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From: David Abrahams (abrahams_at_[hidden])
Date: 2001-03-23 21:27:18


I think it would help if you could be a bit more specific about how you
might use the iterators. For example, what would you iterate over? Pixels,
bytes, scanlines, or something else?

A couple of code samples showing how such an iterator might be used would be
extremely helpful (both to you and to the rest of us) ;-)

----- Original Message -----
From: "Michael D. Crawford" <crawford_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Friday, March 23, 2001 10:17 PM
Subject: [boost] Bitmap pixel iterators?

> I'm about to design a set of iterators to ease direct access into
> graphics buffers. I would like something with the utility and ease of use
> STL iterators, with the difference that the containers will all have the
> type but contain different data formats depending on different flags.
> the pixels will in general iterate over rectangular regions.
> You boost folks are more familiar than I about the design issues with
> iterators so I thought I'd ask for tips on what I should try to achieve
> what I would best avoid.
> You may not know anything about graphics programming, but perhaps you'd
> something to say about iterators in general. If you had to design a set
> iterators to serve any particular purpose at all, what would you want them
> have in common? What can you say about the strengths and weaknesses of
> different iterators that you've encountered, both their abstract API and
> they've actually been implemented in real systems?
> I'll be addressing the ZDCPixmaps from the ZooLib cross-platform
> framework, but on the different platforms these are wrappers around such
> things as Macintosh GWorlds and Windows DIBs and the like. Potentially
> work could be generalized to other graphics APIs by making them templates
> adding traits parameters.
> ZooLib is open source under the MIT License and is available at
> I wrote a graphics editor in ZooLib last year and needed to do a lot of
> access to pixels through raw pointers to read and write graphics files,
> paint and vectors and do alpha blending as well as scaling the images.
> Each time I did this I would need to ask the ZDCPixmap such questions as
> base address, its dimensions, its pixel bit depth, the number of bytes in
> image row (sometimes there are pad bytes to align the rows to make access
> faster at the cost of a little memory), whether the image was inverted (it
> would be on Windows but not on Mac), whether it had an alpha channel as
> as its byte order.
> Then when I twiddled the pixels the operations would all be based on this
> information in complicated ways, for example I ran into the case of
scaling a
> 24-bit packed bitmap into a 32 bit unpacked bitmap and having to add in an
> extra byte in the alpha channel to make up the 32 bits.
> In the course of this work I went from hard-wiring everything into local
> variables to putting some of the functions into library routines and
making it
> reasonably efficient. I never went beyond 8 bits per color channel,
either 24
> bit or 32 bits per pixel.
> I feel I gained a good understanding of the problem but never felt my
> solutions were as clean or as general as I would like, nor did I feel they
> were something I'd want someone else to use. I'd like to write something
> that would be of more general use, at first in my own application but
> eventually to be folded into the ZooLib library for everyone to use.
> ZooLib provides easy-to-use GetPixel and SetPixel calls that will access
> pixel at a time and set or fetch a 24-bit RGB value. But it's quite slow
> large data sets because it needs to make a lot of complicated decisions
> regarding the pixel format each time a pixel is accessed; one might have
> indexed, 24-bit RGB packed pixels or 32-bit aligned RGB pixels, and one
> have to use a color table to convert between RGB values and an index.
> all the platforms it supports there are many formats it might encounter.
> The most basic purpose of the iterator is to cache this format information
> provide routines that are directly optimized for whatever the result of a
> particular caching instance happens to be.
> I found pointers to member functions to be helpful in an earlier try at
> but as I traced in the debugger through the pointer to member code, I saw
> in one development system at least, dereferencing one actually takes a
> significant number of assembler opcodes.
> Here's what I envision I'll have when I'm done, I know it's a pretty tall
> - I want them to be extremely fast - the main objective of writing these
> things is speed. With my first, primitive shot at this I had
> animated alpha blending of semitransparent objects a couple hundred pixels
> across working responsively on a 150 MHz PowerPC 604 Macintosh. It would
> great if one could use them in a video game.
> - They also need to be quite platform independent; in my case this is
aided by
> the encapsulation of platform-specific details in the implementation
member of
> the ZDCPixmap class.
> - They must be easy to use. I would like someone to feel comfortable
> from GetPixel/SetPixel to using my iterators. Someone who is not as good
> programmer as me must be able to pick them up quickly and readily learn
how to
> use them correctly.
> - They will be used in a multithreaded environment (ZooLib provides
> cross-platform locking and thread classes), although I'm not clear to what
> extent you'd want multiple threads hitting the same iterator. One
> use would be to create an iterator to a rectangular area, than have an
> algorithm that would spawn threads that would divide the operation it
> out on that iterator among two or more CPUs.
> - The iterator could range over an entire image or a rectangular part of
> - You can have const iterators for reading from bitmaps
> - You can have non-const ones for writing into them
> - You can have iterators that are optimized for sequential access or
> access. Sequential access may potentially be faster.
> - One can use these to support masking operations, where the mask may be
> entirely separate image or an alpha layer, and may be on or off, or an
alpha blend
> - There are also Region data objects, which aren't strictly bitmaps, but
> logically are more like lists of rectangles that the graphics system
> boolean operations with as well as scaling. One can convert a bitmap into
> region and vice-versa.
> I'd want to iterate over the pixels that are contained in the part of a
> specified by a region. One some platforms like Macintosh Quickdraw the
> regions use clever algorithms and data structures for speed and efficient
> of memory, but because Apple has patented its region algorithm most other
> platforms use much less sophisticated representations.
> - I'll have algorithms that take the boundary points of iterators as their
> inputs. One such algorithm, possibly complicated, would scale a
> area from one bitmap into a differently sized and proportioned are into
> another, which might be the same bitmap.
> - One would have the option of using iterators that give you native access
> raw pixel data, say 16 bits with 5 bits of RGB and 1 bit of alpha, or one
> might have automatic conversion to comfortable common formats, like
> of color value classes that would then have helpful member functions to
> operate on the color.
> - One could also use iterators from two different bitmaps that had
> different formats and combine them together in a graphics operation, for
> example to blend a 16 bit 5,5,5,1 RGBA graphic into a 4-bit indexed
> - It would be extremely helpful if new pixel or bitmap formats could be
> without ripping into the guts of the thing. This is likely to occur as
> overall ZooLib library is bound to new OS and GUI systems.
> Well I guess that's enough to stimulate some conversation.
> Mike
> --
> Michael D. Crawford
> GoingWare Inc. - Expert Software Development and Consulting
> crawford_at_[hidden]
> Tilting at Windmills for a Better Tomorrow.
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