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I am looking for design literature discussing the thematic graphical modification of text. Often, images (from a theme) are modified to make a letter / word discernible in them. Two examples of this "modification" -
A Harry Potter Themed "Always" (link) - In this picture, the word "Always" has been written using Harry Potter themed icons, such as wizards, the Deathly Hallows, Quidditch Rings etc.
Google Doodles - Google Doodles are modifications of the word "Google" in the Google homepage logo to commemorate an event or festival. For example, the doodle for 40 years of Sesame Street (link) replaced the 'o' with Cookie Monstar's eyes.
My question is - What is this graphical modification of text called? What are some keywords I can use to search for design literature discussing this modification?