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rudimentary support for BMPs with alpha masks #43

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rudimentary support for BMPs with alpha masks #43

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5e71cb5
rudimentary support for BMPs with alpha masks
samcday Mar 25, 2024
512c09e
basic blending support for transparent BMPs
samcday Mar 27, 2024
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8 changes: 8 additions & 0 deletions src/header/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -201,6 +201,14 @@ impl ChannelMasks {
blue: 0x0000FF,
alpha: 0,
};

/// Argb888 color masks.
pub const ARGB888: Self = Self {
red: 0xFF000000,
green: 0xFF0000,
blue: 0xFF00,
alpha: 0xFF,
};
}

/// Describes how the BMP file is compressed.
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1 change: 1 addition & 0 deletions src/iter.rs
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Expand Up @@ -56,6 +56,7 @@ where
ColorType::Rgb555 => Rgb555::from(RawU16::from_u32(color)).into(),
ColorType::Rgb565 => Rgb565::from(RawU16::from_u32(color)).into(),
ColorType::Rgb888 | ColorType::Xrgb8888 => Rgb888::from(RawU24::from_u32(color)).into(),
ColorType::Argb8888 => Rgb888::from(RawU24::from_u32(color >> 8)).into(),
};

Some(Pixel(position, color))
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45 changes: 45 additions & 0 deletions src/lib.rs
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Expand Up @@ -209,6 +209,7 @@ pub use raw_iter::{RawPixel, RawPixels};
pub struct Bmp<'a, C> {
raw_bmp: RawBmp<'a>,
color_type: PhantomData<C>,
alpha_bg: Rgb888,
}

impl<'a, C> Bmp<'a, C>
Expand All @@ -225,9 +226,18 @@ where
Ok(Self {
raw_bmp,
color_type: PhantomData,
alpha_bg: Rgb888::BLACK,
})
}

/// If this image contains transparent pixels (pixels with an alpha channel), then blend these
/// pixels with the provided color. Note that this will only be used when drawing to a target.
/// It will not be applied when querying pixels from the image.
pub fn with_alpha_bg<BG: Into<Rgb888>>(mut self, alpha_bg: BG) -> Self {
self.alpha_bg = alpha_bg.into();
self
}

Comment on lines +233 to +240
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I think this should be a regular setter (set_alpha_bg<...>(&mut self, alpha_bg: BG)) instead of this builder style method. Using a mutable reference instead of having to transfer ownership makes it more flexible.

An even better solution would be to add a wrapper type around the Bmp type, that sets the background color, but this would be a lot more work:

pub fn with_alpha_bg<BG: Into<Rgb888>>(&self, alpha_bg: BG) -> BmpWithAlphaBg<'_, C> {
    BmpWithAlphaBg {
        bmp: self,
        alpha_bg: alpha_bg.into(),
    }
}

This would allow the Bmp object to be drawn with any background color without needing a mutable reference or even ownership of the object. You could then easily implement something like:

fn draw_button<D: DrawTarget>(target: &mut D, text: &str, icon: &Bmp<'_, D::Color>, background: D::Color) -> Result<(), D::Error> {
    ....
}

This would still kind of work with a mutable reference to the icon, but wouldn't work with the original transfer of ownership in the with_alpha_bg method.

/// Returns an iterator over the pixels in this image.
///
/// The iterator always starts at the top left corner of the image, regardless of the row order
Expand Down Expand Up @@ -362,6 +372,37 @@ where
RawColors::<RawU32>::new(&self.raw_bmp)
.map(|raw| Rgb888::from(RawU24::new(raw.into_inner())).into()),
),
ColorType::Argb8888 => {
target.fill_contiguous(
&area,
RawColors::<RawU32>::new(&self.raw_bmp).map(|raw| {
// integer blending approach from https://stackoverflow.com/a/12016968
let v = raw.into_inner();
let mut alpha = v & 0xFF;
let inv_alpha = 256 - alpha;
alpha += 1;
if alpha == 0 {
// pixel is completely transparent, use bg color
self.alpha_bg
} else if alpha == 255 {
// pixel is completely opaque, just use its color
Rgb888::from(RawU24::new(v >> 8))
Comment on lines +383 to +389
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I didn't have time to thoroughly check the blending code, but the checking for alpha == 0 and alpha == 255 must happen before 1 is added.

} else {
// pixel has transparency, blend with BG color
let col = Rgb888::from(RawU24::new(v >> 8));
Rgb888::new(
((alpha * col.r() as u32 + inv_alpha * self.alpha_bg.r() as u32)
>> 8) as u8,
((alpha * col.g() as u32 + inv_alpha * self.alpha_bg.g() as u32)
>> 8) as u8,
((alpha * col.b() as u32 + inv_alpha * self.alpha_bg.b() as u32)
>> 8) as u8,
)
}
.into()
}),
)
}
}
}

Expand Down Expand Up @@ -421,6 +462,10 @@ where
.raw_bmp
.pixel(p)
.map(|raw| Rgb888::from(RawU24::from_u32(raw)).into()),
ColorType::Argb8888 => self
.raw_bmp
.pixel(p)
.map(|raw| Rgb888::from(RawU24::from_u32(raw >> 8)).into()),
}
}
}
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3 changes: 3 additions & 0 deletions src/raw_bmp.rs
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Expand Up @@ -157,6 +157,7 @@ pub enum ColorType {
Rgb565,
Rgb888,
Xrgb8888,
Argb8888,
}

impl ColorType {
Expand Down Expand Up @@ -184,6 +185,8 @@ impl ColorType {
if let Some(masks) = header.channel_masks {
if masks == ChannelMasks::RGB888 {
ColorType::Xrgb8888
} else if masks == ChannelMasks::ARGB888 {
ColorType::Argb8888
} else {
return Err(ParseError::UnsupportedChannelMasks);
}
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