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diff --git a/libgimpcolor/gimpbilinear.c b/libgimpcolor/gimpbilinear.c
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+/* LIBGIMP - The GIMP Library
+ * Copyright (C) 1995-1997 Peter Mattis and Spencer Kimball
+ *
+ * This library is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include <glib-object.h>
+
+#include "libgimpmath/gimpmath.h"
+
+#include "gimpcolortypes.h"
+
+#include "gimpbilinear.h"
+
+
+/**
+ * SECTION: gimpbilinear
+ * @title: GimpBilinear
+ * @short_description: Utility functions for bilinear interpolation.
+ *
+ * Utility functions for bilinear interpolation.
+ **/
+
+
+gdouble
+gimp_bilinear (gdouble x,
+ gdouble y,
+ gdouble *values)
+{
+ gdouble m0, m1;
+
+ g_return_val_if_fail (values != NULL, 0.0);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0.0)
+ x += 1.0;
+ if (y < 0.0)
+ y += 1.0;
+
+ m0 = (1.0 - x) * values[0] + x * values[1];
+ m1 = (1.0 - x) * values[2] + x * values[3];
+
+ return (1.0 - y) * m0 + y * m1;
+}
+
+guchar
+gimp_bilinear_8 (gdouble x,
+ gdouble y,
+ guchar *values)
+{
+ gdouble m0, m1;
+
+ g_return_val_if_fail (values != NULL, 0);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0.0)
+ x += 1.0;
+ if (y < 0.0)
+ y += 1.0;
+
+ m0 = (1.0 - x) * values[0] + x * values[1];
+ m1 = (1.0 - x) * values[2] + x * values[3];
+
+ return (guchar) ((1.0 - y) * m0 + y * m1);
+}
+
+guint16
+gimp_bilinear_16 (gdouble x,
+ gdouble y,
+ guint16 *values)
+{
+ gdouble m0, m1;
+
+ g_return_val_if_fail (values != NULL, 0);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0.0)
+ x += 1.0;
+ if (y < 0.0)
+ y += 1.0;
+
+ m0 = (1.0 - x) * values[0] + x * values[1];
+ m1 = (1.0 - x) * values[2] + x * values[3];
+
+ return (guint16) ((1.0 - y) * m0 + y * m1);
+}
+
+guint32
+gimp_bilinear_32 (gdouble x,
+ gdouble y,
+ guint32 *values)
+{
+ gdouble m0, m1;
+
+ g_return_val_if_fail (values != NULL, 0);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0.0)
+ x += 1.0;
+ if (y < 0.0)
+ y += 1.0;
+
+ m0 = (1.0 - x) * values[0] + x * values[1];
+ m1 = (1.0 - x) * values[2] + x * values[3];
+
+ return (guint32) ((1.0 - y) * m0 + y * m1);
+}
+
+GimpRGB
+gimp_bilinear_rgb (gdouble x,
+ gdouble y,
+ GimpRGB *values)
+{
+ gdouble m0, m1;
+ gdouble ix, iy;
+ GimpRGB v = { 0, };
+
+ g_return_val_if_fail (values != NULL, v);
+
+ x = fmod(x, 1.0);
+ y = fmod(y, 1.0);
+
+ if (x < 0)
+ x += 1.0;
+ if (y < 0)
+ y += 1.0;
+
+ ix = 1.0 - x;
+ iy = 1.0 - y;
+
+ /* Red */
+
+ m0 = ix * values[0].r + x * values[1].r;
+ m1 = ix * values[2].r + x * values[3].r;
+
+ v.r = iy * m0 + y * m1;
+
+ /* Green */
+
+ m0 = ix * values[0].g + x * values[1].g;
+ m1 = ix * values[2].g + x * values[3].g;
+
+ v.g = iy * m0 + y * m1;
+
+ /* Blue */
+
+ m0 = ix * values[0].b + x * values[1].b;
+ m1 = ix * values[2].b + x * values[3].b;
+
+ v.b = iy * m0 + y * m1;
+
+ return v;
+}
+
+GimpRGB
+gimp_bilinear_rgba (gdouble x,
