Colour change in gallery

[wolfie]

Ok this is now bringing up more problems and I'm guessing no one is actually seeing what I see.

I've just tried IE and Chrome and as everyone is saying the saturation/colours do not change, but, and this is a big but, viewing this particular photo and for all I know all my photos are being displayed incorrectly. ie. oversaturated.
However using Firefox the enlarged image is almost as intended.

Harry

Edited
Clive I was typing my message as you posted.
Thank goodness someone else is seeing almost what I see.
Not sure where I go from here.

Harry

[robski]

Hi Harry

The first thing to remember about jpeg compression is that is designed to take advantage of the weaknesses of the Human vision system. i.e severely nobbles the Colour (chroma channels Cr , Cb) before affecting the sharpness which is maintained in the black and white component ( luminance channel).

If you save a colour image in the bmp format and then save the same image in a heavily compressed jpeg format and then compare the two you will often notice a slight colour shift.

I've taken a look at the internals of your original and wpf resampled file.

The first thing to note is that the ICC profile information has been striped out of the wpf down sampled version. This could have some impact on browsers that now attempt to implement the profile ( I don't pretend to be up to speed on this aspect ).

The chroma sub-sampling is the same in both cases. (2 x 2) this is where 2 pixels on a Cr line and the 2 on the next row are merged in one. The same is applied to the Cb channel. In simple terms the colour components of a 1024 x1024 image have become 512 x 512. The luminance sub-sampling of ( 1 x 1 ) means the the black and white component is unchanged.


For you to understand the next difference I need to explain part of the jpeg process that occurs for the luminance and chroma channels. The image is broken down into 8 x 8 pixel blocks. A Discrete Cosine transform (DCT) is applied to each block. Yes this sounds very complicated but in essence it is measuring the strength of 64 frequencies in that block and replacing 64 image values with 64 strength values. The values in the top left hand corner of the matrix represent lowest frequency components and the bottom right hand corner the highest frequency components.

The next step is what has the biggest impact on the amount of compression when you operate the quality slider in Photoshop or similar setting in other programs. A Quantization table ( containing 64 values ) is used to divide the 64 strength values by. A higher value in the table has the effect of reducing the strength of that frequency component to zero. There are different sets of tables for each quality (slider) setting. Just out of interest there are no standard sets of values in the tables and they differ with different software and camera. That is why the tables used are embedded in the jpeg file so that any application can decode the image correctly.

Heavy compression is obtained by reducing most of the higher frequency components of the image to zero and if you go far enough the 8 x 8 block becomes a single blob.

Below are the table values for each file. In the down sampled version you can see much higher values in the chroma table which will reduce the chroma quality.

PS. Just to add a couple of illustrations of Y (luminance) and Chroma channels (Cr & Cb) and the frequency components ( or patterns if that helps the explanation ) that the DCT process is looking for in a 8 pixel x 8 pixel block. Yes - Jpeg compression involves a number of complex stages ( some of the concepts can be hard to grasp like converting apples to oranges because that works better for you ). What you have above is only part of the story. For example the image is not stored as RGB but converted to a YCrCb colour space which is similar to Lab colour space found in Photoshop.

[wolfie]

Many thanks for the response Rob, I'm afraid I don't fully understand, but accept what you say.

Harry

[robski]

I've added a PS to my previous post. The Jpeg format was never designed to be a suitable medium for all types of images. It was designed to throw away what you may not notice has gone. Principally it caters for photographs which do not have hard (sharp ) edge definition. It makes a very bad job of small black text on a white background with lots of diagonals giving lots of jpeg artefacts. Basically the image is broken down into patterns and reconstructed from the same group of patterns. You will note from the illustration in the previous post that the patterns are formed from vertical and horizontal lines only and this is why diagonals are not reconstructed well.

The Jpeg2000 format is far more complex and uses wavelets to de-construct and reconstruct the image which gives better compression and image quality. The uptake of this format has been very slow for a number of reasons and may not become the replacement for the older jpeg format it was hoped for.

[Gidders]

To avoid colour shifts then, I guess this all means in practice - avoid letting WPF resize your images. Does anyone know what the maximum dimensions are before that happens?

[Teuchter]

For my 2p worth .... they both look exactly the same to me and I am using Safari.

[wolfie]

Clive. I think 1000 pixels is the magic number. The photo I uploaded was 1500 x1000 which displays at 1000p unless you click on it.

The 1500p image displays approx correct, whereas the 1000p is over saturated whichever browser you use. At least Firefox displays the larger image nearer to the original.

Harry

[robski]

I can see that the down sized version has stronger colour in firefox, IE and Chrome.

I have uploaded files at 1024 px with WPF not resizing so I think anything above this gets downsized to 1025 px

I had colour vision and related problems explained to me by an optometrist, The biggest factor is genetics. A gene related to colour vision is found in the X chromosome. If this gene is missing or impaired you will have colour vision problems. Men only have one X chromosome (YX) while Women have two X chromosomes (XX). So if a Woman has a problem with this gene in one X chromosome she has a backup of the other X chromosome. This is why there are far less Women with colour vision problems than Men.

Now with regards to how people see the spectrum (rainbow) of colours. Consider the range of colours like the bellows of an accordion. Colour blindness has the effect of compressing the range of colours to a point where you may just see a red or green instead of all the subtle shades of red and green. In some cases the compression maybe so severe that a colour is missing from the spectrum. Women usually see a very broad range of colours and hence the reason why more womem are colour consultants/specialist than men.

[robski]

On a slightly different point I use my right eye to look through the SLR view finder. A couple of years ago I had a problem with my right eye and I tried using the left eye instead. In some lighting conditions I noticed that the left eye saw stronger reds and weaker blues compared to the right eye. I never gave this much through until talking to a fellow photographer on the subject of colour printing. Quite unprompted he said that each of his eyes saw colour differently. I suppose we take it for granted that each eye sees colour in the same way.

[wolfie]

I certainly see a difference, my right eye sees colours more saturated than my left eye, but I just put it down to the slightly differing viewing angle. Never gave a thought to there being a difference between the left & right eye.

Just as a matter of interest I always use my left eye to the viewfinder, so I'm always leaving greasy marks on the LCD.

Harry