drawing only on transparent pixels using blendmode() ?

Questions
1

Is there a way to draw only on transparent parts of an image using blendmode()?

I found the blendmodes thread by Jvm38, but I still don’t quite understand how the multiple parameters work together. Big thanks to jvm38 for the thread though, I found a much faster erase function than my own manual solution!

2

this shoul do what you want

blendMode(
  ONE_MINUS_DST_ALPHA,
  ZERO,
  ZERO,
  ONE_MINUS_DST_ALPHA
)
3

note that

blendMode(
  mutliply_source_image_color_by_this,
  mutliply_destnation_image_color_by_this,
  mutliply_source_image_alpha_by_this,
  mutliply_destnation_image_alpha_by_this,
)

color = sum of the 2 colors after multiplication by the coefficients above
alpha = opacity = sum of the 2 alphas after multiplication by the coefficients above
4

@Jmv38 What would the blendmode be for setting black pixels (0,0,0,255) to alpha zero?

5

the back pixels of the SOURCE image or the DESTINATION image?

6

The image that you’re about to draw (is that source or destination?)

Edit: it’s the source

7

I’m not sure the effect I’m after is possible with blendMode, not without messing up the colour values of the other pixels. At the moment, I use a shader to make black pixels invisible, adding this line to the frag:

col.a = smoothstep(0.,0.1, (col.r + col.g + col.b)); //set alpha to zero for black

i.e. effectively treating black and near-black as an alpha channel.

But I was curious as to whether the same effect was possible just with blendMode.

8

Thanks jvm! The suggested blendmode didn’t do what I wanted (or I may have used it wrong), but thanks to your second post I was able to figure out (or at least stumble upon) the effect I was after!

I used: blendMode(ONE_MINUS_DST_ALPHA,ONE,ONE,ONE_MINUS_SRC_ALPHA)

This appears to draw only on the transparent pixels of the DST image and draws the SRC colors as they are given.

@yojimbo
In my understanding it’s:

SRC = the image you are drawing
DST = the image you are drawing over

Which then together make the output color, which is the eventual resulting color made by multiplying/blending SRC and DST in the way you specify. This will show on the screen.

I’m sure I will be corrected in case I am completely wrong. =)

9

@kirl good to know you could manage it yourself with my little summary!
When i first decided to use blendmode(), it took me weeks before i felt i finally understood it.

10

just curious: is there any chane of faking floodfill with blendmodes, so one color gets replaced by another?

I remember, I tried to make myself a spritesheet with all blendmodes available, but noone had worked as a solution… (spritesheet was done with using every blendmode with every other, like with a for loop, kinda bruteforce method but it worked for a overview)

11

can you give a pecise example of what you mean by floodfill ? If adding colors is involved then yes. If comparison is involved then no. You have to write a shader for this.

12

If you’re talking about the kind of floodfill used in image programs, which fills just the current area and stops when it hits a different colour, I don’t think shaders would do, you have to iterate. I have code for that.