu / width
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S_CROP
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------
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The :ref:`V4L2 crop API <crop-scale>` says:
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"...specification does not define an origin or units. However by convention
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drivers should horizontally count unscaled samples relative to 0H."
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We choose to follow the advise and interpret cropping units as client input
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pixels.
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Cropping is performed in the following 6 steps:
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1. Request exactly user rectangle from the sensor.
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2. If smaller - iterate until a larger one is obtained. Result: sensor cropped
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to 2 : 2', target crop 5 : 5', current output format 6' - 6.
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3. In the previous step the sensor has tried to preserve its output frame as
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good as possible, but it could have changed. Retrieve it again.
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4. Sensor scaled to 3 : 3'. Sensor's scale is (2' - 2) / (3' - 3). Calculate
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intermediate window: 4' - 4 = (5' - 5) * (3' - 3) / (2' - 2)
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5. Calculate and apply host scale = (6' - 6) / (4' - 4)
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6. Calculate and apply host crop: 6 - 7 = (5 - 2) * (6' - 6) / (5' - 5)
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