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What every developer should know about bitmaps


Virtually every developer will use bitmaps at times in their programming. Or if not in their programming, then in a website, blog, or family photos. Yet many of us don't know the trade-offs between a GIF, JPEG, or PNG file – and there are some major differences there. This is a short post on the basics which will be sufficient for most, and a good start for the rest. Most of this I learned as a game developer (inc. Enemy Nations) where you do need a deep understanding of graphics.

Bitmaps fundamentally store the color of each pixel. But there are three key components to this:

  1. Storing the color value itself. Most of us are familiar with RGB where it stores the Red, Green, & Blue component of each color. This is actually the least effective method as the human eye can see subtle differences on some parts of the color spectrum more than others. It's also inefficient for many common operations on a color such as brightening it. But it is the simplest for the most common programming tasks and so has become the standard.
  2. The transparency of each pixel. This is critical for the edge of non-rectangular images. A diagonal line, to render best, will be a combination of the color from the line and the color of the underlying pixel. Each pixel needs to have its level of transparency (or actually opacity) set from 0% (show the underlying pixel) to 100% (show just the pixel from the image).
  3. The bitmap metadata. This is informat about the image which can range from color tables and resolution to the owner of the image.


Bitmaps take a lot of data. Or to be more exact, they can take up a lot of bytes. Compression has been the main driver of new bitmap formats over the years. Compression comes in three flavors, palette reduction, lossy & lossless.

In the early days palette reduction was the most common approach. Some programs used bitmaps that were black & white, so 1 bit per pixel. Now that's squeezing it out. And into the days of Windows 3.1 16 color images (4 bits/pixel) were still in widespread use. But the major use was the case of 8-bits/256 colors for a bitmap. These 256 colors would map to a palette that was part of the bitmap and that palette held a 24-bit color for each entry. This let a program select the 256 colors out of the full spectrum that best displayed the picture.

This approach was pretty good and mostly failed for flat surfaces that had a very slow transition across the surface. It also hit a major problem early on with the web and windowed operating systems – because the video cards were also 8-bit systems with a single palette for the entire screen. That was fine for a game that owned the entire screen, but not for when images from different sources shared the screen. The solution to this is a standard web palette was created and most browsers, etc. used that palette if there was palette contention.

Finally, there were some intermediate solutions such as 16-bits/pixel which did provide the entire spectrum, but with a coarse level of granularity where the human eye could see jumps in shade changes. This found little usage because memory prices dropped and video cards jumped quickly from 8-bit to 24-bit in a year.

Next is lossy compression. Compression is finding patterns that repeat in a file and then in the second case just point back to the first run. What if you have a run of 20 pixels where the only difference in the second run is two of the pixels are redder by a value of 1? The human eye can't see that difference. So you change the second run to match the first and voila, you can compress it. Most lossy compression schemes let you set the level of lossiness.

This approach does have one serious problem when you use a single color to designate transparency. If that color is shifted by a single bit, it is no longer transparent. This is why lossy formats were used almost exclusively for pictures and never in games.

Finally comes lossless. This is where the program compresses the snot out of the image with no loss of information. I'm not going to dive into what/how of this except to bring up the point that compressing the images takes substantially more time than decompressing them. So displaying compressed images – fast. Compressing images – not so fast. This can lead to situations where for performance reasons you do not want to store in a lossless format on the fly.


Transparency comes in three flavors. (If you know an artist who creates web content – have them read this section. It's amazing the number who are clueless on this issue.) The first flavor is none – the bitmap is a rectangle and will obscure every pixel below it.

The second is a bitmap where a designated color value (most use magenta but it can be any color) means transparent. So other colors are drawn and the magenta pixels are not drawn so the underlying pixel is displayed. This requires rendering the image on a selected background color and the edge pixels that should be partially the image and partially the background pixel then are partially the background color. You see this in practice with 256 color icons where they have perfect edges on a white background yet have a weird white halo effect on their edges on a black background.

The third flavor is 8 bits of transparency (i.e. 256 values from 0 – 100%) for each pixel. This is what is meant by a 32-bit bitmap, it is 24-bits of color and 8 bits of transparency. This provides an image that has finer graduations than the human eye can discern. One word of warning when talking to artists – they can all produce "32-bit bitmaps." But 95% of them produce ones where every pixel is set to 100% opacity and are clueless about the entire process and the need for transparency. (Game artists are a notable exception – they have been doing this forever.) For a good example of how to do this right take a look at Icon Experience – I think their bitmaps are superb (we use them in AutoTag).


