OpenGL Multiplicative Blending mode

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I'm trying to draw an image on top of a fully opaque background, using additive blending or multiplicative blending depending on a toggle.



Both work, but the alpha of the image gets ignored when doing multiplicative blending, while in additive it's working.



For additive I have:



glBlendFunc(GL_SRC_ALPHA, GL_ONE);


For multiplicative I have:



glBlendFunc(GL_DST_COLOR, GL_ZERO);


Additive works as expected, multiplicative ignores the alpha.



I am aware I can swap shaders to achieve what I want, but if I could do it through glBlendFunc it would be much better.










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  • How should "multiplicative blending" incorporate the alpha?

    – Nicol Bolas
    Dec 30 '18 at 20:27











  • Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

    – evilpudding
    Dec 30 '18 at 20:37






  • 2





    "in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

    – Nicol Bolas
    Dec 30 '18 at 20:45













  • final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

    – evilpudding
    Dec 30 '18 at 20:55













  • When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

    – evilpudding
    Dec 30 '18 at 21:03


















0















I'm trying to draw an image on top of a fully opaque background, using additive blending or multiplicative blending depending on a toggle.



Both work, but the alpha of the image gets ignored when doing multiplicative blending, while in additive it's working.



For additive I have:



glBlendFunc(GL_SRC_ALPHA, GL_ONE);


For multiplicative I have:



glBlendFunc(GL_DST_COLOR, GL_ZERO);


Additive works as expected, multiplicative ignores the alpha.



I am aware I can swap shaders to achieve what I want, but if I could do it through glBlendFunc it would be much better.










share|improve this question

























  • How should "multiplicative blending" incorporate the alpha?

    – Nicol Bolas
    Dec 30 '18 at 20:27











  • Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

    – evilpudding
    Dec 30 '18 at 20:37






  • 2





    "in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

    – Nicol Bolas
    Dec 30 '18 at 20:45













  • final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

    – evilpudding
    Dec 30 '18 at 20:55













  • When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

    – evilpudding
    Dec 30 '18 at 21:03
















0












0








0








I'm trying to draw an image on top of a fully opaque background, using additive blending or multiplicative blending depending on a toggle.



Both work, but the alpha of the image gets ignored when doing multiplicative blending, while in additive it's working.



For additive I have:



glBlendFunc(GL_SRC_ALPHA, GL_ONE);


For multiplicative I have:



glBlendFunc(GL_DST_COLOR, GL_ZERO);


Additive works as expected, multiplicative ignores the alpha.



I am aware I can swap shaders to achieve what I want, but if I could do it through glBlendFunc it would be much better.










share|improve this question
















I'm trying to draw an image on top of a fully opaque background, using additive blending or multiplicative blending depending on a toggle.



Both work, but the alpha of the image gets ignored when doing multiplicative blending, while in additive it's working.



For additive I have:



glBlendFunc(GL_SRC_ALPHA, GL_ONE);


For multiplicative I have:



glBlendFunc(GL_DST_COLOR, GL_ZERO);


Additive works as expected, multiplicative ignores the alpha.



I am aware I can swap shaders to achieve what I want, but if I could do it through glBlendFunc it would be much better.







opengl






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Jan 1 at 4:27









genpfault

41.9k95398




41.9k95398










asked Dec 30 '18 at 20:21









evilpuddingevilpudding

105




105













  • How should "multiplicative blending" incorporate the alpha?

    – Nicol Bolas
    Dec 30 '18 at 20:27











  • Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

    – evilpudding
    Dec 30 '18 at 20:37






  • 2





    "in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

    – Nicol Bolas
    Dec 30 '18 at 20:45













  • final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

    – evilpudding
    Dec 30 '18 at 20:55













  • When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

    – evilpudding
    Dec 30 '18 at 21:03





















  • How should "multiplicative blending" incorporate the alpha?

    – Nicol Bolas
    Dec 30 '18 at 20:27











  • Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

    – evilpudding
    Dec 30 '18 at 20:37






  • 2





    "in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

    – Nicol Bolas
    Dec 30 '18 at 20:45













  • final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

    – evilpudding
    Dec 30 '18 at 20:55













  • When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

    – evilpudding
    Dec 30 '18 at 21:03



















How should "multiplicative blending" incorporate the alpha?

– Nicol Bolas
Dec 30 '18 at 20:27





How should "multiplicative blending" incorporate the alpha?

– Nicol Bolas
Dec 30 '18 at 20:27













Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

– evilpudding
Dec 30 '18 at 20:37





Maximum alpha should behave like it does, minimum alpha should leave the background unchanged, in the middle it should interpolate. It's what most image editing software does. Is it possible through glBlendFunc?

– evilpudding
Dec 30 '18 at 20:37




2




2





"in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

– Nicol Bolas
Dec 30 '18 at 20:45







"in the middle it should interpolate" Interpolate between what and what? You want linear interpolation between two values, but one of them is neither the source nor destination colors. Write down the math you actually want done, and then see if that fits into OpenGL's blending math.

– Nicol Bolas
Dec 30 '18 at 20:45















final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

– evilpudding
Dec 30 '18 at 20:55







final = dest.rgb * (1.0f - src.a) + (dest.rgb * src.rgb * src.a) I think it's this. Does it make sense?

– evilpudding
Dec 30 '18 at 20:55















When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

– evilpudding
Dec 30 '18 at 21:03







When I said interpolate between them, I meant the maximum and minimum alpha states, where alpha = 0 meant the image doesn't get drawn at all, and alpha = 1 is dest.rgb * src.rgb, which is what glBlendFunc(GL_DST_COLOR, GL_ZERO); yields.

– evilpudding
Dec 30 '18 at 21:03














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