In the current implementation, when text is written over an image, we
have to cut the entire text line out of the image, regardless of how
long the text is. It doesn't look good, but it was a design choice for
the following reasons:
1) To keep the sixel engine as fast as possible
2) Most applications do not write text on the images anyway
To bring the st terminal in line with other terminals that support
sixels, I have now improved the sixel renderer so that the images can
now have gaps, which allows the text to be printed inside the images.
The changes should not affect performance in normal cases. Only when the
renderer has to deal with the text there might be some performance hits
depending on how many gaps there are in the images.
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