A library and viewer for handling ILBM images in SDL applications
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SDL_ILBM is a package containing a library for displaying ILBM images and to manage their state in SDL applications. Additionally, this package contains ilbmviewer, a command-line utility which can be used to view a collection ILBM images inside an IFF file. Furthermore, it supports some of ILBM's advanced (and less commonly used) features, such as color range cycling.


In order to build and use this package the following libraries are required:

  • libiff, in order to parse IFF files
  • libilbm, in order to understand ILBM application chunks, byte-run compression/decompression and to conveniently access ILBM image properties
  • libamivideo, in order to convert Amiga planar graphics data into RGB and to display special screen modes such as EHB and HAM
  • SDL, is used as a portability layer to handle input and graphics

Installation on Unix-like systems

Compilation and installation of this library on Unix-like systems is straight forward, by using the standard GNU autotools build instructions:

$ ./configure
$ make
$ make install

More details about the installation process can be found in the INSTALL file included in this package.

Building with Visual C++

This package can also be built with Visual C++ for Windows platforms. The solution file resides in src/SDL_ILBM.sln that can be opened in Visual Studio to edit or build it. Alternatively, you can also use MSBuild to compile it:

$ MSBuild SDL_ILBM.sln

To make any builds work, you must have a build for libiff, libilbm, libamivideo and SDL 2.0 first. By default, the project file looks for these folders in the parent directory of the current solution.

You can also specify their locations through property parameters. For example, the following instructions specifies where libiff includes and libraries can be found:

$ MSBuild /p:libiffIncludePath:..\..\..\libiff\src /p:libiffLibPath:..\..\..\libiff\src\Debug libilbm.sln

The following property parameters can be used:

  • libiffIncludePath, libiffLibPath
  • libilbmIncludePath, libilbmLibPath
  • libamiVideoIncludePath, libamivideoLibPath
  • SDL2IncludePath, SDL2LibPath

The output is produced in the Debug/ directory.

Using the SDL_ILBM API

The SDL_ILBM package provides an API that can be used to read IFF files containing ILBM images and allows easy usage of these images in SDL applications, including some ILBM's advanced features, such as color cycling. Full API documentation can be found in the doc/apidox directory of this package.

Generating an image set

An ILBM file typically provides one single FORM representing an image, but could also encapsulate multiple images and other types of forms, in so called "IFF scrap files". The SDL_ILBM_Set struct provides an interface to access all ILBM images (and its variants, such as ACBM and PBM images) in a particular IFF file:

#include <set.h>

SDL_ILBM_Set *set = SDL_ILBM_createSetFromFilename("somefile.IFF");

if(set != NULL && set->imagesLength > 0)
    /* Open an image */
    return 0;
    return 1; /* Cannot find any images to display */

The above code fragment generates an image set directly from a file with a given filename. In addition to generating sets from file names, it is also possible to generate a set from a file descriptor (through: SDL_ILBM_createSetFromFd()) or an IFF_Chunk (through: SDL_ILBM_createSetFromIFFChunk()).

When the work is done, a set must be deallocated from heap memory, through:


Displaying a still image

The simplest use case of the SDL_ILBM API is retrieving an ILBM image from the SDL_ILBM_Set and displaying it as a still image, by generating an SDL_Surface and blitting it to another surface or texture.

The following instruction generates an SDL_Surface for the first ILBM image in the image set:

#include <set.h>

SDL_Surface *pictureSurface = SDL_ILBM_createSurfaceFromSet(set, 0, 1, SDL_ILBM_RGB_FORMAT);

The above invocation refers to the image in the set with index 0, uses 1 as lowres pixel scale factor (the width of each source pixel in destination pixels) and "RGB" as output format (meaning that the corresponding SDL surface uses 32-bit true color graphics).

The SDL_ILBM API allows you to pick various options for the lowres pixel scale factor and the format. See sections: 'Choosing a lowres pixel scale factor' and 'Choosing an output format' for more information.

