BlendInvert: invert blend mode

[Tenry]

What about an additional blend mode for inverting?

E.g., for an editor it's useful that the grid does not have a static color (which might be similar to the background) but just inverting what's behind? (black -> white, or white -> black, for example).
Another case would be hit enemies: it's usefull to invert an enemy's sprite for a short time to indicate that it has been hit.


In OpenGL, it seems to be done like this (haven't tested it):

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glBlend(GL_ONE_MINUS_DST_COLOR, GL_ZERO);


[Laurent]

Code: [Select]

glBlend(GL_ONE_MINUS_DST_COLOR, GL_ZERO);

That would be a multiplicative blending, between source and inverted destination.

Given that we'd like to get this:

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pixel.rgb = (1 - source.rgb)
pixel.a = source.a
... and that OpenGL blending follows this formula:

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pixel.rgba = source.rgba * arg1 + dest.rgba * arg2
... I don't think we can get the desired effect.

Blending modes are a way to choose how source and destination are mixed together, not a way to modify how the source is drawn. You have shaders for that

[Tenry]

Code: [Select]

glBlend(GL_ONE_MINUS_DST_COLOR, GL_ZERO);

That would be a multiplicative blending, between source and inverted destination.

Given that we'd like to get this:

Code: [Select]

pixel.rgb = (1 - source.rgb)
pixel.a = source.a
... and that OpenGL blending follows this formula:

Code: [Select]

pixel.rgba = source.rgba * arg1 + dest.rgba * arg2
... I don't think we can get the desired effect.

Blending modes are a way to choose how source and destination are mixed together, not a way to modify how the source is drawn.

Hm... let's assume we are drawing a white image (source) onto a red surface (target). Source values are 1.f, 1.f, 1.f. Target value is 1.f, 0.f, 0.f.
For the ONE_MINUS_DST_COLOR part, 1.f is multiplied with either 1.f-1.f (red) or with 1.f-0.f (green, blue). The other parameter is "ignored" so nothing is added to the result.

1.f * (1.f-1.f) + ZERO = 0.f (RED is now 0% and hence inverted)
1.f * (1.f-0.f) + ZERO = 1.f (GREEN)
1.f * (1.f-0.f) + ZERO = 1.f (BLUE)
And that is inverted. Am I not right?

You have shaders for that

I have never worked with shaders yet...

[Laurent]

You draw a white sprite and what was drawn behind it becomes inverted. Is that what you call "it works"?

People who use a Invert blend mode will most likely expect what they draw to be inverted.

[Tenry]

You draw a white sprite and what was drawn behind it becomes inverted. Is that what you call "it works"?

People who use a Invert blend mode will most likely expect what they draw to be inverted.

Well, in my experience, I usually draw a sprite (or something else) normally, then I have a black/white image, where the white regions means "invert", and the black one "don't revert" (e.g. a grid), and draw it on top of it. If I would draw the second sprite (with the same color) with invert blending, the result is completely inverted.