Add image.premultiply_alpha helper.

core/image/common.odin

@@ -1320,6 +1320,42 @@ blend_pixel :: #force_inline proc(fg: [$N]$T, alpha: T, bg: [N]T) -> (res: [N]T)
 }
 blend :: proc{blend_single_channel, blend_pixel}
 
+// For all pixels of the image, multiplies R, G and B by Alpha. This is useful mainly for games rendering anti-aliased transparent sprites.
+// Grayscale with alpha images are supported as well.
+// Note that some image formats like QOI explicitly do NOT support premultiplied alpha, so you will end up with a non-standard file.
+premultiply_alpha :: proc(img: ^Image) -> (ok: bool) {
+	switch {
+	case img.channels == 2 && img.depth == 8:
+		pixels := mem.slice_data_cast([]GA_Pixel, img.pixels.buf[:])
+		for &pixel in pixels {
+			pixel.r = u8(u32(pixel.r) * u32(pixel.g) / 0xFF)
+		}
+		return true
+	case img.channels == 2 && img.depth == 16:
+		pixels := mem.slice_data_cast([]GA_Pixel_16, img.pixels.buf[:])
+		for &pixel in pixels {
+			pixel.r = u16(u32(pixel.r) * u32(pixel.g) / 0xFFFF)
+		}
+		return true
+	case img.channels == 4 && img.depth == 8:
+		pixels := mem.slice_data_cast([]RGBA_Pixel, img.pixels.buf[:])
+		for &pixel in pixels {
+			pixel.r = u8(u32(pixel.r) * u32(pixel.a) / 0xFF)
+			pixel.g = u8(u32(pixel.g) * u32(pixel.a) / 0xFF)
+			pixel.b = u8(u32(pixel.b) * u32(pixel.a) / 0xFF)
+		}
+		return true
+	case img.channels == 4 && img.depth == 16:
+		pixels := mem.slice_data_cast([]RGBA_Pixel_16, img.pixels.buf[:])
+		for &pixel in pixels {
+			pixel.r = u16(u32(pixel.r) * u32(pixel.a) / 0xFFFF)
+			pixel.g = u16(u32(pixel.g) * u32(pixel.a) / 0xFFFF)
+			pixel.b = u16(u32(pixel.b) * u32(pixel.a) / 0xFFFF)
+		}
+		return true
+	case: return false
+	}
+}
 
 // Replicates grayscale values into RGB(A) 8- or 16-bit images as appropriate.
 // Returns early with `false` if already an RGB(A) image.