Method RenderText

imgui_tiny_app/imgui/imgui_draw.cpp:5752–5955

Note: as with every ImDrawList drawing function, this expects that the font atlas texture is bound. DO NOT CALL DIRECTLY THIS WILL CHANGE WILDLY IN 2026. Use ImDrawList::AddText().

// Note: as with every ImDrawList drawing function, this expects that the font atlas texture is bound.
// DO NOT CALL DIRECTLY THIS WILL CHANGE WILDLY IN 2026. Use ImDrawList::AddText().
void ImFont::RenderText(ImDrawList* draw_list, float size, const ImVec2& pos, ImU32 col, const ImVec4& clip_rect, const char* text_begin, const char* text_end, float wrap_width, ImDrawTextFlags flags)
{
    // Align to be pixel perfect
begin:
    float x = IM_TRUNC(pos.x);
    float y = IM_TRUNC(pos.y);
    if (y > clip_rect.w)
        return;

    if (!text_end)
        text_end = text_begin + ImStrlen(text_begin); // ImGui:: functions generally already provides a valid text_end, so this is merely to handle direct calls.

    const float line_height = size;
    ImFontBaked* baked = GetFontBaked(size);

    const float scale = size / baked->Size;
    const float origin_x = x;
    const bool word_wrap_enabled = (wrap_width > 0.0f);

    // Fast-forward to first visible line
    const char* s = text_begin;
    if (y + line_height < clip_rect.y)
        while (y + line_height < clip_rect.y && s < text_end)
        {
            const char* line_end = (const char*)ImMemchr(s, '\n', text_end - s);
            if (word_wrap_enabled)
            {
                // FIXME-OPT: This is not optimal as do first do a search for \n before calling CalcWordWrapPosition().
                // If the specs for CalcWordWrapPosition() were reworked to optionally return on \n we could combine both.
                // However it is still better than nothing performing the fast-forward!
                s = ImFontCalcWordWrapPositionEx(this, size, s, line_end ? line_end : text_end, wrap_width, flags);
                s = ImTextCalcWordWrapNextLineStart(s, text_end, flags);
            }
            else
            {
                s = line_end ? line_end + 1 : text_end;
            }
            y += line_height;
        }

    // For large text, scan for the last visible line in order to avoid over-reserving in the call to PrimReserve()
    // Note that very large horizontal line will still be affected by the issue (e.g. a one megabyte string buffer without a newline will likely crash atm)
    if (text_end - s > 10000 && !word_wrap_enabled)
    {
        const char* s_end = s;
        float y_end = y;
        while (y_end < clip_rect.w && s_end < text_end)
        {
            s_end = (const char*)ImMemchr(s_end, '\n', text_end - s_end);
            s_end = s_end ? s_end + 1 : text_end;
            y_end += line_height;
        }
        text_end = s_end;
    }
    if (s == text_end)
        return;

    // Reserve vertices for remaining worse case (over-reserving is useful and easily amortized)