Image Embed Issues with Grayscale Image and RGBA PNG

[lanyeeee]

Description

1. Grayscale JPG not display at all
2. Grayscale PNG display incorrectly/with abnormal appearance
3. RGBA PNG not displaying at all

Minimal Reproduction Project

embed-img-to-pdf-err.zip

lopdf Version

0.34.0

Additional Information

In fact I have found the cause of this issue

1. Grayscale Image Not Displaying
   

   lopdf/src/xobject.rs

   Line 75 in 34e2d20

   let bits = color_type.bits_per_pixel() / 3;

   
   The code incorrectly divides bits_per_pixel by 3 for all image types.
   For grayscale images, bits_per_pixel is 8 and should not be divided by 3.

2. RGBA PNG Not Displaying
   

   lopdf/src/xobject.rs

   Line 83 in 34e2d20

   ColorType::Rgba8 => b"DeviceN".to_vec(),

   
   The code incorrectly sets the color_space to DeviceN for RGBA images.
   RGBA should be convert to RGB before embed, then use DeviceRGB color space instead of DeviceN

I have preliminarily resolved this issue, they works good in my local project.

I plan to make a PR, but I noticed this interesting comment in the code:

lopdf/src/xobject.rs

Lines 71 to 75 in 34e2d20

	// It looks like Adobe Illustrator uses a predictor offset of 2 bytes rather than 1 byte as
	// the PNG specification suggests. This seems to come from the fact that the PNG specification
	// doesn't allow 4-bit color images (only 8-bit and 16-bit color). With 1-bit, 2-bit and 4-bit
	// mono images there isn't the same problem because there's only one component.
	let bits = color_type.bits_per_pixel() / 3;

1. The comment discusses predictor offset behavior in Adobe Illustrator
2. However, the code that follows (bits_per_pixel() / 3) seems unrelated to the predictor behavior described in the comment
3. This division by 3 is actually causing issues with grayscale images

Before submitting the PR, I'd like to understand:

1. Is there any historical context for this implementation?
2. Does the Adobe Illustrator predictor behavior still need to be considered?
3. Should we update or remove this comment?