In the Muslim community, the congregation mosque, or masjid al-Jami in Arabic, is a place with expressive connotations. It is a place of worship and a place of gathering. Mosques are buildings that are intended to be acceptable for worship. The mosque has always been the most important building in Islamic society and has remained an important resource for Islamic organizations. The primary functions of a mosque are to be a place of private and public worship, and a center for religious education [1]. The prayer hall, or haram, due to its function as a place of worship for the worshipers, is the most essential architectural element in the mosque [2]. The qibla, one of the walls of the prayer hall, always faces Mecca. Additionally, the courtyard is another essential element that seems to be an uncovered space, usually surrounded by columns or galleries, that serves as a place for worshippers to congregate [2] (Figure 1).

In addition, various factors affected the form, function, and architectural meaning of the mosque in today’s multi-ethnic society, which led to the modification of mosques’ spatial organization and layout pattern, especially the prayer hall [3]. The word classification refers to a schema consisting of different classes and the relationships among them; there is a relation between classification and typology. Vice versa, the name “typology” relates to the classification of objects [4]. Typology is one of the most useful methods for exploring and clarifying the characteristics of the built environment. In architecture, type and typology are important fields of study. It may be just as essential to the field of architecture as the concepts of space and form. Similarly, morphology represents an important item in the current research which involves the relationship between architectural form elements and the studying spatial geometry. Furthermore, morphological characteristics, which deal with the examination of form and its properties, have become current research’s core study [5]. Moreover, shape grammar is a design tool that may be used to describe, analyze, and generate designs. Correspondingly, the classification and analysis of design characteristics in architecture represent the main purposes of shape grammar that were already explained by Hussein et al. [6]. Again, pattern recognition is a scientific discipline that seeks to classify items into groups or classes so that they can be organized and arranged according to similarity measures.

2. Mosque Typo-Morphological Classification for Pattern Recognition Using Shape Grammar Theory and Graph-Based Techniques

The classification of mosques was previously the subject of many studies; it is important to shed light on these studies to understand the various approaches used in this aspect. The studies reviewed, examined, and assessed mosque typologies with a focus on design context, geometric level, and physical design elements to determine the main characteristics that the classification of mosques depends on (Table 1). Some of the studies are descriptive and are documentary studies, organized chronologically or geographically to provide a clear understanding of the mosque’s design principles, and they lead to a typological classification. In this context, a classification made by Hoag [7] gives a history of Islamic architecture from the 7th century to the 17th century in the Middle East, Africa, Spain, and India. It classified mosques into eighteen styles, including photos and plans of the most important and well-known mosques and shows how the design of mosques modified over time all over the world. On the other hand, a descriptive study by Mo’nis [8] was dedicated to providing a clear idea about the history of mosques and classified them into seven categories as Moroccan style, Andalusia style, Egyptian style, Turkish style, Seljuk style, Indian style, and Iranian style. Additionally, contemporary mosques have been described and classified in qualitative analysis. Four main types were explored, named vernacular, traditional, modern, and historical designs ^[9][10][24][9,10,24]. Furthermore, the Arab hypostyle, the Persian four-Iwan, the Indian three-dome, and the Turkish central-dome mosque are the four primary styles of mosques that have been addressed in research by Hakim [10] in western and central Asia and North Africa. Many classifications have been conducted through typological analysis. The most common classification examined by Hillenbrand was focused on three main types, such as the Arabic type, Persian type, and Turkish type [12], while the study by Aazam [14] developed these three basic types in more detail and named them as (a) the hypostyle with dome emphasis and courtyard type, (b) the central-domed type, (c) the hypostyle with domical vaulting and an extensive courtyard type, and (d) the detached pavilion within a walled garden enclosure type. Parallel to this, four typologies have been described through an analysis of the main axis characteristics and its directionality: the Arabic type as the horizontal axis, the Ottoman type as the vertical axis, the Iwan type as cross axis, and the Indian type as multi-axis [13]. In addition to the three basic patterns described by Hillenbrand [12], the study by Khachan [15] added three more patterns and identified six types of traditional mosques throughout the world: hypostyle, central dome, Iwan, Indian, Chinese, and Southeast Asian mosques. Furthermore, the analytical method by Mustafa and Hassan [16], which was conducted to quantify the influence of spatial configuration on the functional efficiency of mosque layouts revealed six forms of the Ottoman style: single-dome layout, earring layout, multiple-dome layout, duplication-dome layout, courtyard-dome layout, and earring-dome-courtyard layout.  However, an analysis of the typological plan and main components in various traditional mosques in Peninsular Malaysia has identified the forms and categories of the mosque on the building structure. Thus, two traditional mosque plans—square and rectangular—have been finalized by Ahmad [17]. Particularly, the focus of the descriptive analysis by Loukma and Stefanidou [18] on Macedonian mosques is the typological features that divided mosques into two main categories, single-unit and and multi-space, depending on various areas. Likewise, mosques in Saudi Arabia are classified spatially and typologically by analyzing their space shape, enclosure, symmetry, and complexity. As a result, basic prayer halls are classified as closed or semi-closed if they are attached to a courtyard as well as symmetrical or asymmetrical based on the plan’s layout [19]. Based on the typological study, the plan layout typology of historical mosques in Erbil, with determining the extent to which these typologies are connected with Islamic known styles, Mustafa and Ismael [20] focuses on nine different styles, beginning with the Prophet’s Mosque style and ending with the Ottoman style, and narrows upon the three basic forms of mosques—the hypostyle, the Iwan, and the central-domed form. Once again, eight categories were found by Rashid [21], such as the hypostyle hall, a prayer hall with mud-brick, a huge courtyard that is enclosed by a prayer hall, a prayer hall with a sloping roof, a mosque with four Iwan and a courtyard, a prayer hall with a big dome covering its central part, a prayer hall covered by a very large central dome supported by a series of half-domes and frequently accessed from an axially oriented entrance. More specifically, some studies attempted to classify mosques depending on morphological analyses, architectural shape, structural system, and form components. The study by Ardalan [22] focuses on the main five general types of mosques with their morphological characteristics such as hypostyle, hypostyle with a dome over the qibla axial line, hypostyle with multi-dome, Iwan style, and central-domed mosque. Moreover, four basic types have been illustrated by Al-Omary [23], dedicating the courtyard and non-courtyard types to the shape and form of a whole mosque layout which is first, Arabic style, second, the Seljuk type which has no courtyard, third, the Ottoman style, and lastly, the Iwan type. Accordingly, Mosques are classified into six types, morphologically, regarding their geographical distribution, including primitive mosques containing courtyards and flat-roofed prayer halls, Islamic countries mosques’ shape, early Arabic mosques, Iranian mosques with four Iwans, and Turkish central-dome mosques with Indian-style mosques [5]. On the other side, Al-Jameel [25] presents a new classification of congregational mosques based on a digital approach to pattern recognition. For a more accurate and objective classification, the cases chosen for the study cover the majority of the Islamic period and region and a competitive computational model was used. The study examines an unsupervised texture image classification algorithm to achieve five categories which are the flat roof, domed roof, articulated roof, domed roof with courtyard and domed-gallery roof, and a domed roof with courtyard and sloped-gallery roof. Furthermore, a morphological analysis conducted by Cherif and Allani-Bouhoula [26] on Tunisian mosques concluded that there is no single model of the mosque in Tunisia rather than a morphological diversity characterized by a range of varying spatial configurations. It is concluded from the foregoing that the prayer hall is the most important element in mosques which has a great influence on classification and categorization due to its distinctive architectural characteristics such as size, layout, proportion, and form.