e8b40f3c2b
The Gordian knot that 7fec5074f9 attempted
to unravel was caused by the fact that there are several
data-precision-dependent (JSAMPLE-dependent) fields and methods in the
exposed libjpeg API structures, and if you change the exposed libjpeg
API structures, then you have to change the whole API. If you change
the whole API, then you have to provide a whole new library to support
the new API, and that makes it difficult to support multiple data
precisions in the same application. (It is not impossible, as example.c
demonstrated, but using data-precision-dependent libjpeg API structures
would have made the cjpeg, djpeg, and jpegtran source code hard to read,
so it made more sense to build, install, and package 12-bit-specific
versions of those applications.)
Unfortunately, the result of that initial integration effort was an
unreadable and unmaintainable mess, which is a problem for a library
that is an ISO/ITU-T reference implementation. Also, as I dug into the
problem of lossless JPEG support, I realized that 16-bit lossless JPEG
images are a thing, and supporting yet another version of the libjpeg
API just for those images is untenable.
In fact, however, the touch points for JSAMPLE in the exposed libjpeg
API structures are minimal:
- The colormap and sample_range_limit fields in jpeg_decompress_struct
- The alloc_sarray() and access_virt_sarray() methods in
jpeg_memory_mgr
- jpeg_write_scanlines() and jpeg_write_raw_data()
- jpeg_read_scanlines() and jpeg_read_raw_data()
- jpeg_skip_scanlines() and jpeg_crop_scanline()
(This is subtle, but both of those functions use JSAMPLE-dependent
opaque structures behind the scenes.)
It is much more readable and maintainable to provide 12-bit-specific
versions of those six top-level API functions and to document that the
aforementioned methods and fields must be type-cast when using 12-bit
samples. Since that eliminates the need to provide a 12-bit-specific
version of the exposed libjpeg API structures, we can:
- Compile only the precision-dependent libjpeg modules (the
coefficient buffer controllers, the colorspace converters, the
DCT/IDCT managers, the main buffer controllers, the preprocessing
and postprocessing controller, the downsampler and upsamplers, the
quantizers, the integer DCT methods, and the IDCT methods) for
multiple data precisions.
- Introduce 12-bit-specific methods into the various internal
structures defined in jpegint.h.
- Create precision-independent data type, macro, method, field, and
function names that are prefixed by an underscore, and use an
internal header to convert those into precision-dependent data
type, macro, method, field, and function names, based on the value
of BITS_IN_JSAMPLE, when compiling the precision-dependent libjpeg
modules.
- Expose precision-dependent jinit*() functions for each of the
precision-dependent libjpeg modules.
- Abstract the precision-dependent libjpeg modules by calling the
appropriate precision-dependent jinit*() function, based on the
value of cinfo->data_precision, from top-level libjpeg API
functions.
54 lines
1.8 KiB
C
54 lines
1.8 KiB
C
/*
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* jdsample.h
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*
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* This file was part of the Independent JPEG Group's software:
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* Copyright (C) 1991-1996, Thomas G. Lane.
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* libjpeg-turbo Modifications:
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* Copyright (C) 2022, D. R. Commander.
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* For conditions of distribution and use, see the accompanying README.ijg
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* file.
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*/
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#define JPEG_INTERNALS
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#include "jpeglib.h"
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#include "jsamplecomp.h"
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/* Pointer to routine to upsample a single component */
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typedef void (*upsample1_ptr) (j_decompress_ptr cinfo,
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jpeg_component_info *compptr,
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_JSAMPARRAY input_data,
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_JSAMPARRAY *output_data_ptr);
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/* Private subobject */
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typedef struct {
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struct jpeg_upsampler pub; /* public fields */
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/* Color conversion buffer. When using separate upsampling and color
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* conversion steps, this buffer holds one upsampled row group until it
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* has been color converted and output.
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* Note: we do not allocate any storage for component(s) which are full-size,
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* ie do not need rescaling. The corresponding entry of color_buf[] is
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* simply set to point to the input data array, thereby avoiding copying.
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*/
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_JSAMPARRAY color_buf[MAX_COMPONENTS];
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/* Per-component upsampling method pointers */
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upsample1_ptr methods[MAX_COMPONENTS];
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int next_row_out; /* counts rows emitted from color_buf */
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JDIMENSION rows_to_go; /* counts rows remaining in image */
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/* Height of an input row group for each component. */
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int rowgroup_height[MAX_COMPONENTS];
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/* These arrays save pixel expansion factors so that int_expand need not
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* recompute them each time. They are unused for other upsampling methods.
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*/
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UINT8 h_expand[MAX_COMPONENTS];
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UINT8 v_expand[MAX_COMPONENTS];
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} my_upsampler;
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typedef my_upsampler *my_upsample_ptr;
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