early-access version 1432

externals/ffmpeg/doc/swscale.txt

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+    The official guide to swscale for confused developers.
+   ========================================================
+
+Current (simplified) Architecture:
+---------------------------------
+                        Input
+                          v
+                   _______OR_________
+                 /                   \
+               /                       \
+       special converter     [Input to YUV converter]
+              |                         |
+              |         (8-bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:0:0 )
+              |                         |
+              |                         v
+              |                  Horizontal scaler
+              |                         |
+              |     (15-bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:1:1 / 4:0:0 )
+              |                         |
+              |                         v
+              |          Vertical scaler and output converter
+              |                         |
+              v                         v
+                         output
+
+
+Swscale has 2 scaler paths. Each side must be capable of handling
+slices, that is, consecutive non-overlapping rectangles of dimension
+(0,slice_top) - (picture_width, slice_bottom).
+
+special converter
+    These generally are unscaled converters of common
+    formats, like YUV 4:2:0/4:2:2 -> RGB12/15/16/24/32. Though it could also
+    in principle contain scalers optimized for specific common cases.
+
+Main path
+    The main path is used when no special converter can be used. The code
+    is designed as a destination line pull architecture. That is, for each
+    output line the vertical scaler pulls lines from a ring buffer. When
+    the ring buffer does not contain the wanted line, then it is pulled from
+    the input slice through the input converter and horizontal scaler.
+    The result is also stored in the ring buffer to serve future vertical
+    scaler requests.
+    When no more output can be generated because lines from a future slice
+    would be needed, then all remaining lines in the current slice are
+    converted, horizontally scaled and put in the ring buffer.
+    [This is done for luma and chroma, each with possibly different numbers
+     of lines per picture.]
+
+Input to YUV Converter
+    When the input to the main path is not planar 8 bits per component YUV or
+    8-bit gray, it is converted to planar 8-bit YUV. Two sets of converters
+    exist for this currently: One performs horizontal downscaling by 2
+    before the conversion, the other leaves the full chroma resolution,
+    but is slightly slower. The scaler will try to preserve full chroma
+    when the output uses it. It is possible to force full chroma with
+    SWS_FULL_CHR_H_INP even for cases where the scaler thinks it is useless.
+
+Horizontal scaler
+    There are several horizontal scalers. A special case worth mentioning is
+    the fast bilinear scaler that is made of runtime-generated MMXEXT code
+    using specially tuned pshufw instructions.
+    The remaining scalers are specially-tuned for various filter lengths.
+    They scale 8-bit unsigned planar data to 16-bit signed planar data.
+    Future >8 bits per component inputs will need to add a new horizontal
+    scaler that preserves the input precision.
+
+Vertical scaler and output converter
+    There is a large number of combined vertical scalers + output converters.
+    Some are:
+    * unscaled output converters
+    * unscaled output converters that average 2 chroma lines
+    * bilinear converters                (C, MMX and accurate MMX)
+    * arbitrary filter length converters (C, MMX and accurate MMX)
+    And
+    * Plain C  8-bit 4:2:2 YUV -> RGB converters using LUTs
+    * Plain C 17-bit 4:4:4 YUV -> RGB converters using multiplies
+    * MMX     11-bit 4:2:2 YUV -> RGB converters
+    * Plain C 16-bit Y -> 16-bit gray
+      ...
+
+    RGB with less than 8 bits per component uses dither to improve the
+    subjective quality and low-frequency accuracy.
+
+
+Filter coefficients:
+--------------------
+There are several different scalers (bilinear, bicubic, lanczos, area,
+sinc, ...). Their coefficients are calculated in initFilter().
+Horizontal filter coefficients have a 1.0 point at 1 << 14, vertical ones at
+1 << 12. The 1.0 points have been chosen to maximize precision while leaving
+a little headroom for convolutional filters like sharpening filters and
+minimizing SIMD instructions needed to apply them.
+It would be trivial to use a different 1.0 point if some specific scaler
+would benefit from it.
+Also, as already hinted at, initFilter() accepts an optional convolutional
+filter as input that can be used for contrast, saturation, blur, sharpening
+shift, chroma vs. luma shift, ...