Many formats have a resolution, normally described as DPI (Dots Per Inch). When viewing a photograph this generally is not an issue. But take the example of a chart rendered as a bitmap. You want the text in the chart to be readable, and you may want it to print cleanly on a 600 DPI printer, but on the screen you want the 600 dots that take up an inch to display using just 96 pixels. The resolution provides this ability. The DPI does not exist in some formats and is optional in others (note: it is not required in any format, but it is unusual for it to be missing in PNG).

The important issue of DPI is that when rendering a bitmap the user may want the ability to zoom in on and/or to print at the printer's resolution but display at a lower resolution – you need to provide the ability for the calling program to set the DPI. There's a very powerful charting program that is useless except for standard viewing on a monitor – because it renders at 96 DPI and that's it. Don't limit your uses.

File formats

Ok, so what file formats should you use? Let's go from most to least useful.

PNG – 32-bit (or less), lossless compression, small file sizes – what's not to like. Older versions of some browsers (like Internet Explorer) would display the transparent pixels with an off-white color but the newer versions handle it properly. Use this (in 32-bit mode using 8 bits for transparency) for everything.

ICO – This is the icon file used to represent applications on the desktop, etc. It is a collection of bitmaps which can each be of any resolution and bit depth. For these build it using just 32-bit png files from 16x16 up to 256x256. If your O/S or an application needs a lesser bit depth, it will reduce on the fly – and keep the 8 bits of transparency.

JPEG – 24-bit only (i.e. no transparency), lossy, small file sizes. There is no reason to use this format unless you have significant numbers of people using old browsers. It's not a bad format, but it is inferior to PNG with no advantages.

GIF – 8-bit, lossy, very small file sizes. GIF has two unique features. First, you can place multiple GIF bitmaps in a single file with a delay set between each. It will then play through those giving you an animated bitmap. This works on every browser back to the 0.9 versions and it's a smaller file size than a flash file. On the flip side it is only 8 bits and in today's world that tends to look poor (although some artists can do amazing things with just 8 bits). It also has a set color as transparent so it natively supports transparency (of the on/off variety). This is useful if you want animated bitmaps without the overhead of flash or if bandwidth is a major issue.

BMP (also called DIB) – from 1 up to 32-bit, lossless, large file sizes. There is one case to use this – when speed is the paramount issue. Many 2-D game programs, especially before the graphics cards available today, would store all bitmaps as a BMP/DIB because no decompression was required and that time saving is critical when you are trying to display 60 frames/second for a game.

TIFF – 32-bit (or less), lossless compression, small file sizes – and no better than PNG. Basically the government and some large companies decided they needed a "standard" so that software in the future could still read these old files. This whole argument makes no sense as PNG fits the bill. But for some customers (like the federal government), it's TIFF instead of PNG. Use this when the customer requests it (but otherwise use PNG).

Everything Else – Obsolete. If you are creating a bitmap editor then by all means support reading/writing every format around. But for other uses – stick to the 2+4 formats above.

What every developer should know series:

  • What every developer should know series
  • Tags Start & End

    Tags Can Start & End Anywhere

    Appendix B

    .NET code for multi-page image output

    Appendix A

    Java code for multi-page image output

    Data Bin Search

    The Data Bin can now be searched to find a table, column, node or other piece of data without scrolling through it all.

    Shrink to Fit

    This will shrink the contents of a cell until it fits the defined cell size.

    Time Zone Conversion

    A new Windward macro has been added to help with converting dates and times from UTC time to the local time zone.

    Image Output Format

    New image output formats added.

    PostScript Output Format

    PostScript, commonly used with printers and printing companies, has been added as an additional output format.

    New and Improved Datasets (Designer, Java Engine, .NET Engine)

    Datasets have been re-written from scratch to be more powerful and easier to use.

    Stored Procedure Wizard (Designer)

    This works for all tag types that are connected to a SQL-based data source (Microsoft SQL Server, Oracle, MySQL, or DB2).

    Boolean Conditional Wizard (Designer)

    Before, conditional statements could only be written manually. Now they can also be built using our intuitive Wizard interface.

    Reorganized Ribbon

    The ribbon menus have been re-organized and consolidated to improve the report design workflow.

    XPath 2.0 as Data Source

    Adds various capabilities such as inequalities,descending sort, joins, and other functions.

    SQL Select Debugger

    SQL Select  Debugger

    • The look and feel was improved
    • Stored Procedure Wizard
    • Improved Exceptions pane

    Tag Editor/Tag Selector

    Added a Query tab as a field for typing or pasting in a select statement

    • Color Coding of Keywords
    • TypeAhead
    • Evaluate is now "Preview"

    Rename a Datasource

    All tags using that Data source will be automatically updated with that name.

