Histogram, what is cache???

GD
Posted By
Gustavo_Del_Vechio
Jun 1, 2005
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1917
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15
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Closed
Hello,
I’m trying to understand histogram palete…
I have not knew to understand about what is cache and histogram statistics, but one is not important now…

About cache:
Could somebody please explain me what would be 1 cache???

In memory and cache, I can choose 8 caches???
When I need to use 1 cache, when I need to use 8 caches???

What does 1 cache do on my pc??? It affect visualy a photo???

Thank’s
Gustavo.

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Michael_D_Sullivan
Jun 1, 2005
The cache level can be set in preferences.

The various levels of cache are used when viewing at 50% or less zoom level. From what I understand (perhaps imperfectly), cache level 2 stores a simplified view of the image that is used at 50% zoom; presumably it averages every four pixels for display as one pixel; cache level 3 would repeat this for 25% zoom, etc. The histogram pallette uses the cache to display an approximation of the effect of various manipulations on an image, and a more accurate representation is obtained by clicking on the "uncached refresh" icon, which uses the full image data instead of the cache to update the histogram.

And no, the cache level does not affect your photo.

Chris Cox, please correct me if my understanding is incorrect.
GD
Gustavo_Del_Vechio
Jun 1, 2005
Michael,
By what I have understood…
How much more cache, more detail an image will have in displaying??? (this, in less zoom than 100 percent)

Is correct my understanding???
TD
Thee_DarkOverLord
Jun 1, 2005
Is correct my understanding???

Yoda you are?
GH
Gernot_Hoffmann
Jun 1, 2005
In addition to Michael’s explanation:

Cache Level 1 means: all pixels are used for statistics (PhS help text).
Cache Level >1 means: a smaller number of samples is used (not specified in my PhS7 help text).

The histogram has K=256 classes (0 to 255 from left to right). According to standard methods for technical measurement it’s not necessary to use more than S=K^2 samples (roughly 65000). The samples can be taken by equally distributed random xy-pixel coordinates from the WHOLE image (don’t know how PhS
handles this).
Taking more pixel doesn’t change the appearance of the histo- gram for practical purposes.

Of course the final image quality is not affected by the cache settings.

Best regards –Gernot Hoffmann
GD
Gustavo_Del_Vechio
Jun 2, 2005
Hi, Friend,s
I still didn´t know to undersantd…
Cache is only for hystogram displaying?? is it??

Could please somebody expalin me again, but in other words???
MD
Michael_D_Sullivan
Jun 2, 2005
Cache is used for histogram display (unless you have turned this off). Cache is (probably) used to generate statistics.
Cache is used to speed the display of images at 50% zoom and lower.
GD
Gustavo_Del_Vechio
Jun 3, 2005
Michael….

What does chache when imagem is 50%????
MD
Michael_D_Sullivan
Jun 5, 2005
First, this is the image cache, not the cache file.

The level 2 cache is a 50% sized version of the main image (level 1). If you change the image’s brightness, contrast, color, etc., it’s simpler and faster to display the manipulations on a version of the image that has fewer pixels, so at 50% zoom, you see the results of the manipulation on the smaller image faster, instead of updating the main image and then shrinking it. Presumably the level 3 cache is used for 25% zoom, etc.
GD
Gustavo_Del_Vechio
Jun 6, 2005
What´s relationship between cache and buffer????

What´s relationship between cache and scratch disk???
MD
Michael_D_Sullivan
Jun 7, 2005
Cache, scratch disk, and buffer are all (generically) the same concept: a temporary, short-term storage location. The temporary storage concept is used for several purposes. A temporary copy of your entire image, with all layers, and history/undo information is stored in a file in the scratch disk. Also included in the scratch disk file is the image cache, also known as the image pyramid, which is working copies of the current image at 100%, 50%, and lower resolutions. (My guess is that the cache level 3 image is at 25%, cache level 4 is at 12.5%, etc., because that would make sense and be fast, but I don’t have any inside knowledge.) Not being a PS developer or an Adobe employee, I can only read between the lines of postings by Chris Cox, but it seems to me that when you make changes to an image, those changes are made to the main (cache level 1) image and to the lower-level cached images as well, instead of recreating the cached images from the full image. This allows reduced scale images (50% or less) to be displayed faster, but at the cost of slightly reduced accuracy, by using the modified cached image instead of computing all the changes to the full image and then converting it down to fewer pixels. For example, if you reduced the brightness of an image by one or another means, PS reduces the brightness of the main image but also applies the same transformations to the 50% image in the cache instead of converting the main image down to 50%. If you are zoomed to 50%, you will see results faster using the cached image.

Neither Adobe nor I have used the term "buffer" in this connection. A buffer, to a programmer, is simply a temporary storage location in RAM for something that has been read from, or is to be written to, a file. In essence, it’s an easy way to refer to a file’s contents. In the hard disk drive world, a buffer or disk cache is an intermediary storage location (using RAM in the drive) that temporarily holds data being read from, or written to, disk but is seen by the OS and programs as being the disk itself. So, for example, a program such as PS might tell the OS to open a file for reading; the drive then pulls the first X MB of data from the file into a temporary buffer or cache and tells the OS "here’s the data!"; the OS then tells PS that the data can be accessed from a certain location; PS then uses OS conventions to "read x" bytes from the disk, and the disk drive’s cache spits up those bytes instead of actually reading them. Or, depending on the computer language being used, the program (such as PS) may open up a buffer, or temporary storage location, to pull down data from the "file" in the disk drive’s buffer, store it in RAM in a particular format, and supply bytes as needed as though it is coming from the hard drive file.
GD
Gustavo_Del_Vechio
Jun 7, 2005
Hello, Michael….

I think I am undersanding about caches…
Please, read below and say me if is correct:

Cache is a local where photoshop puts all information whick will need (if necessary) faster. This is snapshots, or most recent actions, etc…
But, if you have been working with 1 cache, will not import if you is working in 12,5 or 50 or 100 zoom, photoshop makes and record back actions using all pixels (Level 1)
If you select 2 cache levels, in the level 1 we have an 100% image version, and in cache 2, we have an image 50%, whick has fewer pixels therefore it redraws histogram faster…

If i am correct, please, answer my question below….

If i use more caches, does my computer work slowly than when I have just 1 cache??

Because I think photoshop needs to make (example 4 cache levels) 4 versions of image immediately when I make na action…

Thank’s
Gustavo
MD
Michael_D_Sullivan
Jun 8, 2005
I think you are largely correct on the cache. I don’t know if higher cache levels slow the computer. They do take additional memory and/or scratch file space to store. I have seen some advice not to use more than cache level 4 if you don’t have a lot of memory and a fast computer. I don’t have either, so I stay at cache level 4. Try some experiments with different cache levels and report back on speed differences!
GD
Gustavo_Del_Vechio
Jun 9, 2005
Hello, Michael…

Thank’s for you explanation..
Could I ask just more 1 question about it???

I finnaly have understood about caches. But, what happen when do I check "using cache for histogram" option???????

Thank’s again

Gustavo
MD
Michael_D_Sullivan
Jun 9, 2005
When you select that option, it uses the cached view of the file (fewer pixels and faster to update) for displaying the histogram. While doing a Curves or Level operation, for example, the histogram will be updated as you manipulate the image, based on the reduced view of the file in the cache. This will only approximate the actual effect on the file, but it’s fast. When you want to see the true effect on the full image, click on the little icon above the histogram.
GD
Gustavo_Del_Vechio
Jun 9, 2005
Ok, Michael, I have understood.

Thank´s for you explanation….

Gustavo

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