V4L/DVB (13651): sh_mobile_ceu_camera: document the scaling and cropping algorithm
The sh_mobile_ceu_camera driver implements an advanced algorithm, combining scaling and cropping on the client and on the host. Due to its complexity the algorithm deserves separate documentation. create mode 100644 Documentation/video4linux/sh_mobile_ceu_camera.txt Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
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Guennadi Liakhovetski
Mauro Carvalho Chehab
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Documentation/video4linux/sh_mobile_ceu_camera.txt
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Documentation/video4linux/sh_mobile_ceu_camera.txt
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Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver
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=======================================================================
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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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-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-- -\
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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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| ---/
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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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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 = ((3') - (3)) / ((2') - (2))
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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 = ((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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If old scale applied to new crop is invalid produce nearest new scale possible
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1. Calculate current combined scales.
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scale_comb = (((4') - (4)) / ((6') - (6))) * (((2') - (2)) / ((3') - (3)))
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2. Apply iterative sensor S_CROP for new input window.
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3. If old combined scales applied to new crop produce an impossible user window,
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adjust scales to produce nearest possible window.
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width_u_out = ((5') - (5)) / scale_comb
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if (width_u_out > max)
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scale_comb = ((5') - (5)) / max;
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else if (width_u_out < min)
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scale_comb = ((5') - (5)) / min;
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4. Issue G_CROP to retrieve actual input window.
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5. Using actual input window and calculated combined scales calculate sensor
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target output window.
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width_s_out = ((3') - (3)) = ((2') - (2)) / scale_comb
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6. Apply iterative S_FMT for new sensor target output window.
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7. Issue G_FMT to retrieve the actual sensor output window.
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8. Calculate sensor scales.
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scale_s = ((3') - (3)) / ((2') - (2))
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9. Calculate sensor output subwindow to be cropped on CEU by applying sensor
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scales to the requested window.
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width_ceu = ((5') - (5)) / scale_s
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10. Use CEU cropping for above calculated window.
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11. Calculate CEU scales from sensor scales from results of (10) and user window
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from (3)
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scale_ceu = calc_scale(((5') - (5)), &width_u_out)
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12. Apply CEU scales.
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--
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Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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