The above SDL_Surface instance can be blitted to another surface as follows:

#include <SDL.h>

if(pictureSurface != NULL)
    SDL_BlitSurface(pictureSurface, NULL, screenSurface, NULL);

Consult SDL's documentation for more details on how to work with SDL_Surfaces for displaying 2D graphics.

Displaying an animatable/cyclable image

Besides displaying still images, it may also be desired to animate an image by using its color cycling properties. The image module encapsulates relevant functionality to do this conveniently.

The following command line instruction composes an SDL_ILBM_Image instance for the first image in an SDL_ILBM_Set, using the same display settings as in the previous example:

#include <image.h>

SDL_ILBM_Image *image = SDL_ILBM_createImageFromSet(set, 0, 1, SDL_ILBM_RGB_FORMAT);

Blitting an SDL_ILBM_Image instance to an SDL_Surface can be done as follows:

SDL_ILBM_blitImageToSurface(image, NULL, destSurface, NULL);

The above function invocation blits the entire image to the given destination surface.

The SDL_ILBM_blitImageToSurface() function is basically a wrapper around SDL's SDL_BlitSurface() function and returns the same exit values (0 in case of success or a value below zero in case of an error). It can also blit from and to sub areas of surfaces by defining the srcrect and dstrect parameters (which have been set to NULL in the example).

Cycling the colors of an animatable/cyclable image

We can shift the color palette of an image (according to the color range specifications in the image) by running:


and restoring the palette back to normal, with:


Each time the palette changes, we must reblit/redraw the surface (e.g. through SDL_ILBM_blitImageToSurface()) to make the color cycling changes visible.

To fully animate a cyclable image, we must regularly reinvoke these functions (for smooth playback: it should be done at least 50 times per second). For example, it can be done by integrating these invocations into the main loop or by invoking them in a timer.

The SDL_ILBM_Image struct internally maintains a table of range times to determine which colors should be changed at a certain time, so there is no need to worry about this. The only obligation is that the SDL_ILBM_cycleColors() function (and corresponding redraw function) should be invoked regularly.

Creating a display window for images

We may also want to construct a window that has the appropriate dimensions for displaying an image. This library provides a display module that can be used to perform some of the required "house keeping" tasks.

The following instruction constructs a display that has the same dimensions as an image's page width and height:

#include <display.h>

SDL_ILBM_Display *display = SDL_ILBM_createDisplay(image, FALSE /* the window's dimensions are equal to the page dimensions */);

The page width and page height correspond to the physical dimensions of an Amiga display. Sometimes the logical width and height of an image could be bigger than the dimensions of the physical screen. In such cases, the Amiga display clips the image and the screen offsets need to be adjusted to display other sub areas of the image.

Modern displays have much bigger display resolutions and can easily display such images entirely on one screen. By setting the stretch parameter to TRUE, a window will be constructed whose dimensions are equal to the logical sizes of the image:

SDL_ILBM_Display *display = SDL_ILBM_createDisplay(image, TRUE /* stretch the window to match an image's logical dimensions */);

We can also use the display module to conveniently construct an SDL window whose dimensions are identical to the display:

SDL_Window *window = SDL_ILBM_createWindow("My Cool ILBM Image", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, display, 0)

The above function is basically a wrapper around SDL's SDL_CreateWindow() function and takes similar parameters. Furthermore, it has the same return values.

In addition to a window, an SDL application also typically needs a renderer which purpose is to render graphics to the window:

#include <SDL.h>

SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, 0);

SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);

As with the SDL window, the renderer's dimensions typically need to correspond to the display's dimensions as well. We can use the following wrapper function to accomplish this:

SDL_ILBM_renderSetLogicalSize(renderer, display);

The above function is a wrapper around SDL's SDL_RenderSetLogicalSize() function taking similar parameters and returning the same exit values.

To render graphics to the window, we must allocate a texture in hardware memory. As with the window and renderer, the texture's dimensions also typically need to correspond to the display. The following command creates a streaming texture with the same dimensions as the display:

SDL_Texture *texture = SDL_ILBM_createTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_STREAMING, display);

The above function is a wrapper around SDL's SDL_CreateTexture() function, takes similar parameters, and returns the same exit values.

