How to render text at a specific height in pixels?

Question

I would like a way to create an image of text that is a specific height in pixels. In the following code, the text is sized to fit in the specified raster. Since there are five letters, we expect each letter, with padding, to consume about 1/5 the raster width. When cropped, the height is a bit larger than that, but that makes sense because letter height will generally be greater than letter width. The problem with this approach is that letter height depends in a complicated way on the widths of the letters used. Is there a simple way to ensure that text is rendered at a specific height (e.g. for the letter "x") in pixels?

rastersize = 256;
tmp0 = Rasterize["jello", RasterSize -> rastersize {1, 1}]
tmp1 = ImageCrop[tmp0]
ImageDimensions /@ {tmp0, tmp1}
rastersize /5.

Answer 1

The PDF trick

Exporting reimporting a PDF of text yields curve primitives which can be rasterized with some control:

Remove[rasterizeWithHeight]
rasterizeWithHeight[t_,h_,"pdf"] := Cases[
  ImportString[ExportString[t,"PDF"],{"PDF","Pages"}],
  Graphics[x_,___]:> 
  Rasterize[Graphics[x,PlotRangePadding->0,ImagePadding->0], 
  RasterSize->{_,h}],\[Infinity]
][[1]]

This is system dependent and often chops off parts of glyphs. Here's a side by side with PlotRangePadding->0 on the left, and default padding on the right:

Perhaps we could do better by manually computing the extrema of the Bezier curves.

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In general, your approach could be to look for accurate data sources of font metrics, or to compute the measurements yourself. There's JLink which exposes the FontMetrics class, but it's unclear whether that would yield better results.

Answer 2

Okay, I'll give it a go, but it just seems unlikely that this is what you want. All I really have to go on is what you said:

I would like a way to create an image of text that is a specific height in pixels

Some sample text:

text = "jello"

When we rasterize, the system will make choices about the image data based on how the image will display. So, we first need to get a feel for that part of the process.

experiment = Rasterize[text];
Options[experiment, ImageResolution]

{ImageResolution->144.}

ImageDimensions[experiment]

{78, 34}

So, to get a height of 5 pixels, we need to choose a value for the ImageResolution option that will give us a height of 5 instead of 34.

textAt5PixelHeight = Rasterize[text, ImageResolution -> (5*144/34)]

ImageDimensions@textAt5PixelHeight

{12, 5}

Now, it sounds like you're wanting to crop any white border. It also sounds like you don't necessarily want to specify a total height, but the height for a particular character. These requirements add complications, but some reasonable approach can be devised. But the bigger problems will be dealing with different fonts, font sizes, and possibly other contextual things that affect the display. So, rather than press forward, I'd like to get your feedback on whether this is actually even in the right ballpark.