Getting Correct Alpha Values

General
1

I have been struggling with what should be very simple.
I have a PNG image file with alpha, it is a square picture with a circle in the center, the top half of the circle is opaque white, the bottom half is pure BLUE opaque, the corners of the image (outside the circle) are clear.
I load it from disk and assign it to an ImageWell and it draws correctly.
But if I access the Image (not the image well) then Image’s RGBSurface to get pixel values here’s what I see.
in the White area the color is &h00FFFFFF (which seems correct, opaque white)
in the blue area the color is &h000000FF (which seems correct, opaque blue)
in the clear area the color is &h00000000 (which seems totally wrong, it is saying opaque black)

I can also report that the image says Transparent = 0 and HasAlphaChannel=true.

Any ideas where I have gone wrong?

2

&h00000000 is right for premultiplied transparent color.

because alpha is 0, the actual color there is gone as all components are multiplied with zero.

3

I don’t think that is correct. according to Xojo’s Docs for Color (https://documentation.xojo.com/api/graphics/color.html)

Alpha Channel Support
The Color type has a read-only “Alpha” property. The alpha channel is the transparency of the color represented as an integer between 0 (opaque) and 255 (transparent).

So &h00000000 means OPAQUE BLACK. Which is definitely not clear.
I modified my image to have a circle of Opaque Black and checked the color returned there, it likewise returned &h00000000
so it is drawing different but returning the same value (in fact nowhere in the image do I get an alpha value of anything other than 00.)
Or am I missing something on that?

4

Oh, sorry, it may be that Color in Xojo inverts alpha.

6

Depends on how you are reading and writing colors
using &H or &C seem to not be the same when Alpha is involved

&c000000FF is opaque black, but resolves to &HFF000000

so you need to be aware of the method to know if the first byte is RED or ALPHA and if the last byte is BLUE or ALPHA

Dim c As Color = &c000000ff
MsgBox Str(c.red)+"/"+Str(c.Green)+"/"+Str(c.blue)+"/"+Str(c.alpha)+"/"+str(c)
7

&h is RRGGBBAA
&c is AARRGGBB

8

Thanks, I get that &c and &h will represent the values differently, but the problem is, at the core, that when I execute
c = theImage.RGBSurface.Pixel(x,y) the value for pixels which are plotting as clear are exactly the same values as for pixels which are opaque black.

I have now found that if I change a pixel to color.rgba(0,0,0,254) then the pixel reads as &hFE000000, but…
If I set it to color.rgba(0,0,0,255) then the pixel reads as &h00000000.

I’ve gone through every imaginable manner of manipulating the image but I NEVER read an alpha of transparent (255)

9

I really want to help you here, because something at the back of my brain is telling me that I dealt with this before. I just can’t remember the cause or how I fixed it.

One thing you can try is getting the mask of the picture and checking that:

dim pictureMask as Picture = myPicture.CopyMask()
10

How about splitting the concept of the and instead use the attributes of a Xojo color

if you get

c = theRGBSurface.pixel(x,y)
Then c.alpha should tell you what the alpha value is
And c.red c.green and c.blue should give you the color values

11

I have gone through many gyrations even doing the following:

system.DebugLog("Pixel XY = "+str(theImage.RGBSurface.Pixel(x,y).alpha))

The result is correct for points with alpha of 0 (opaque) to 254 (almost totally transparent), BUT if the point is totally transparent (with alpha of 255) then the returned value is 0 (opaque)

It is infuriating. I’ve done every imaginable combination.

12

I’ve now felt in this trap as well. Creating the picture using 2 or 3 parameters makes no difference: every transparent pixel of the original PNG file, obtained using RGBSurface.Pixel, returns an alpha of 0 (like non-transparent pixels), although the picture is drawn with correct transparency using DrawPicture.
Curious it’s not considered a big problem since one year.