    Connecting to a Data Source

    New single interface to replace 2 separate dialog boxes

    Tag Tree

    Displays of all the tags in the template, structured as they are placed in the template. This provides a simple & intuitive way to see the structure of your template. Also provides the capability to go to any tag and/or see the properties of any tag.

    Added Javelin into the RESTful Engine

    Support for Google Application Engine Integration

    The ability to integrate the Windward Engine into Google’s cloud computing platform for developing and hosting web applications dubbed Google Applications Engine (GAE).

    Additional Refinement for HTML Output

    • Improved indentation for ordered and unordered lists
    • Better handling of template header and footer images
    • Better handling for background images and colors

    Redesigned PDF Output Support

    This new  integration will allow for processing of complex scripts and bi-directional  text such as Arabic.  Your PDF output  will be much tighter and more closely match your template, and we’ll be able  to respond rapidly to PDF requests and fixes.

    PowerPoint Support

    Includes support for new ForEach and slide break handling, table header row repeat across slide breaks, and native Microsoft support for charts and images.

    Tags are Color Coded

    Tags are color coded in the template by type, making it easy to visually identify them.

    Increased Performance

    Version 13’s core code has been reworked and optimized to offer a reduced memory footprint, faster PDF generation and full documentation of supported features and limitations in the specifications for DOCX, XLSX and PPTX.

    Advanced Image Properties

    Documents can include advanced Word image properties such as shadows, borders, and styles.

    Improved HTML Output

    Windward has updated HTML output to reflect changing HTML standards.

    Version 13 New Data Sources

    Windward now works with a slew of new datasources: MongoDB, JSON, Cassandra, OData, Salesforce.com

    Generate Code

    The Generate Code tool in the designer allows you to open an existing template and, with a click of a button, automatically create a window with the code needed to run your current template with all data sources and variables. Simply copy this code and paste into your application's code in the appropriate place. You now have Windward integrated into your application.

    You only need to do this once. You do not do this for each template. Instead, where it has explicit files for the template and output, change that to parameters you pass to this code. Same for the parameters passed to Windward. This example uses explicit values to show you what to substitute in where.

    Pivot Tables Adjusted in Output

    Any pivot tables in an XLSX template are carried over to the XLSX output. The ranges in the pivot ranges are adjusted to match the generated output. So your final XLSX will have pivot tables set as expected in the generated file.

    This makes creating an XLSX workbook with pivot tables trivial.

    Imported Template Can be Set to Match the Parent Styles

    In an imported sub-template, if its properties for a style (ex. Normal) differ from the parent template's properties for the style, the use in the sub-template can be set to either use the properties in the sub-template, or the properties in the parent.

    You set to retain when you don't want the child template's styling to change when imported. You set to use the parent when you want the styling of the imported template to match the styling in the parent.

    Any explicit styling is always retained. This only impacts styling set by styles.

    Tags can be Placed in Text Boxes

    Tags can be placed in text boxes. Including linked text boxes. This gives you the ability to set the text in a textbox from your data.

    Tags can be Placed in Shapes & Smart Art

    Tags can be placed in shapes & smart art. This gives you the ability to set the text in a shape from your data.

    HTML Output Supports Embedded Images

    When generating HTML output, the engine can either write bitmaps as distinct files the generate HTML references, or it can embed the images in the HTML providing a single file for the output.

    Footnotes & Endnotes can Have Tags

    You can place tags in pretty much any part of a template, including in footnotes & endnotes.

    Document Locking Supported in DOCX & XLSX

    Any parts of a DOCX or XLSX (PowerPoint does not support this) file that are locked in the template, will be locked the same in the output.

    Specify Font Substitution

    If a font used in the template does not exist on the server generating a report, the font to substitute can be specified.
    In addition, if a glyph to be rendered does not exist in the font specified, you can specify the replacement font. This can be set distinctly for European, Bi-Directional, and Far East fonts.

    Process Multiple Datasources Simultaneously

    If you need this - it's essential. And if you don't need it, it's irrelevant.

    Windward enables you to build a document by applying multiple datasources to the template simultaneously. When Windward is merging the data into a template, it processes the template by handling each tag in order, and each tag pulls from different datasources. (As opposed to processing all of one datasource, then processing the next.)

    This allows the select tag to use data from another datasource in its select. For example, if you are pulling customer information from one data source, you can then pull data from the sales datasource using the customer ID of the customer presently processing to pull the sales information for that customer. If you're interested in patching together your data from multiple datasources, read this post on our blog.

    David Thielen

    President/CEO at Windward Studios

    From his early years as a Senior Developer at Microsoft, to legendary designer of the popular Enemy Nations strategy game, to reporting and document generation guru, Dave has never lost his passion for building superb software and teams.

    Written by:_
    David Thielen
    President/CEO at Windward Studios


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