--- /dev/null
+ Cropping and Scaling algorithm, used in the sh_mobile_ceu_camera driver
+ =======================================================================
+
+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
+---------------------------------
+
+-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 API at http://v4l2spec.bytesex.org/spec/x1904.htm 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)
+
+--
+Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Code Review / kvmfornfv.git / blobdiff