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vips.h: Fix image watermarking #259

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vips.h: Fix image watermarking #259

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156 changes: 128 additions & 28 deletions vips.h
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Original file line number Diff line number Diff line change
Expand Up @@ -487,52 +487,152 @@ vips_add_band(VipsImage *in, VipsImage **out, double c) {
}

int
vips_watermark_image(VipsImage *in, VipsImage *sub, VipsImage **out, WatermarkImageOptions *o) {
VipsImage *base = vips_image_new();
VipsImage **t = (VipsImage **) vips_object_local_array(VIPS_OBJECT(base), 10);
vips_watermark_image(VipsImage *in, VipsImage *watermark, VipsImage **out, WatermarkImageOptions *o) {
// Inspired by https://gist.github.com/jcupitt/abacc012e2991f332e8b

// add in and sub for unreffing and later use
t[0] = in;
t[1] = sub;
VipsImage *base = vips_image_new();
// These images will all be unreffed when context is unreffed.
VipsImage **t = (VipsImage**) vips_object_local_array(VIPS_OBJECT(base), 18);

if (has_alpha_channel(in) == 0) {
if (has_alpha_channel(in) == 0) {
vips_add_band(in, &t[0], 255.0);
// in is no longer in the array and won't be unreffed, so add it at the end
t[8] = in;
// Add at the end to unref input image when done.
t[17] = in;
} else {
t[0] = in;
}

if (has_alpha_channel(sub) == 0) {
vips_add_band(sub, &t[1], 255.0);
// sub is no longer in the array and won't be unreffed, so add it at the end
t[9] = sub;
if (has_alpha_channel(watermark) == 0) {
vips_add_band(watermark, &t[1], 255.0);
// Add at the end to unref input image when done.
t[18] = watermark;
} else {
t[1] = watermark;
}

// Place watermark image in the right place and size it to the size of the
// image that should be watermarked
if (
vips_embed(t[1], &t[2], o->Left, o->Top, t[0]->Xsize, t[0]->Ysize, NULL)) {
g_object_unref(base);
// image that should be watermarked. Necessary for following operations because
// libvips enlarges images that are smaller than the other operand-image with
// zero-pixels. This is especially bad for multiplications as we effectively get
// a mask cutting out everything that's outside the smaller image.
if( vips_embed(t[1], &t[2], o->Left, o->Top, t[0]->Xsize, t[0]->Ysize, NULL)) {

g_object_unref(base);
return 1;
}

// Create a mask image based on the alpha band from the watermark image
// and place it in the right position
if (
vips_extract_band(t[1], &t[3], t[1]->Bands - 1, "n", 1, NULL) ||
vips_linear1(t[3], &t[4], o->Opacity, 0.0, NULL) ||
vips_cast(t[4], &t[5], VIPS_FORMAT_UCHAR, NULL) ||
vips_copy(t[5], &t[6], "interpretation", t[0]->Type, NULL) ||
vips_embed(t[6], &t[7], o->Left, o->Top, t[0]->Xsize, t[0]->Ysize, NULL)) {
g_object_unref(base);
// dst = in
// src = watermark

// Extract the alpha channels.
//
// dst_a255, src_a255
//
// ^^^^^^^^ ^^^^^^^^
// t3 t4
if( vips_extract_band(t[0], &t[3], 3, NULL) ||
vips_extract_band(t[2], &t[4], 3, NULL)) {

g_object_unref(base);
return 1;
}

// Blend the mask and watermark image and write to output.
if (vips_ifthenelse(t[7], t[2], t[0], out, "blend", TRUE, NULL)) {
// Extract the RGB bands.
//
// dst_rgb, src_rgb
//
// ^^^^^^^ ^^^^^^^
// t5 t6
if( vips_extract_band(t[0], &t[5], 0, "n", 3, NULL ) ||
vips_extract_band(t[2], &t[6], 0, "n", 3, NULL)) {

g_object_unref(base);
return 1;
}

// Compute normalized input alpha channels.
//
// dst_a = dst_a255 / 255, src_a = src_a255 / 255
//
// ^^^^^ ^^^^^
// t7 t8
if( vips_linear1(t[3], &t[7], 1.0 / 255.0, 0.0, NULL) ||
vips_linear1(t[4], &t[8], 1.0 / 255.0, 0.0, NULL)) {

g_object_unref(base);
return 1;
}

// Compute normalized output alpha channel:
//
// References:
// - http://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
// - https://github.com/jcupitt/ruby-vips/issues/28#issuecomment-9014826
//
// out_a = src_a + dst_a * (1 - src_a)
//
// ^^^^^ ^^^^^ ^^^^^
// t8 t7 t8
// ^^^^^^^^^^^
// t9
// ^^^^^ ^^^^^^^^^^^^^^^^^^^
// t11 t10

if( vips_linear1(t[8], &t[9], -1.0, 1.0, NULL) ||
vips_multiply(t[7], t[9], &t[10], NULL) ||
vips_add(t[8], t[10], &t[11], NULL)) {

g_object_unref(base);
return 1;
}

// Compute output RGB channels:
//
// Wikipedia:
//
// out_rgb = (src_rgb * src_a + dst_rgb * dst_a * (1 - src_a)) / out_a
//
// ^^^^^^^ ^^^^^^^ ^^^^^ ^^^^^^^ ^^^^^ ^^^^^ ^^^^^
// t13 t6 t8 t5 t7 t8 t11
// ^^^^^^^^^^^^^^^
// t12
//
// `vips_ifthenelse` with `blend=TRUE`:
// https://jcupitt.github.io/libvips/API/current/libvips-conversion.html#vips-ifthenelse
//
// out = (cond / 255) * in1 + (1 - cond / 255) * in2
//
// ^^^^ ^^^^
// └─── src_a255 = t4 ───┘
//
// Substitutions:
//
// cond --> src_a255 = t4
// in1 --> src_rgb = t6
// in2 --> dst_rgb * dst_a = t5 * t7 (premultiplied destination RGB)
//
// Division by `out_a` is not required as `vips_ifthenelse` doesn’t require
// normalized inputs.

if( vips_multiply(t[5], t[7], &t[12], NULL) ||
vips_ifthenelse(t[4], t[6], t[12], &t[13], "blend", TRUE, NULL)) {

g_object_unref(base);
return 1;
}

// unnormalise the alpha, reattach
if( vips_linear1(t[11], &t[14], 255.0, 0.0, "uchar", TRUE, NULL) ||
vips_bandjoin2(t[13], t[14], &t[15], NULL)) {

g_object_unref(base);
return 1;
}

// Return a reference to the output image.
g_object_ref(t[15]);
*out = t[15];

g_object_unref(base);
return 0;
}
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