Why does TextOverLay support hdr, but Bitmap not support?

[peerless2012]

I am using effect to render subtitles, and the TextOverLay in OverlayShaderProgram can be hdr on any version, but it's parent class BitmapOverlay can be hdr only on api34+ ?

If i want to draw subtitles use CanvasOverlay (Subclass of Bitmap Overlay), what should i do?

And if i draw content on TexOverLay, what should I do?

Both of the two places, i don't want to interrupt the HDR or DV render, i don't want the video trans HDR to SDR.

Any ideas of this question?

[droid-girl]

We use Bitmap and BitmapFactory under the hood and these APIs only supports HDR starting level 34.
We have a lower priority item on our roadmap to add a workaround for APIs < 34, but it is a lower priority item at the moment

[peerless2012]

Why TextOverlay don't have api level check?

[droid-girl]

If you want to render subtitles over HDR content, we recommend using TextOverlay. We included luminanceMultiplier in the OverlaySettings and will adjust the value based on the format of the content. This value is normalized so that 1 provides the default HDR luminance and 0.5 provides the default SDR luminance.
The reason BitmapOverlay has a version check is because of the limitations of Bitmap and BitmapFactory and it's support for retrieving gainmap from UltraHDR image.
If you need to draw subtitles, using TextOverlay should provide you with all functionality you need as we only adjust luminance.

[peerless2012]

Unfortunately, my render result is pieces of Bitmap, I have to render them on CanvasOverlay or TexOverLay.

I am still confused, the TextOverlay's Bitmap is create by Bitmap.create, and CanvasOverlay's Bitmap is also create by Bitmap.create, why these two have differences?

[droid-girl]

TextOverlay uses a SpannableString to create an overlay. Under the hood, this creates a BitmapOverlay by drawing the text onto an ARGB_8888 Bitmap. We add OverlaySettings to pass the luminance to adjust the text. So if you input is a string, we recommend to use TextOverlay effect. This is a specific optimization we made only to TextOvelay for now to account for text brightness on HDR videos.
The main difference in this case is that TextOverlay has an implementation to adjust for luminance.

[peerless2012]

Is it possible to add luminance to CanvasOverlay? It's use Bitmap as a canvas to draw other bitmaps.

Or the core difference is TextOverLay use StaticLayout, which support hdr?

[droid-girl]

Adding luminance to CanvasOverlay is a lower priority item for the team at the moment.
This commit added the support for luminance to the TextOverlay. You could try to create your own implementation of CanvasOverlay based on this change. I hope it helps

[peerless2012]

Forgive my poor english, the answer is still not clear, as TextOvderlay is base on BitmapOverlay, they all use Bitmap.create() to create a bitmap, I can't figure out why BitmapOverlay has api34 check, but TextOverlay doesn't need.

[claincly]

The API 34 check is likely added as a part to support gain map images, and that change didn't touch TextOverlay.

I wonder what you proble are trying to solve? Do you need CanvasOverlay to support HDR? If so I think what Kristina linked should give you some hint forward?

[peerless2012]

I am developing a libass impl for media3 (without modify media3), the libass will render result to Bitmap pieces, I draw these bitmaps use CanvasOverlay.

Since CanvasOverlay base on BitmapOverlay, but it seems support hdr on api34 and above. And TextOverLay is also base on BitmapOverlay, but it support hdr on all versions, so that's what I am confused.