mlib_ImageSubsampleAverage

, mlib_ImageSubsampleAverage_Fp

- subsamples an image with a box filter

Synopsis

cc [ flag... ] file... -lmlib [ library... ]
#include <mlib.h>

mlib_status mlib_ImageSubsampleAverage(mlib_image *dst,
     const mlib_image *src, mlib_d64 xscale, mlib_d64 yscale);

mlib_status mlib_ImageSubsampleAverage_Fp(mlib_image *dst,
     const mlib_image *src, mlib_d64 xscale, mlib_d64 yscale);

Description

Each function scales an image down with an adaptive box filter.

The subsampling algorithm performs the scaling operation by averaging all the pixel values from a block in the source image that correspond to the destination pixel.

The width and height of the source block for a destination pixel are computed as:

  blockX = (int)ceil(1.0/xscale);
  blockY = (int)ceil(1.0/yscale);

If we denote a pixel's location in an image by its column number and row number (both counted from 0), the destination pixel at (i, j) is backward mapped to the source block whose upper-left corner pixel is at (xValues[i], yValues[j]), where

  xValues[i] = (int)(i/xscale + 0.5);
  yValues[j] = (int)(j/yscale + 0.5);

The width and height of the filled area in the destination are restricted by

  dstW = (int)(srcWidth * xscale);
  dstH = (int)(srcHeight * yscale);

where srcWidth and srcHeight are width and height of the source image.

Since the block size in source is defined from scale factors with roundup, some blocks (the rightmost and the bottommost blocks) may overrun the border of the source image by 1 pixel. In this case, such blocks are moved by 1 pixel to left/up direction in order to be inside of the source image.

Parameters

The function takes the following arguments:

dst

Pointer to destination image.

src

Pointer to source image.

xscale

X scale factor. 0.0 < xscale ≤ 1.0.

yscale

Y scale factor. 0.0 < yscale ≤ 1.0.

Return Values

The function returns MLIB_SUCCESS if successful. Otherwise it returns MLIB_FAILURE.

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

mlib_ImageSubsampleBinaryToGray(3MLIB), mlib_ImageFilteredSubsample(3MLIB), mlib_ImageZoomTranslate(3MLIB), mlib_ImageZoomTranslate_Fp(3MLIB), attributes(5)