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| Help in analysing PNG http://gimpchat.com/viewtopic.php?f=8&t=18891 |
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| Author: | GIMP101 [ Mon Nov 02, 2020 11:26 am ] |
| Post subject: | Help in analysing PNG |
GIMP Version: 2.8.22 Operating System: Windows OS Version: Windows 10 GIMP Experience: New User Hello, I hope somebody can help me! I am trying to find a hidden image/data in a PNG. As far as I can tell using GIMP, the image is single layer sRGB with no alpha channel. A Compress Or Die 'heat map' of the image clearly shows a 'spiral' of high density non repeating 'data' over a localised section of the image; how would I find out exactly what this is? EXIF/metadata fields are empty except for the comments section where there are six characters '<<6&FF' - could this relate to colour palette information in some way? Any comments or help would be most appreciated. Thanks |
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| Author: | sallyanne [ Mon Nov 02, 2020 11:35 am ] |
| Post subject: | Re: Help in analysing PNG |
Are you able to upload the image? |
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| Author: | GIMP101 [ Mon Nov 02, 2020 12:39 pm ] |
| Post subject: | Re: Help in analysing PNG |
Yes, of course I can. But I would really like to try to 'solve' it myself first using GIMP as opposed to being given the answer by a super experienced GIMP user! I can post a link to the image soon ... |
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| Author: | Nidhogg [ Mon Nov 02, 2020 1:19 pm ] |
| Post subject: | Re: Help in analysing PNG |
It relates to RGBA color model, the FF in the end is the alpha channel. |
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| Author: | GIMP101 [ Mon Nov 02, 2020 2:28 pm ] |
| Post subject: | Re: Help in analysing PNG |
Thank you. I will experiment with RGBA and see what happens ... |
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| Author: | ofnuts [ Tue Nov 03, 2020 7:39 am ] |
| Post subject: | Re: Help in analysing PNG |
"<<" is the shift-left operator in C and other programming languages, so this could mean to shift 6 bits left and then keep the lower 8 bits (& FF). Shifting left is multiplying by 2⁶=64. |
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| Author: | GIMP101 [ Tue Nov 03, 2020 7:54 am ] |
| Post subject: | Re: Help in analysing PNG |
Thank you. Yes, I was thinking that maybe the '<<' indicated a bitwise left shift operator but if so (and please correct me if I am wrong), I'm not sure how to go from there to three 8-bit numbers for RGB and then to use GIMP to display just the colour represented by those numbers ... |
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| Author: | Nidhogg [ Tue Nov 03, 2020 2:12 pm ] |
| Post subject: | Re: Help in analysing PNG |
Displaying all pixels sharing same color can be done from Menu: Select/By Color. Drag the threshold slider to 0.0, and paint a small area with same color as your target color, on which you will click with the color selection tool. Searching specific colors in an image would be a cool plug-in. If it's already possible somehow I'd really like to know. |
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