The above instructions are based on SDL's 1.2 to 2.0 migration guide instructions. For more detailed information on these function calls, consult the SDL manual.

Blitting an ILBM image to a texture

In addition to SDL_Surfaces, we may also want to blit an image to a texture so that we can render an image to a window. The above code snippet illustrates how to do this with the streaming texture that we have constructed previously:

void *pixels;
int pitch;

if(SDL_LockTexture(texture, NULL, &pixels, &pitch) < 0)
    fprintf(stderr, "Cannot lock texture: %s\n", SDL_GetError());

if(!SDL_ILBM_blitDisplayToTexture(display, SDL_PIXELFORMAT_RGBA8888, pixels, pitch))
    fprintf(stderr, "Cannot blit image to texture!\n");


Rendering a texture to a window

In addition to rendering an image to a texture, we must also render a texture to a window so that its changes become visible. This library offers a convenience function that renders a texture to a display, while taking a display offset into account and clipping the image where necessary:

SDL_ILBM_renderCopy(renderer, texture, 20 /* x offset */, 20 /* y offset */, display);

The function shown above is a wrapper around SDL's SDL_RenderCopy() function taking similar pararmeters and returning the same exit values.

After rendering a texture, we must always call the following function to swap the display buffers so that the rendered graphics become visible:


Choosing a lowres pixel scale factor

On PCs, resolutions refer to the amount of pixels per scanline and the amount of scanlines per screen. On the Amiga, resolutions only refer to the amount of pixels per scanline and only a few fixed resolutions can be used.

For example, a high resolution screen has twice the amount of pixels per scanline compared to a low resolution screen. A super hires screen has double the amount of pixels per scanline compared to a high resolution screen. Moreover, a low resolution pixel is twice a wide as a high resolution pixel and so on.

Vertically, there are only two options. In principle, there are a fixed amount of scanlines on a display. The amount of scanlines can be doubled, using a so-called interlace screen mode. However, interlace screen modes have a big drawback -- they draw the odd scanlines in one frame and the even ones in another. On displays with a short after glow, flickering may be observed.

If we would convert an Amiga image directly to a PC display, we may observe odd results in some cases. For example, a non-interlaced high resolution image looks twice as wide on a PC display than on an Amiga display. To give it the same look, we must correct its aspect ratio by doubling the amount of scanlines on the PC display.

  • By picking 1 as a lowres pixel scale factor, each pixel in the bitplane surface translates to a pixel in the converted chunky/RGB surface, breaking the aspect ratio for certain kinds of images.

  • By picking 2, we can retain the aspect ratio for lowres and hires images. For example, a lowres, non-interlaced picture is twice as wide and twice as high as the original image. For a high resolution, non-interlaced image, the amount of scanlines are doubled.

  • By picking 4, we can retain the aspect ratio for super hires images in addition to hires and lowres images. Lowres, non-interlaced images are four time as wide and four times a high as the original.

  • By picking 0, the API attempts to find the smallest lowres pixel scale factor that retains the aspect ratio.

Choosing an output format

We can convert planar graphics surfaces to surfaces with the following output formats:

  • SDL_ILBM_CHUNKY_FORMAT composes a chunky surface having a 256-color palette in which each byte represents a pixel referring to a color in the palette. A limitation of this output format is that it cannot display images using the HAM screen mode or images with 24 or 32 bitplanes (essentially true-color images).

  • SDL_ILBM_RGB_FORMAT composes an RGB surface in which each pixel is encoded as 4 bytes representing the red color, green and blue color intensity. The remaining byte is used as an alpha layer for transparency. It can display all possible images, but is more memory consuming.

  • SDL_ILBM_AUTO_FORMAT chooses the most memory efficient option -- it will choose SDL_ILBM_CHUNKY_FORMAT unless the bitplane surface requires more than 256 colors. If the latter is the case, then it will choose SDL_ILBM_RGB_FORMAT.

ILBM viewer command-line utility

This package also includes a command-line utility ilbmviewer that exposes most of the API's features as command-line options. For more information, run:

$ ilbmviewer --help


This library is available under the zlib license