Transparency Issue with MultipassBitmap in GeUserArea

T1001

Hello,

I'm encountering an issue with transparency when drawing a MultipassBitmap over a background in a custom GeUserArea. The transparent areas appear black unless I save the bitmap before drawing it.

Code Snippet:

// ARGBfbuf is a Float32 image buffer with ARGB data
// w = width, h = height

auto Bmp = MultipassBitmap::Alloc(w, h, COLORMODE::ARGBf);
if (!Bmp)
{
    // Handle allocation failure
    return;
}
Bmp->SetColorProfile(ColorProfile::GetDefaultSRGB());

// Set parameters for the MultipassBitmap
Bmp->SetParameter(MPBTYPE_NAME, "bitmap");
Bmp->SetParameter(MPBTYPE_SAVE, FALSE);
Bmp->SetParameter(MPBTYPE_SHOW, TRUE);

// Add root alpha channel to the MultipassBitmap
auto rootAlpha = Bmp->AddAlpha(nullptr, COLORMODE::GRAYf);
if (!rootAlpha)
{
    // Handle error
    return;
}
// Set parameters for the root alpha
rootAlpha->SetParameter(MPBTYPE_NAME, "rootAlpha");
rootAlpha->SetParameter(MPBTYPE_SAVE, FALSE);
rootAlpha->SetParameter(MPBTYPE_SHOW, TRUE);

auto layer = Bmp->GetLayerNum(0);
if (!layer)
{
    // Handle error
    return;
}

// Set parameters for the layer
layer->SetParameter(MPBTYPE_NAME, "layer0");
layer->SetParameter(MPBTYPE_SAVE, FALSE);
layer->SetParameter(MPBTYPE_SHOW, TRUE);

// Add alpha channel to the layer
auto alpha = layer->AddAlpha(nullptr, COLORMODE::GRAYf);
if (!alpha)
{
    // Handle error
    return;
}

// Set parameters for the alpha layer
alpha->SetParameter(MPBTYPE_NAME, "alpha0");
alpha->SetParameter(MPBTYPE_SAVE, FALSE);
alpha->SetParameter(MPBTYPE_SHOW, TRUE);

for (Int32 y = 0; y < h; ++y)
{
    Float32* ptr = ARGBfbuf + (y * w * 4);

    // Copy image line to the layer
    layer->SetPixelCnt(0, y, w, reinterpret_cast<UChar*>(ptr), COLORBYTES_ARGBf, COLORMODE::ARGBf, PIXELCNT_0);

    // Copy alpha line to the alpha layer
    alpha->SetPixelCnt(0, y, w, reinterpret_cast<UChar*>(ptr), COLORBYTES_ARGBf, COLORMODE::GRAYf, PIXELCNT_0);
}

// In the GeUserArea render function:

// Draw background image
DrawBitmap(Background, 0, 0, GetWidth(), GetHeight(),
           0, 0, Background->GetBw(), Background->GetBh(),
           BMP_NORMAL);

// Draw bitmap with transparency
DrawBitmap(Bmp, 0, 0, GetWidth(), GetHeight(),
           0, 0, Bmp->GetBw(), Bmp->GetBh(),
           BMP_NORMAL | BMP_APPLY_COLORPROFILE | BMP_TRANSPARENTALPHA);

Issue:

Transparent areas (where alpha = 0.0f) appear black instead of showing the background.
Workaround:

Saving the bitmap before drawing fixes the issue:

// Save and delete the file
String path = GeGetStartupWritePath() + "/temp.tga";
Bmp->Save(path, FILTER_TGA, nullptr, SAVEBIT_ALPHA);
GeFKill(path);

// Then draw as before
DrawBitmap(Bmp, ...);

Additional Information:

I tried using BaseBitmap instead of MultipassBitmap, but BaseBitmap does not support floating-point color modes (COLORMODE::ARGBf), which I need for my application.
Even when using BaseBitmap and saving the image, the transparency issue persists.

Questions:

Why does saving the MultipassBitmap make the transparency work?
How can I achieve proper transparency without saving the bitmap?
Is there a way to handle floating-point ARGB data with transparency using BaseBitmap, or is MultipassBitmap the only option?
Do the SetParameter calls on the Bmp, Layer, root alpha, and alpha layers have any effect on this issue, and am I using them correctly?

Any insights or solutions would be greatly appreciated!

ferdinand

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About your First Question

Hey, you did a great job for your first your topic(s), I assume you did read our forum rules - thanks. But you should really avoid having more than one question in a topic, because things tend to become very hard to answer then and our answers very long (as evident by my answer here). Try to find a singular main question you have. You can then optionally ask thematically closely related follow-up questions once the main question has been answered.

Before we go here into details, I would recommend having a look at the topic Overlapping images with transparency with BaseBitmap as it is quite tangential to what you seem to be trying to do.

The general problem is that this "writing multiple layers with embedded alpha values" scenario is not something the Cinema Image API (BaseBitmap, MultipassBitmap) has been really geared for. The major use case for creating multiple layers with alpha values in the Cinema API, Bodypaint, is covered by cinema::PaintBitmap. But that is sort of its own eco-system. BaseBitmap and MultipassBitmap are meant for loading and displaying data.

The reasons why your code is likely not working, is because you do not write the master alpha of the multipass bitmap. That was also what I was struggling with in the other thread. You create the channel as auto rootAlpha = Bmp->AddAlpha(nullptr, COLORMODE::GRAYf); but never write to it. Probably in the assumption that this would happen automatically, but it does not.
That is probably due to the Maxon Image API image saver maxon::ImageSaverClasses::TGA or generally the switch to the underlying Image API (BaseBitmap::GetImageRef()) making some things snap into place there. You must understand that the Cinema API types BaseBitmap and MultipassBitmap are just a shallow wrapper for the underlying Maxon Image API.
You could save into one of our in-memory UrlSchemes, e.g., an MFS or Ramdisk, when it is the disk activity you fear. When you want to avoid the general overhead of having to serialize and deserialize the image, and want to stay in the Cinema Image API, you will probably have to curate your master alpha channel manually. Or just do what I did in the other thread and manually blend things. There are also some options in the Maxon Image API, but my answer is getting longer and longer ...
I don't think that this is true. Both BaseBitmap and the underlying ImageInterface (which you usually encounter as an ImageRef in code) support floating point image data. They are defined under the Graphics label as maxon:Pix...
They are largely correct but neither necessary nor the cause (not sure why you set MPBTYPE_SAVE to false though).

This topic is riddled a bit by too many questions. Let's try to take a more direct angle here and just try to solve what you are trying to do. I assume you want to have some form of UI that stacks multiple images with transparent areas, similar to what the Asset Browser is doing here to display overlay icons over its assets.

The Asset browser simply achieves this by drawing the images on top of each other via cinema::GeUserArea::DrawImageRef, i.e., it uses the Maxon Image API directly. Here you could also load in SVG data via maxon::VectorImageClasses if you wanted to. We also have specialized buffer handlers for pixel blending or you could manage a multi layer image in the Maxon Image API via maxon::ImageBaseInterface. But I would avoid these two latter options when possible.

Maybe you could describe what you want to achieve concretely?

Given the 32bit/float requirement, I assume you want to draw some texture tiles with some meta information superimposed on them? Please also try to share executable code with us, so that I have a starting point when I write a code example. Please also share demo data when necessary (e.g., an HDRI texture).

Cheers,
Ferdinand