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Fix alpha blending for WebP images

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* src/image.c (webp_load): Fix WebP transparency by doing manual alpha
blending instead of naive masking.  (Bug#59242)
This commit is contained in:
Stefan Kangas 2023-08-25 20:26:45 +02:00
parent aaaee4ebe1
commit eb61bbacef
1 changed files with 38 additions and 34 deletions
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72
src/image.c
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@ -10450,22 +10450,36 @@ webp_load (struct frame *f, struct image *img)
}
/* Create the x image and pixmap. */
Emacs_Pix_Container ximg, mask_img = NULL;
Emacs_Pix_Container ximg;
if (!image_create_x_image_and_pixmap (f, img, width, height, 0, &ximg, false))
goto webp_error2;
/* Create an image and pixmap serving as mask if the WebP image
contains an alpha channel. */
if (features.has_alpha
&& !image_create_x_image_and_pixmap (f, img, width, height, 1,
&mask_img, true))
/* Find the background to use if the WebP image contains an alpha
channel. */
Emacs_Color bg_color;
if (features.has_alpha)
{
image_destroy_x_image (ximg);
image_clear_image_1 (f, img, CLEAR_IMAGE_PIXMAP);
goto webp_error2;
Lisp_Object specified_bg
= image_spec_value (img->spec, QCbackground, NULL);
/* If the user specified a color, try to use it; if not, use the
current frame background, ignoring any default background
color set by the image. */
if (STRINGP (specified_bg))
FRAME_TERMINAL (f)->defined_color_hook (f,
SSDATA (specified_bg),
&bg_color,
false,
false);
else
FRAME_TERMINAL (f)->query_frame_background_color (f, &bg_color);
bg_color.red >>= 8;
bg_color.green >>= 8;
bg_color.blue >>= 8;
}
/* Fill the X image and mask from WebP data. */
/* Fill the X image from WebP data. */
init_color_table ();
img->corners[TOP_CORNER] = 0;
@ -10480,21 +10494,21 @@ webp_load (struct frame *f, struct image *img)
{
for (int x = 0; x < width; ++x)
{
int r = *p++ << 8;
int g = *p++ << 8;
int b = *p++ << 8;
int r, g, b;
/* The WebP alpha channel allows 256 levels of partial
transparency. Blend it with the background manually. */
if (features.has_alpha || anim) {
float a = (float) p[3] / UINT8_MAX;
r = (int)(a * p[0] + (1.0 - a) * bg_color.red) << 8;
g = (int)(a * p[1] + (1.0 - a) * bg_color.green) << 8;
b = (int)(a * p[2] + (1.0 - a) * bg_color.blue) << 8;
p += 4;
} else {
r = *p++ << 8;
g = *p++ << 8;
b = *p++ << 8;
}
PUT_PIXEL (ximg, x, y, lookup_rgb_color (f, r, g, b));
/* An alpha channel associates variable transparency with an
image. WebP allows up to 256 levels of partial transparency.
We handle this like with PNG (which see), using the frame's
background color to combine the image with. */
if (features.has_alpha || anim)
{
if (mask_img)
PUT_PIXEL (mask_img, x, y, *p > 0 ? PIX_MASK_DRAW : PIX_MASK_RETAIN);
++p;
}
}
}
@ -10507,16 +10521,6 @@ webp_load (struct frame *f, struct image *img)
/* Put ximg into the image. */
image_put_x_image (f, img, ximg, 0);
/* Same for the mask. */
if (mask_img)
{
/* Fill in the background_transparent field while we have the
mask handy. Casting avoids a GCC warning. */
image_background_transparent (img, f, (Emacs_Pix_Context)mask_img);
image_put_x_image (f, img, mask_img, 1);
}
img->width = width;
img->height = height;