+ gdouble y,
+ GimpRGB *values)
+{
+ gdouble m0, m1;
+ gdouble ix, iy;
+ gdouble a0, a1, a2, a3, alpha;
+ GimpRGB v = { 0, };
+
+ g_return_val_if_fail (values != NULL, v);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0)
+ x += 1.0;
+ if (y < 0)
+ y += 1.0;
+
+ ix = 1.0 - x;
+ iy = 1.0 - y;
+
+ a0 = values[0].a;
+ a1 = values[1].a;
+ a2 = values[2].a;
+ a3 = values[3].a;
+
+ /* Alpha */
+
+ m0 = ix * a0 + x * a1;
+ m1 = ix * a2 + x * a3;
+
+ alpha = v.a = iy * m0 + y * m1;
+
+ if (alpha > 0)
+ {
+ /* Red */
+
+ m0 = ix * a0 * values[0].r + x * a1 * values[1].r;
+ m1 = ix * a2 * values[2].r + x * a3 * values[3].r;
+
+ v.r = (iy * m0 + y * m1)/alpha;
+
+ /* Green */
+
+ m0 = ix * a0 * values[0].g + x * a1 * values[1].g;
+ m1 = ix * a2 * values[2].g + x * a3 * values[3].g;
+
+ v.g = (iy * m0 + y * m1)/alpha;
+
+ /* Blue */
+
+ m0 = ix * a0 * values[0].b + x * a1 * values[1].b;
+ m1 = ix * a2 * values[2].b + x * a3 * values[3].b;
+
+ v.b = (iy * m0 + y * m1)/alpha;
+ }
+
+ return v;
+}
+
+/**
+ * gimp_bilinear_pixels_8:
+ * @dest: Pixel, where interpolation result is to be stored.
+ * @x: x-coordinate (0.0 to 1.0).
+ * @y: y-coordinate (0.0 to 1.0).
+ * @bpp: Bytes per pixel. @dest and each @values item is an array of
+ * @bpp bytes.
+ * @has_alpha: %TRUE if the last channel is an alpha channel.
+ * @values: Array of four pointers to pixels.
+ *
+ * Computes bilinear interpolation of four pixels.
+ *
+ * When @has_alpha is %FALSE, it's identical to gimp_bilinear_8() on
+ * each channel separately. When @has_alpha is %TRUE, it handles
+ * alpha channel correctly.
+ *
+ * The pixels in @values correspond to corner x, y coordinates in the
+ * following order: [0,0], [1,0], [0,1], [1,1].
+ **/
+void
+gimp_bilinear_pixels_8 (guchar *dest,
+ gdouble x,
+ gdouble y,
+ guint bpp,
+ gboolean has_alpha,
+ guchar **values)
+{
+ guint i;
+
+ g_return_if_fail (dest != NULL);
+ g_return_if_fail (values != NULL);
+
+ x = fmod (x, 1.0);
+ y = fmod (y, 1.0);
+
+ if (x < 0.0)
+ x += 1.0;
+ if (y < 0.0)
+ y += 1.0;
+
+ if (has_alpha)
+ {
+ guint ai = bpp - 1;
+ gdouble alpha0 = values[0][ai];
+ gdouble alpha1 = values[1][ai];
+ gdouble alpha2 = values[2][ai];
+ gdouble alpha3 = values[3][ai];
+ gdouble alpha = ((1.0 - y) * ((1.0 - x) * alpha0 + x * alpha1)
+ + y * ((1.0 - x) * alpha2 + x * alpha3));
+
+ dest[ai] = (guchar) alpha;
+ if (dest[ai])
+ {
+ for (i = 0; i < ai; i++)
+ {
+ gdouble m0 = ((1.0 - x) * values[0][i] * alpha0
+ + x * values[1][i] * alpha1);
+ gdouble m1 = ((1.0 - x) * values[2][i] * alpha2
+ + x * values[3][i] * alpha3);
+
+ dest[i] = (guchar) (((1.0 - y) * m0 + y * m1) / alpha);
+ }
+ }
+ }
+ else
+ {
+ for (i = 0; i < bpp; i++)
+ {
+ gdouble m0 = (1.0 - x) * values[0][i] + x * values[1][i];
+ gdouble m1 = (1.0 - x) * values[2][i] + x * values[3][i];
+
+ dest[i] = (guchar) ((1.0 - y) * m0 + y * m1);
+ }
+ }
+}
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