"Wrong" colours in png files

Trond · Post by **Trond** » Thu May 11, 2006 5:12 pm

When I use this code the png file looks very white. When I load the image using GDI+ it looks normal. Is this is a problem in PB or with something else?

Code: Select all

UsePNGImageDecoder()

OpenWindow(0, 0, 0, 512, 384, "", #PB_Window_ScreenCentered | #PB_Window_SystemMenu)
SetWindowLong_(WindowID(0), #GWL_EXSTYLE, GetWindowLong_(WindowID(0), #GWL_EXSTYLE) | #WS_EX_LAYERED)

LoadImage(0, "clock.png")

hDC = StartDrawing(ImageOutput(0))
  GetObject_(ImageID(0), SizeOf(BITMAP), @BitmapInfo.BITMAP)
  ContextOffset.POINT
  BlendMode.BLENDFUNCTION
  BlendMode\SourceConstantAlpha = 255
  BlendMode\AlphaFormat = 1
  UpdateLayeredWindow_(WindowID(0), 0, 0, @BitmapInfo+4, hDC, @ContextOffset, 0, @BlendMode, 2)
StopDrawing()

Repeat
  Select WaitWindowEvent()
    Case #PB_Event_CloseWindow
      Break
  EndSelect
ForEver

Num3 · Post by **Num3** » Thu May 11, 2006 8:56 pm

PNG's have a gamma information on them, so probably it depends if PB reads it and applies the gamma correction...

Trond · Post by **Trond** » Thu May 11, 2006 9:13 pm

Hmm, I could have swore I unchecked "save gamma information", but I'll try to fiddle with it.

mskuma · Post by **mskuma** » Thu May 11, 2006 11:44 pm

Maybe this is not required anymore.. I saved out a 32bit PNG in Photoshop & tried it with the above code and found no problem.

Trond · Post by **Trond** » Fri May 12, 2006 8:57 am

Did you save gamma?

mskuma · Post by **mskuma** » Fri May 12, 2006 9:38 am

Show me where the save gamma option is when saving a PNG in Photoshop & I'll try it ... usually it's a non-issue for me using PS.

Trond · Post by **Trond** » Fri May 12, 2006 10:16 am

I used GIMP for mine. But on Photoshop for mac there´s a checkbox in the save as dialog. I´ll try a few different png´s when I get home.

Edit: No, there wasn´t a checkbox for that. But did you check "build in colour profile"? I think gamma is myabe part of a colour profile.

mskuma · Post by **mskuma** » Fri May 12, 2006 10:58 am

You can't save ICC profile info in a PNG using Photoshop. Sounds like you're better off using Photoshop on your Mac than using GIMP.

Maybe try using something like irfanview to re-save out your PNG. Maybe you can get some clues >here< (see the section "File size and optimisation software", specifically PNG chunks).

Use Photoshop, it's better

Trond · Post by **Trond** » Fri May 12, 2006 4:22 pm

Hmm, I tried some images from the internet and 7 displayed incorrectly. 2 displayed correctly.

To see an example of incorrect display, try this: http://www.w3.org/Graphics/PNG/inline-alpha.html

To see an example of correct display, try this: http://www.ncf.ca/~bh447/pngdemo/bird.png

Edit: When I loaded the broken one using GDI+ it displayed correctly. Could this be a bug in PB's PNG decoder?

mskuma · Post by **mskuma** » Fri May 12, 2006 11:02 pm

Using the following code with the examples you've mentioned, the PNGs seem to display correctly, which would suggest PB's PNG decoder is ok:

Code: Select all

UsePNGImageDecoder() 
LoadImage(0,"alphatest.png") 
OpenWindow(0,0,0,640,480,"",#PB_Window_SystemMenu|#PB_Window_ScreenCentered) 
StartDrawing(WindowOutput(0)) 
  Box(10,10,620,460,RGB(212,220,234))
  DrawAlphaImage(ImageID(0),100,50) 
StopDrawing() 
Repeat 
  ev=WaitWindowEvent() 
Until ev=#PB_Event_CloseWindow

What do you think?

netmaestro · Post by **netmaestro** » Fri May 12, 2006 11:11 pm

mskuma - that code shows correctly here. I've used the alphalayer of png and tiff regularly since beta 1 of PB 4 and with the exception of one early bug that was fixed months ago, never had a problem. I'd be surprised if there were a bug in the decoder.

mskuma · Post by **mskuma** » Sat May 13, 2006 4:52 am

Yes it was thanks to netmaestro who showed us all the way of how to use this function in the early beta days. Many thanks to you sir.

Trond · Post by **Trond** » Sat May 13, 2006 11:39 am

I tried resaving it with IrfanView then it didn't show at all using my code above. It worked (but no alpha transparency) when I loaded it with GDI+.

I then tried resaving it (from the original) with Pixel. It then had that same whiteness to it. As usual, no problem when I used GDI+ for loading.

I then tried resaving it (from the original) with Corel PhotoPaint. It did not show at all. It worked (but no alpha transparency) when I loaded it with GDI+.

Since one png image works and another not, what else can be wrong than the decoder or the png files?

josku_x · Post by **josku_x** » Sat May 13, 2006 11:42 am

I think there's something wrong with the PB PNG decoder. If most of the images show better in GDI+, I'd go with it.

mskuma · Post by **mskuma** » Sat May 13, 2006 12:01 pm

I could be wrong, but I think there's absolutely nothing wrong with the decoder - did you guys try the code I posted above & see netmaestro's result also? Using the 'defective' image, I could get the same result (i.e. alpha/transparency effects) as exhibited on the original web page (i.e. http://www.w3.org/Graphics/PNG/inline-alpha.html). [edit: Note that the alphatest.png image is from that page].